Ieee 525 pdf free download

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Johnson, and N. Kasztenny, M. Thompson, and N. Zocholl, A. Guzman, and D. Stringer, L. Lawhead, T. Wilkerson, J. Biggs and G. Guzman, N. Fischer, and C. Thompson and R. Hodder, B. Kaszdenny, N. Fischer, and Y. If you use Firefox, add a. Lecture 2 Lecture 2 handwritten notes. Lecture 5 Lecture 5 handwritten notes CT modeling and response. Zocholl and D. This web page points to a link for ordering it.

Lecture 17 Lecture 17 handwritten notes Reference material See section Lecture 23 Introduction to Bus Protection. Lecture 26 Lecture 26 handwritten notes Transformer connection compensation matrices for common connections PDF Transformer connection compensation matrices for common connections Mathcad Transformer connection compensation matrices for common connections Mathcad Prime Transformer protection examaple for lecture 27 PDF Transformer protection example for lecture 27 Mathcad Transformer protection example for lecture 27 Mathcad Prime Reference Material Link to paper: L.

PDF extension to the file that is downloaded. During a fault, the portion of fault current which is conducted by a ground electrode in the earth causes a rise of the electrode voltage with respect to remote earth see IEEE Std and [B26].

Both electromagnetic coupling and conduction can contribute to substantial ground voltage rise differences, particularly at the higher frequencies typical of many transients occurring on a high-voltage power system. Even well designed grounding grids that extend over the large areas needed for high-voltage switchyards have sufficient inductance to cause high voltage differences.

Electromagnetic coupling to the ground grid is directly proportional to the rate of change of flux and the length and orientation of the current-carrying conductor and inversely proportional to the height of the conductor above the ground grid.

Conduction of power system transients to the ground grid is typically provided through metallic grounding of transformer neutrals and capacitive paths, such as bushings, coupling capacitors, and CCVTs.

These are low-impedance high-energy paths that can induce common-mode voltages on control circuits see IEEE Std Other phenomena that generate transients occur in power systems. Some examples are undesirable time spans between the closing of the poles of a circuit breaker, fault occurrence, fault clearing, load tap-changing, line reactor de-energizing, series capacitor gap flashing, arcing ground faults, failing equipment, lightning, GIS surges, and capacitor reinsertion.

Normally, the magnitudes of such transients are less than those of other phenomena described herein. The selection of the shield grounding locations and the effects of single and multiple grounds are points to be considered for the proper installation of shielded cable. Multiconductor cable applications in the operating range of 2 kV to 5 kV require careful judgment, and each installation should be evaluated based on the existing and anticipated conditions.

Shielding can be used to monitor or test cable installation for additional assurance of insulation integrity. A shield screen material is applied directly to the insulation and in contact with the metallic shield. It can be semiconducting material or, in the case of at least one manufacturer, a stress control material.

At the high voltages associated with shielded cable applications, a voltage gradient would exist across any air gap between the insulation and shield.

The voltage gradient may be sufficient to ionize the air, causing small electric arcs or partial discharge. These small electric arcs burn the insulation and eventually cause the cable to fail. The semiconducting screen allows application of a conducting material over the insulation to eliminate air gaps between insulation and ground plane.

Various shield screen material systems include: a Extruded semiconducting thermoplastic or thermosetting polymer b Semiconducting woven fabric tape c Semiconducting coating paint used with semiconducting woven fabric tape d Extruded high-dielectric-constant thermoplastic or thermosetting polymer, referred to as a stress control layer NEMA and AEIC standards require shielded power cable to be partial-discharge or corona tested.

This test evaluates the effectiveness of the conductor and shield insulation screen materials and application, and verifies the absence of voids within the insulation. A cyclic aging test is required by AEIC as a qualification of the cable design to ensure gaps do not develop between tape layers as a result of expansion and contraction cycles.

If all elements of the shield are not removed, excessive leakage current with tracking or flashover may result. Accidental removal of the shield ground can cause a cable failure and a hazard to personnel. The length of cable run should be limited by the acceptable voltage rise of the shield if the shield is grounded at only one point.

The derating of ampacity due to multiple-point short circuited shields has a negligible effect in the following cases for three-phase circuits: a Three-conductor cables encased by a common shield or metallic sheath b Single-conductor shielded cables containing kcmil copper or smaller installed together in a common duct c Triplexed or three-conductor individually shielded cables containing kcmil copper or smaller d Single-conductor lead sheathed cables containing kcmil copper or smaller installed together in a common duct Because of the frequent use of window type or zero-sequence current transformers for ground overcurrent protection, care must be taken in the termination of cable shield wires at the source.

If the shield wire is passed through the window-type current transformer, it should be brought back through this current trans- former before connecting to ground in order to give correct relay operation.

Table can be used to calculate the induced shield voltage. A maximum voltage of 25 V, under normal operating conditions, is a commonly accepted limit. For cables installed three per conduit, use arrangement II in table. All three phases of a circuit should be installed in the same conduit. When it is necessary to run only one phase per conduit, then nonmetallic or nonmagnetic metallic conduit should be used. If nonmagnetic metallic conduits are used, the reduction of cable ampacity due to conduit heating should be considered.

Also, no magnetic metal, such as clamps or reinforcing bar, should form a closed ring around the conduit. Table 2 gives the maximum lengths of single conductor cable with shields grounded at one point to stay within the 25 V maximum for the conditions stated. Other conditions will permit different lengths. For example, cables operated at less than rated ampacity will allow longer lengths. Direct-buried cables operating at their rated ampacity, with all other conditions being the same, will require shorter lengths to stay below the 25 V maximum.

The length listed is the duct length. For further details on shielding and grounding of instrumentation and control circuits, see IEEE Std Common-mode noise may be caused by one or more of the following: a Electrostatic induction. With equal capacitance between the signal wires and the surroundings, the noise voltage developed will be the same on both signal wires. With the magnetic field linking the signal wires equally, the noise voltage developed will be the same on both signal wires.

NOTE — When electromagnetic shielding is intended, the term electromagnetic is usually included to indicate the difference in shielding requirement as well as material. To be effective at power system frequencies, electromagnetic shields would have to be made of high-permeability steel. Such shielding material is expensive and is not normally applied. Other less expensive means for reducing low-frequency electromagnetic induction, as described herein, are preferred.

Isolating the circuits from ground effectively opens the ground common-mode voltage path through the signal circuit. If an intervening amplifier is a single-ended amplifier, the low side of the signal circuit is not broken and is grounded at the terminal. Therefore, the situation is not changed, so the same procedure should be followed with the terminal as indicated above. A guarded isolated differential amplifier provides isolation of both input terminals from the chassis or ground and from the output.

This amplifier is capable of high-common-mode rejection and provides the input-output isolation so that the output ground will not affect the input circuit. Typically, the common-mode rejection ratio of an isolated differential amplifier used in instrumentation systems is about dB and is the ratio of common-mode voltage applied to the amount of normal-mode voltage developed in the process.

When an ungrounded transducer is used, it may be possible to obtain satisfactory results by leaving the transducer circuit ungrounded, connecting the cable shield to the amplifier guard shield, and grounding the shield at either the transducer end or the amplifier end. However, it is considered that connecting the cable shield to the amplifier guard shield and grounding both transducer cable shield and circuit at the transducer will result in a less noisy, more stable system.

See 6. A properly grounded shield will greatly reduce the capacitance between the signal conductors and external sources of electrostatic noise so that very little noise voltage can be coupled in the signal circuit.

By alternately presenting each conductor to the same electromagnetic field, voltages of equal magnitude and opposite polarity are induced in each conductor with respect to ground. The frequency of twisting lay affects noise reduction ability and, therefore, should be considered in specifying twisted pair cable. The materials normally used for shielding of instrumentation cable are nonferrous and cannot shield against power frequency electromagnetic fields. The steels normally used in conduit or tray are not of high enough permeability to provide very effective shielding at power frequencies.

However, some benefit may accrue from the use of rigid steel conduit or steel trays with solid bottoms and tightly fitting solid steel covers. However, physical separation in itself, unless carefully analyzed, may not achieve the desired degree of immunity. Cables should be run in accordance with clause An exception to this is when the shield is used for the excitation of a neutralizing transformer.

If the shield is grounded at some point other than where the signal equipment is grounded, charging currents may flow in the shield because of differences in voltage between signal and shield ground locations. If the shield is grounded at more than one point, differences in ground voltage will drive current through the shield. In either case, shield current can induce common- mode noise current into the signal leads, and by conversion to normal-mode noise, voltage proportional to signal circuit resistance unbalance can reduce accuracy of signal sensing.

In a system with grounded transducer and isolated- input differential amplifier, the cable shield should connect to the amplifier guard shield, but grounding the shield at the amplifier will reduce the amplifier's common-mode rejection capability. Grounding the shield only at the transducer will maintain the shield at the same ground voltage as the transducer, which will minimize shield-induced common-mode current while permitting the amplifier to operate at maximum common-mode rejection capability.

Also see 6. They can also be used in a combined isolating-drainage transformer configuration. They neutralize extraneous longitudinal voltages resulting from ground voltage rise or longitudinal induction, or both, while simultaneously allowing ac or dc metallic signals to pass. For noncomputer type applications, such as annunciators, shielding may not be required. Separation of digital input cables and digital output cables from each other and from power cables may be required.

Where digital inputs originate in proximity to each other, twisted pair multiple conductor cables with overall shield should be used or multiple conductor cable with common return may be permitted, and overall shielding may not be required. Digital output cables of similar constructions may also be permitted. Individual twisted and shielded pairs should be considered for pulse-type circuits. When two lengths of shielded cable are connected together at a terminal block, an insulated point on the terminal block should be used for connecting the shields.

Furthermore, the use of shielded, twisted pairs into balanced terminations greatly improves transient suppression. It is never acceptable to use a common line return both for a low-voltage signal and a power circuit [B13]. A separate ungrounded power supply should be furnished for the group of RTDs installed in each piece of equipment. Normally, the maximum voltage will exist at the instant of interruption. Magnitude is very dependent on supply circuit impedance.

If impedance is high, voltage will be proportionally high. The higher the speed of interruption, the higher the surge voltage generated. Voltages in excess of 10 kV have been observed across a V coil in laboratory tests, but 2. DC circuit energization has an effect on adjacent circuits where capacitive coupling exists. Full battery voltage appears initially across the impedance of the adjacent circuit and then decays exponentially in accordance with the resistance- capacitance RC time constant of the circuit [B38].

The extensive use of surge capacitors on solid-state equipment and the longer cable runs associated with extra-high voltage EHV stations have substantially increased the capacitance between control wiring and ground. Inadvertent momentary grounds on control wiring cause a discharge or a redistribution of charge on this capacitance. Although this seldom causes failure, the equipment may malfunction. Saturation of current transformers by high-magnitude fault currents, including the dc offset, can result in the induction of very high voltages in the secondary windings.

This phenomenon is repeated for each transition from saturation in one direction to saturation in the other. The voltage appearing in the secondary consists of high-magnitude spikes with alternating polarity persisting for an interval of a few milliseconds every half cycle [B38].

The most significant interference comes from voltages or currents, or both, induced in the circuits as a result of exposure to nearby conductors in which transient currents or voltages appear as a result of switching or faults.

Although voltages used in the transmission of power have been increasing over the years, the level of control voltages and signal power has had a tendency to remain constant or even decrease. Since induced interference increases with the use of higher voltages and increased fault current levels, the ratio of unwanted signal noise to useful signal will be increased if precautions are not taken to protect the signal circuits. Transient voltages on cables cannot be completely eliminated, but can be limited in magnitude.

In the interest of compatibility with solid-state relaying systems, one suggested limit is the peak of the surge withstand capability SWC test described in IEEE Std C Many different things can be done separately or in combination to reduce the magnitude of the transients, depending upon economics and equipment configuration.

The following methods are primarily confined to control cable installation. Because mutual capacitance and mutual inductance are greatly influenced by circuit spacing, small increases in distance may produce substantial decreases in interaction between circuits [B8]. Where possible, control cables should be routed perpendicular to high-voltage buses [B17], [B38]. When control cables must be run parallel to high-voltage buses, maximum practical separation should be maintained between the cables and the buses [B8].

NOTE — Tests indicate that in some cases, nonshielded control cables may be used without paralleling ground cables when they are parallel and are located at a distance greater than 50 ft Great care should be exercised in routing cables through areas of potentially high ground grid current either 60 Hz or high-frequency currents [B17]. When practical, control cables may be installed below the main ground grid. All cables from the same equipment should be close together, particularly to the first manhole or equivalent in the switchyard [B17].

High-voltage cables should not be in duct runs or trenches with control cables [B8], [B17], [B38]. Radial arrangement of control circuitry will reduce transient voltages. Circuits routed into the switchyard from the control house must not be looped from one piece of apparatus to another in the switchyard with the return conductor in another cable.

All supply and return conductors must be in a common cable to avoid the large electromagnetic induction possible because of the very large flux-linking-loop that the loop arrangement provides [B8], [B38]. The ground grid, even when designed with a very low resistance, cannot be considered as an equal-voltage surface. Substantial grid voltage differences may occur that will be directly influenced by a number of factors, for example, grid resistance, grid geometry and distribution of ground currents see IEEE Std , earth resistivity see [B51] and IEEE Std , and frequency of the transient [B14].

Since it is impractical to eliminate grid voltage differences, their effects must be neutralized. Neutralization can be accomplished by a low-resistance shield conductor parallel to, and in proximity to, the affected control circuits. Such a conductor may be the shield of a shielded control cable, unused conductors of an unshielded control cable, or a separate shield conductor. These conductors will carry currents proportional to the grid voltage differences and induce a counter voltage in the control circuits, thus effecting neutralization.

Grounding, neutralizing, and shielding methods that have been found to be effective are as follows: a In trench systems, shield conductors which are connected to the substation grid as necessary should be attached to the top sides of the trench.

This places the shield conductors between the transient source and the control cables [B51]. These shield conductors should have sufficient conductivity to carry fault currents without damage and have adequate mechanical strength.

This ground bus provides a convenient means of grounding individual cable shields. For equivalent conductivity, several smaller shield conductors are more effective than a single large conductor. Provisions should be made for replacement with shield conductors should the unused conductors later be used for active circuits. To be most effective, shield conductors must be in the closest possible proximity to the control cables, particularly where unshielded cables are used.

Making the ground connection at the relay or control building has the following advantages: 1 Voltage rise is minimized near the relay equipment. To facilitate one point grounding, all capacitor banks of a given voltage should be at one location. For a concentric shield, this ratio should be one. For an adjacent shield wire, the ratio must always be less than one. If the resistance of the shield is considered, then the cancelling voltage generated by the shield current is reduced by the ratio of the self-inductive reactance of the shield to the total complex self-impedance of the shield.

The resistance becomes significant at low frequencies where the inductive reactance of the shield is low and can generally be neglected at high frequencies see IEEE Std Protection may take the form of surrounding the sensitive circuits with an equal-voltage surface to prevent capacitive coupling to high-voltage conductors and magnetic shielding to mitigate the effect of strong magnetic fields. When shielded control cable is used, grounding the shield at both ends is recommended [B13].

Care should be exerted in keeping the shield intact, as a broken or separated shield can greatly reduce the shield efficiency. If only one end of the shield is grounded, large transient shield-to-ground and conductor-to-ground voltages may be present at the ungrounded end [B8], [B35].

Grounding a shield at both ends allows shield current to flow. The shield current resulting from magnetic induction creates a counter-flux which will tend to cancel the flux that created the shield current.

The net effect of the shield on the lead is to reduce the noise level. An exception to this is that the current flowing in shields not produced by flux linking the lead will cause the surge or noise voltage on the lead to be higher than it would be if there were no shield [B38], [B13], [B51]. The lower the shield impedance, the greater is the amount of transient voltage cancellation because of greater current flow.

Generally, a lower surge impedance permits larger induced transient currents to flow in the shield [B35]. A grounding conductor may be run parallel to the shielded cable to help protect the shield from being damaged when fault currents are present [B51]. If electrostatic shields are required, they should be within the outer shield [B35]. Auxiliary power and yard lighting circuits should not be installed without adequate shielding near shunt capacitor banks [B8]. Experience has shown that in high-voltage substations, steps should be taken to reduce the transients in auxiliary power cables, lighting cables, etc.

Three specific types of conductors are normally used: insulated single conductor, coaxial cable, and triaxial cable. It can also be used as the interconnecting lead for short bypasses. Bare conductors and coaxial cables should be avoided for these applications, since either one can introduce excessive leakage currents or excessive stray capacitance. The stray capacitance can cause a reduction in bandwidth, and the leakage currents can cause a loss in carrier power.

To reduce stray capacitance and leakage currents, either of the following methods may be used: a An insulated single conductor should be run as directly as possible between its required terminations. It should be mounted on insulators and fed through bushings at each end.

The conductor insulation should be unbroken between its ends to maintain low leakage. The insulated single conductor should be isolated from the flexible metal conduit with non- conductive washers spaced about 6 in mm apart.

If the conductor has a significant portion of its length outside the flexible metal conduit, it should be mounted on insulators and fed through bushings at its ends as in item a. A typical insulated carrier lead, 0. If both shield ends are grounded, large surge currents can flow under certain conditions, causing saturation of the impedance-matching transformer and resulting in an inoperative carrier channel.

This cable provides an additional heavy shield which does not carry signal currents. The outer shield is capable of carrying large induced surge currents under fault conditions and is grounded at both ends.

This arrangement provides very effective shielding against both magnetic and electrostatic induction so that surges induced in the signal leads are small.

Chlorosulfonated polyethylene and silicone rubber compounds are examples of materials that have been used in high-temperature cables or where cable fire propagation is a consideration.

This voltage can be reduced by lowering the surge impedance achieved by mounting the CCVTs as close to the ground as permitted by clearance standards and by providing multiple low-resistance conductors between the CCVT base and the station ground grid, and between phases.

All secondary circuits from the CCVTs should be radial and contained within a single shielded cable to provide cancellation of the differences in ground grid voltage [B8]. The secondary cables should follow the ground conductor as closely as possible. Cable penetration fire stops, fire breaks, system enclosures, and cable coatings This clause provides guidance for the selection and application of cable penetration fire stops, cable fire breaks, cable system enclosures cocoons , and coatings for cable systems.

NOTE — Several types of fire stops, cable system enclosures, fire barriers, and coatings are made from materials that are thermal insulators. Their use can result in significant cable derating, which should be considered in sizing cables. NOTE — To be effective, fire barriers must have sufficient fire resistance to withstand the effects of the most severe fire that may be expected to occur in the area adjacent to the fire barrier, and must provide a complete barrier to the spread of fire.

Modifications or additions of cables through the fire stop should not compromise the integrity of the fire stop. A special example of this method is using a solid section of tray which is then filled with sealant. The sealant or compound should be compatible with the cable outer surface material. The void around the cable should be filled with a fire-resistive seal.

Cable penetration fire stops should be used when sleeve or tray penetrations are used beneath control boards or other panels. Fire detection systems This clause provides guidance or information for the selection of fire detection systems for cable systems. Automatic fire detection systems may be installed in areas of high cable concentration. The zone of influence is determined by extending lines from the bottom of the side rails of the lowest cable tray at a 30 degree angle from vertical see figure 1.

Fire detection systems may also be considered in areas of lesser cable concentration that provide vital service, or areas where, because of its location, a cable fire may go unnoticed for a relatively long period of time. Fire-extinguishing systems This clause provides guidance for the selection and application of fire-extinguishing systems protecting cable systems.

Fire-extinguishing systems may be utilized for the protection of cable systems. Installation and handling This clause provides guidance for the construction methods, materials, and precautions in handling and installing cable systems.

Optical cable is addressed separately in this clause. During storage, the ends of the cables should be sealed against the entrance of moisture or contamination. Reels should be stored on solid ground to prevent the flanges from sinking into the earth. Handling or pulling cables in extremely low temperatures can cause damage to the cable sheathing, jacketing or insulation.

To prevent damage of this nature, store cables in a heated building at least 24 h prior to installation. The lubricant should not set up so as to prevent the cable from being pulled out of the conduit at a later time. Cable lubricants should not support combustion. Turning the reel and feeding slack cable to the duct entrance will reduce the pulling tension.

Whenever a choice is possible, the cable should be pulled so that the bend or bends are closest to the reel. The worst condition possible is to pull out of a bend at or near the end of the run. Cable splices should not be placed directly on racks or hangers.

It is desirable that the system be designed so that additions and changes can be made with ease, economy, and minimum outages. If necessary, cables should be protected by fire-resistant material.

The radius of the feeder device should not be less than the minimum bending radius of the cable. If a feeder device is not used, the cable should be hand-guided into the raceway. The ends of all other cables should be properly sealed during and after installation in wet locations. Cables such as aluminum, mineral-insulated, paper, and varnished cambric should be resealed after pulling, regardless of location. If water has entered the cable, a vacuum should be pulled on the cable or the cable should be purged with nitrogen to extract the water.

Projections and sharp edges on pulling hardware should be taped or otherwise covered to protect against snagging at conduit joints and to prevent damage to the conduit. A mandrel should be pulled through all underground ducts prior to cable pulling. Any abrasions or sharp edges that might damage the cable should be removed. Minimum bending radius should never be less than that recommended by the manufacturer. To prevent damage by deformation due to excessive bearing pressure or cable tension, vertically run cables should be supported by holding devices in the tray, in the ends of the conduit, or in boxes inserted at intervals in the conduit system.

Raceway fill, maximum sidewall pressure, jam ratio, and minimum bending radius are design limits which should be examined in designing a proper cable pull.

These design limits are prerequisites needed in designing a cable raceway system. Once these limits are determined for a particular cable, the raceway system can then be designed.

If the system has already been designed, modifications may be required in order to pull the cable without damage. Conduit and duct system design should consider the maximum pulling lengths of cable to be installed.

The maximum pulling length of a cable or cables is determined by the maximum allowable pulling tension and sidewall pressure. The pulling length will be limited by one of these factors. Pull points or manholes should be installed wherever calculations show that expected pulling tensions exceed either maximum allowable pulling tension or sidewall pressure. A sample calculation for determining cable pulling tensions is shown in the annex.

If the fill limitations and cable area are known, the raceway area can be calculated and an adequate size can be selected. The cable manufacturer should be consulted when tensions exceeding these limits are expected.

When pulling by basket grip over a nonleaded jacketed cable, the pulling tension should not exceed lb 4. When using a basket-weave type pulling grip applied over a lead-sheathed cable, the force should not exceed lb 6. The maximum allowable sidewall pressure is lb per ft of radius for multiconductor power and control cables and single-conductor power cables 6 AWG and larger, subject to verification by the cable manufacturer.

The recommended maximum allowable sidewall pressure for single-conductor power cable 8 AWG and smaller is lb per ft of radius subject to verification by the cable manufacturer. Jam ratio is defined for three cables of equal diameter as the ratio of the conduit inside diameter D to the cable outside diameter d. The jam ratio is a concern because jamming in the conduit could cause damage to one or more of the cables.

The possibility of jamming is greater when the cables change direction. Therefore, the inside diameter of the conduit at the bend is used in determining the jam ratio. The values given are usually stated as a multiple of cable diameter and are a function of the cable diameter, and whether the cable is nonshielded, shielded, armored, or single or multiple conductor.

It is very accurate where the incoming tension at a bend is equal to or greater than 10 times the product of cable weight per foot times the bend radius 10 w r expressed in feet.

If the tension into a bend is less than 10 w r, the exact equations can be found in reference [B41]. Cases in which the exact equations may become necessary are where light tensions enter large radii bends. Usually equation 19 is precise enough for normal installations. The glass fibers are usually well protected by buffer tubes inside the cable itself. Even though the glass in the fiber is actually stronger higher tensile strength per unit area than a metal conductor, there is very little cross-sectional area in a fiber available for strength and support.

For this reason, most optical cables have other components to provide the strength for cable support during pulling, handing, etc. The maximum allowable pulling tension on optical cable can vary from as low as 50 lb force The maximum tension for a particular optical cable should be obtained from the cable manufacturer. This maximum recommended pulling tension should be noted on any drawings, installation instruction, etc.

The theory of pulling tension is the same for optical cable as it is metallic conductor cable. Pulling tension can be calculated based on cable weight, conduit system design, and lubricated coefficient of friction. Probably the most common installation mistake is making tight bends in the cable. Tight bends, kinks, knots, etc. Minimum bending radius in traditional optical cable is usually in the range of 20 times the cable diameter, considerably higher than electrical cable; however, new fiber technologies are lowering this minimum bending radius.

This bending radius should be considered by the engineer when specifying conduit bends and pull box openings or sizing guide pulleys, sheaves, mid-assist capstans, etc. Optical cables are often pulled for much longer distances than electrical cables.

Continuous fiber pulls of over ft m are not uncommon. These long pulls minimize the number of splices in optical cable which is desirable for fiber performance. The light weight of the cable makes these long pulls possible, although proper lubrication and a good conduit installation are also necessities. Pulling lubricants with some unique features are required by the special nature of optical cable pulling, i. Lightweight optical cable rubs on all sides of the conduit through the natural undulation of long straight runs.

Many common lubricants flow to the bottom of the raceway and lose effectiveness in this type of pulling. As with electrical cable, specific coefficients of friction depend on cable jacket type, conduit type, and the lubricant as well. One of the types of conduit used for buried optical cable is the continuous-reeled type. Such continuous duct is popular because it is inexpensive and offers enough protection to allow the use of the less expensive cable constructions. While these undulations may look minor, they can result in hundreds of degrees of bend per thousand foot of pull, and vastly increase pulling tensions even with an extremely low friction coefficient.

Short-length optical cable pulls may not require lubricant; however, for long or complex fiber pulls, lubricant is critical to making an efficient, high quality installation. Some of the requirements for optical cable pulling lubricant are: a Compatibility with polyethylene no stress cracking and other types of cable jacket b Complete and even coating on the cable for friction reduction at all friction points c Consistent low coefficient of friction over time Acceptance testing of installed cables This clause provides guidance for the testing of cables after installation, but before their connection to equipment, and includes cable terminations, connectors, and splices.

It should be noted, however, that these tests may not detect damage that may eventually lead to cable failure in service, e. Cable ends should be properly cleaned of all conducting material. Cable test results, environmental conditions, and data should be recorded and filed for maintenance reference. These cables may be tested as part of the system checkout. Unshielded high-voltage cables should not be subjected to high-voltage dc tests; insulation resistance tests are suggested.

Raceways This clause provides guidance for both a means of supporting cable runs between electrical equipment and physical protection to the cables. Raceway systems consist primarily of cable tray and conduit. If they are installed direct buried in soil, consideration should be given to the zinc coating having a limited life, and corrosion may be rapid after the zinc coating is consumed or damaged.

When used in cinder fills, the conduit should be protected by non-cinder concrete at least 2 in thick.

When used where excessive alkaline conditions exist, the conduit should be protected by a coat of bituminous paint or similar material. PVC-coated steel conduit may be used in corrosive environments. Plugs should be used to seal spare conduits in wet locations. Since ABS and PVC conduit may have different properties, a review should be made of their brittleness and impact strength characteristics. Coefficient of expansion should also be considered for outdoor applications.

Flammability of such conduits is of particular concern in indoor exposed locations. Burning or excessive heating of PVC in the presence of moisture may result in the formation of hydrochloric acid which can attack reinforcing steel, deposit chlorides on stainless steel surfaces, or attack electrical contact surfaces.

The use of exposed PVC conduit indoors should generally be avoided, but may be considered for limited use in corrosive environments. Aluminum conduit may be exposed in wet and dry locations. Aluminum conduit should not be embedded in concrete or direct buried in soil unless coated bitumastic compound, etc.

Aluminum conduit may be used, exposed or concealed, where a strong magnetic field exists; however, conduit supports should not form a magnetic circuit around the conduit if all the cables of the electrical circuit are not in the same conduit.

Protection should be provided against attack by insects, rodents, or other indigenous animals. Cost savings may be realized when comparing above grade trays, conduit, and troughs to similar below-grade systems.

Care should be taken in routing above grade systems to minimize interference with traffic and equipment access, and to avoid a reduction in minimum electrical clearances. These systems are more vulnerable to fires, mechanical damage, environmental elements, and seismic forces, and offer greater susceptibility to electrostatic and electromagnetic coupling than if the cables were below grade.

The judicious location of these boxes may result in considerable savings. Liquid-tight flexible conduit is commonly used for this application.

Flexible conduit length should be as short as practical, but consistent with its own minimum bending radius, the minimum bending radius of the cable to be installed, and the relative motion expected between connection points. A separate ground wire should be installed if the flexible conduit is not part of the grounding and bonding system.

Drain fittings and air vents in the equipment enclosure should also be considered. Expansion couplings should be installed in the conduit run or at the enclosure to prevent damage caused by frost heaving or expansion.

Reinforcing steel in the manhole walls should not form closed loops around individual nonmetallic conduit entering the manhole. Nonmetallic spacers should be used. When this is not practical, lean concrete or porous fill can be used between the frost line and the duct bank.

Each threaded joint should be cleaned to remove all of the cutting oil before the compound is applied. The compound should be applied only to the male conduit threads to prevent obstruction.

The connections should be of minimum lengths and should employ at least the minimum bending radii established by the cable manufacturer. Loading calculations should include the static weight of cables and a concentrated load of lb N at midspan. The tray load factor safety factor should be at least 1. For horizontal elbows, rung spacing should be maintained at the center line.

A minimum clearance of 9 in In selecting material for trays, the following should be considered: a A galvanized tray installed outdoors will corrode in locations such as near the ocean or immediately adjacent to a cooling tower where the tray is continuously wetted by chemically treated water.

If an aluminum tray is used for such applications, a corrosive-resistant type should be specified. Special coatings for a steel tray may also serve as satisfactory protection against corrosion.

The use of a non-metallic tray should also be considered for such applications. Ice, snow, and wind loadings must be added to loads described in Aluminum alloys T6, T6, and M34 are acceptable, with careful recognition of the differences in strength. Mill-galvanized steel should normally be used only for indoor applications in noncorrosive environments. Hot-dipped galvanized-after-fabrication steel should be used for outdoor and damp locations.

Tray load capacity is defined as the allowable weight of wires and cables carried by the tray. This value is independent of the dead load of the tray system. In addition to and concurrent with the tray load capacity and the dead load of the tray system, any tray should neither fail nor be permanently distorted by a concentrated load of lb N at midspan at the center line of the tray or on either side rail. This results in cable crossing and void areas, which take up much of the tray cross-sectional area.

This will result in a tray loading in which no cables will be installed above the top of the side rails of the cable tray, except as necessary at intersections and where cables enter or exit the cable tray systems. This restraint is generally applicable to instrumentation cables, but may also apply to power and control cables. Conduit connections through the tray bottom or side rail should be avoided.

When cable trays are used as raceways for solidly grounded or low-impedance grounded power systems, consideration should be given to the tray system ampacity as a conductor. Inadequate ampacity or discontinuities in the tray system may require that a ground conductor be attached to and run parallel with the tray, or that a ground strap be added across the discontinuities or expansion fittings.

The ground conductor may be either bare, coated, or insulated, depending upon metallic compatibility. Tray sections should be supported near section ends and at fittings such as tees, crosses, and elbows. Wireways are for exposed installations only and should not be used in hazardous areas.

Consideration should be given to the wireway material where corrosive vapors exist. In outdoor locations, wireways should be of raintight construction. Taps from wireways should be made with rigid, intermediate metal, electrical metallic tubing, flexible-metal conduit, or armored cable. Direct burial, tunnels, and trenches This clause provides guidance for the installation of cables that are direct buried or installed in permanent tunnels or trenches.

The excavation is then backfilled. A layer of sand is usually installed below and above the cables to prevent mechanical damage. Care must be exercised in backfilling to avoid large or sharp rocks, cinders, slag, or other harmful materials. A warning system to prevent accidental damage during excavation is advisable. Several methods used are treated wood planks, a thin layer of colored lean concrete, a layer of sand, strips of plastic, and markers above ground. Untreated wood planks may attract termites, but overtreatment may result in leaching of chemicals harmful to the cables.

Spare cables or empty capped ducts for future cables may be installed before backfilling. This system has low initial cost, but does not lend itself to changes or additions, and provides limited protection against the environment. Damage to cables is more difficult to locate and repair in a direct burial system than in a permanent trench system.

This system has the advantages of minimum interference to traffic and drainage, good physical protection, ease of adding cables, shielding effect of the ground mat, and the capacity for a large number of cables. Disadvantages include high initial cost and danger that fire could propagate between cable trays and along the length of the tunnel. If fire stops are provided, hazards can be minimized. Duct entrances may be made at the bottom of open-bottom trenches or through knockouts in the sides of solid trenches.

Trenches may be made of cast-in-place concrete, fiber pipes coated with bitumastic, or precast material. Where trenches interfere with traffic in the substation, vehicle crossovers—permanent or temporary—may be provided as needed.

Warning posts or signs may be used to warn vehicular traffic of the presence of trenches. The trenches may interfere with surface drainage and can be sloped to storm sewers, sump pits, or French drains. Open-bottom trenches may dissipate drainage water but are vulnerable to rodents. A layer of sand applied around the cables in the trench may protect the cables from damage by rodents. Trenches at cable entrances into control buildings should be sloped away from the building for drainage purposes.

The trenches also should have a barrier to prevent fire or rodents from entering the control building. The tops of the trench walls may be used to support hangers for grounded shield conductors. The covers of trenches may be used for walkways. Consideration should be given to grounding metal walkways and also to providing safety clearance above raised walkways.

Free download 525 pdf ieee asana windows app download

Ieee 525 pdf free download	The resulting overvoltages can exceed two per unit. GVR is the voltage rise proportional to the magnitude of the ground current and to the ground resistance. Trenches cast into concrete floors should be covered. They can also be used in a combined isolating-drainage transformer configuration. Bolin F.
Ieee 525 pdf free download	One of downlowd types of conduit used for buried optical cable is the continuous-reeled type. The cable manufacturer should be consulted when tensions exceeding these limits are expected. Cannon A. Also see 6. Lightweight optical cable rubs on all sides of the conduit through the natural undulation of long straight runs. Raceways This clause provides guidance for both a means of supporting cable runs between electrical equipment and physical protection to the cables.
Ieee 525 pdf free download	PAS, pp. Where possible, control cables should be routed perpendicular to high-voltage buses [B17], [B38]. Patel Https://saadpcsoftware.com/gba-emulator-ios-download/5392-equalizer-download-for-windows-10.php. Stringer, L. Where digital https://saadpcsoftware.com/sonic-cd-download-pc/9200-android-link-download.php originate in proximity to each other, twisted pair multiple conductor cables with overall shield should be used or multiple conductor cable with common return may be permitted, and overall shielding may not be required. If a feeder device is not used, the cable should be pcf into the raceway. More info cables are often pulled for much longer distances than electrical fref.
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The second example shows the connections involved in PLC communications. Signals from the remote source arrive first at the multiplexer. In this example, five circuits are then produced. The first is a standard telephone voice circuit. The third circuit serves the revenue meters and digital fault recorder through a circuit-sharing device. Circuits 4 and 5 represent examples of dedicated circuits. Circuit 4 is dedicated to the RTU communications, and Circuit 5 is dedicated to protective relay communications.

Figure 5 —Example telephone communications block diagram 5. For noncomputer-type applications, such as annunciators, shielding may not be required. Separation of digital input cables and digital output cables from each other and from power cables may be required. Where digital inputs originate in proximity to each other, twisted pair multiple conductor cables with overall shield should be used or multiple conductor cable with common return may be permitted, and overall shielding may not be required.

Digital output cables of similar constructions may also be permitted. Individual twisted and shielded pairs should be considered for pulse-type circuits. When two lengths of shielded cable are connected together at a terminal block, an insulated point on the terminal block should be used for connecting the shields. A separate ungrounded power supply should be furnished for the group of RTDs installed in each piece of equipment.

Crosstalk, electromagnetic interference EMI , and transient spikes can seriously affect the transmission of digital signals. An overall shield limits exterior interferences but will not protect against internal coupling and cross-talk.

In general, communications cable shields are grounded at one end to prevent ground loop potentials and the associated noise. In cases where equipment designs require grounds at both ends, capacitors can be used between the shield and ground to block dc voltages.

Isolation amplifiers have also been employed. One or more of the following protection devices may be installed to protect against power-frequency GPR. They can also be used in a combined isolating-drainage transformer configuration. They neutralize extraneous longitudinal voltages resulting from ground voltage rise or longitudinal induction, or both, while simultaneously allowing ac or dc metallic signals to pass.

Fiber-optic cable 6. These fibers transmit infrared laser light with wavelengths of nm to nm. Single-mode cables are more efficient at signal transfer and are used for transmission distances nominally greater than 1 km. Multi-mode fibers have larger cores typically about Multi-mode cables are less expensive to install, less efficient than single-mode cables, and are used for shorter runs within substations. The termination devices are less expensive than for single-mode. Due to impurities in the glass fibers the light signal degrades within the fiber, depending upon the wavelength of the transmitted light and the distance over which it is to be transmitted.

When the signal is transmitted over great distances, optical regenerators may be required to boost signal strength. In addition to the choice of single mode or multi- mode, the number of fibers can range from two to hundreds.

Cable is available with surrounding loose buffer tube, an internal dielectric tension member, a duct that is integral with the cable, and armor. Cable diameter is a function of the construction and ranges from 4 mm to more than 20 mm.

Additional information about available cable constructions is available at various websites,6 e. One of the types of conduit used for buried fiber-optic cable is the continuous-reeled type. Such continuous duct is popular because it is inexpensive and offers enough protection to allow the use of the less expensive cable constructions. However, conduit and duct offers protection from crushing, ground disruption, rodents, and other environmental abuse. In addition, the cable is easier to replace or upgrade in the future.

Several methods and types of conduit systems are used. The inner ducts can be smooth walled or corrugated either longitudinally or horizontally. For these situations follow the guidelines from the cable manufacturer. The glass fibers are usually well protected by buffer tubes, duct, armor, etc. Even though the glass in the fiber is actually stronger higher tensile strength per unit area than a metal conductor, there is very little cross- sectional area in a fiber available for strength and support.

For this reason, most fiber-optic cables have other components to provide the strength for cable support during pulling, handling, etc. Fiber-optic cables in substations can be installed in the same manner as metallic conductor cables; however, this practice requires robust fiber-optic cables that can withstand normal construction handling and still protect the fibers inside. There are important differences to be considered in the handling and installation of fiber-optic cable, as compared to metallic conductor cable.

In cable tray and trench, fiber-optic cable may be subjected to stress due to the weight of other cables which can induce microbending into the fiber-optic cable. Therefore, it is more-common practice to place the fiber-optic cable in a separate duct installed in the tray, trench or conduit usually plastic , or use a cable construction with an integral duct. This not only protects the cable, but also allows easier identification from metallic cables. Depending on the cable construction, the maximum allowable pulling tension on fiber-optic cable can vary from N 45 lb to more than N lb.

The maximum allowable tension for a particular fiber- optic cable should be obtained from the cable manufacturer. This maximum recommended pulling tension should be noted on any drawings, installation instruction, etc. The theory of pulling tension is the same for fiber-optic cable as it is for metallic conductor cable.

Pulling tension can be calculated based on cable weight, conduit system design, and coefficient of friction. Probably the most common installation mistake is making tight bends in the cable. Tight bends, kinks, knots, etc. Minimum bending radius in fiber-optic cable is typically in the range of 20 times the cable diameter. This bending radius should be considered by the engineer when specifying conduit bends and pull box openings or sizing guide pulleys, sheaves, mid-assist capstans, etc. Fiber-optic cables are often pulled for much longer distances than metallic conductor cables.

These long pulls minimize the number of splices in fiber-optic cable which is desirable for fiber performance. The light weight of the cable, internal tension members, and tube or duct in the cable itself, makes these long pulls possible. Proper lubrication and good conduit installation are also necessities. The special nature of fiber-optic cable pulling, i. Lightweight fiber-optic cable rubs on all sides of the conduit through the natural undulation of long straight runs.

Many common lubricants flow to the bottom of the raceway and lose effectiveness in this type of pulling. As with metallic conductor cable, specific coefficients of friction depend on cable jacket type, conduit type, and the lubricant as well. Short-length fiber-optic cable pulls may not require lubricant; however, for long or complex cable pulls, lubricant is critical to making an efficient, high quality installation.

The requirements for fiber-optic cable pulling lubricant are the same as those for metallic conductor cable: a Compatibility with cable outer covering, tube, or duct 11 For information on references, see Clause 2.

This ensures that all connections have been performed properly, and the fiber has not been damaged during installation. Low-voltage power cable rated V and V is currently in use. When cable classifications are mixed, the power cable ampacity is calculated as if all the cables were power cables. Segregating low-voltage power cables in the substation cable trench or cable tray system is generally not necessary. Consideration of circuit voltage drop may lead to cables larger than the available space in typical service panels and connectors.

This may lead to multiple conductors per phase, conductor reducing terminal connectors, or interior panel space and bending radius constraints. These cables may be tested as part of the system checkout.

The low-voltage power cable insulation resistance tests should measure the insulation resistance between any possible combination of conductors in the same cable and between each conductor and station ground, with all other conductors grounded in the same cable. Power cable 1 kV to 35 kV Medium-voltage power cables are designed to supply power to substation utilization devices, other substations, or customer systems rated higher than V.

NOTE—Oil-filled and gas-insulated cables are excluded from this definition and are not covered in this guide. These factors make it prudent to consult industry codes. It can be semiconducting material or, in the case of at least one manufacturer, a stress control material. At the high voltages associated with shielded cable applications, a voltage gradient would exist across any air gap between the insulation and shield.

The voltage gradient may be sufficient to ionize the air, causing small electric arcs or partial discharge. These small electric arcs burn the insulation and eventually cause the cable to fail. The semiconducting screen allows application of a conducting material over the insulation to eliminate air gaps between insulation and ground plane.

Various shield screen material systems include the following: a Extruded semiconducting thermoplastic or thermosetting polymer b Extruded high-dielectric-constant thermoplastic or thermosetting polymer, referred to as a stress control layer 8.

The selection of the shield grounding locations and the effects of single and multiple grounds are points to be considered for the proper installation of shielded cable. Methods for achieving this segregation include the following: a Installation of medium-voltage cables in raceways that are separated from low-voltage power and control cables and from instrumentation cables.

Installation of different voltage classes of medium-voltage power cables in separate raceways is also recommended.

Cables installed in stacked cable trays should be arranged by descending voltage levels, with the higher voltages at the top. Special considerations should be given to cable installed in areas where ambient temperatures differ from these values. Cable should be suitable for operation in wet and dry locations.

The proper design of cable systems requires the consideration of many factors. These factors include ambient temperature, conductor temperature, earth thermal resistivity, load factor, current loading, system fault level, voltage drop, system nominal voltage and grounding, method of installation, and number of conductors being installed. The selection of power cables may include consideration of the cost of losses.

Conductors are typically specified based on this standard. Copper conductor may be uncoated or coated with tin, lead alloy, or nickel.

Normally uncoated conductor is used, but coated conductor may be used to ease stripping of the insulation from the conductor and to make soldering easier. Note that soldering is not a typical termination method for utilities.

For the same diameter, aluminum conductors have a lower conductivity than copper. Control and instrumentation cable conductor is almost always copper. Aluminum conductor should be considered for larger power cables.

Factors that influence the selection of either copper or aluminum for conductors include: a Aluminum metal has historically been less expensive than copper. One circular mil is defined as the area of a circle 1 mil 0.

The AWG number increases as the cross-sectional area decreases. Solid conductors may be used for sizes up to 12 AWG. Solid conductors larger than this would be stiff and difficult to install, therefore stranded construction is normally used for these larger conductors. The number of strands and size of each strand for a given size is dependent on the use of the conductor.

Common stranding classes are summarized in Table C. The number of strands per conductor is standardized and is summarized in Table C. Class B stranding is normally used for substation installations. Table C. M Cords and cables composed of 34 AWG copper wires.

The maximum temperature usually occurs at the conductor-insulation interface. The maximum allowable insulation temperature limits cable ampacity. Maximum allowable insulation temperature has been determined through testing and experience for the commonly used materials and is a function of time. The steady-state load, short- time cyclic load, emergency load and fault conditions are usually considered in determining the ampacity required for a cable.

Losses I2R in the conductor and magnetically induced losses in the insulation shield and the raceway are the principal causes of the insulation temperature rise. The magnitude of circulating currents flowing in shields grounded at more than one point depends on the mutual inductance between the cable shielding and the cable conductors, the mutual inductance to the conductors in other cables, the current in these conductors, and the impedance of the shield.

Below-grade cables are usually installed in trench or duct, or direct buried. Above-grade cables are usually installed in conduit, wireway, tray, or suspended between supports. Cables may be routed through foundations, walls, or fire barriers, and raceway may be partially or totally enclosed. The installation that results in the highest insulation temperature should be used to determine the ampacity of a cable routed through several configurations.

If a number of cables are installed in close proximity to each other and all are carrying current, each cable will be derated. The reason for derating is reduced heat dissipation in a group of cables, compared with a single isolated cable or conduit. Group correction factors should be used to find reduced ampacity of cables in the group. The cable materials themselves can affect heat transfer and ampacity. The thermal conductivity of earth surrounding below grade cables is one of the most important parameters in determining ampacity.

However, many engineers have found it acceptable to use typical values. The ampacity of below-grade cable is also dependent upon the load factor, which is the ratio of the average current over a designated period of time to the peak current occurring in that period. Methods for determining ampacity and the tables of ampacities for a large number of typical cable and below-grade and above-grade installation configurations are included in IEEE Std In addition, IEEE Std includes guidance for determining ampacities for configurations not included in the tables.

Finite element techniques have been used to calculate below grade cable ampacity. These techniques will allow the designer to account for specific cable construction and installation details. Most codes include derating factors that account for multiple conductors per raceways. Cable ampacity should be equal to or larger than the trip rating of the rating of the circuit overload protection, which is typically 1.

The voltage drop requirements should be such that the equipment operates within its design limits. Voltage drop for motor feeders should be considered for both starting and running conditions to ensure the motor operates within its design limits. For situations like this, Equation C. Alternatively, computer software may be used to determine the exact solution.

Hand calculations will typically be done using the approximate solution. Voltage drop is commonly expressed as a percentage of the source voltage. An acceptable voltage drop is determined based on an overall knowledge of the system. Voltage drop is normally based on full load current. However, there is often diversity in the load on lighting and receptacle circuits, and the actual load that may occur on a receptacle circuit can not be accurately predicted. It is recommended that a voltage drop be calculated initially at the maximum conductor operating temperature because the ampacity is based on this too.

In cases where a cable will be sized based on voltage drop and one size is marginal for voltage drop, voltage drop may be recalculated at the expected cable operating temperature. Calculations are commonly used for larger size, high current cables since there may be many variables that affect the impedance. For small conductor sizes, table values may be used with only a small error. For other sizes, refer to manufacturer catalogs. Equation C. When using tables, it may be necessary to adjust the values to account for a different operating temperature or cable type.

The ac resistance is determined from the following Equation C. The skin effect factor is approximated using Equation C. Ycp increases as spacing between conductors is decreased. The factor is calculated using Equation C. Losses are produced in cable shields due to eddy currents produced in the shield as a function of conductor proximity. Equations for calculating these losses are given in the Neher and McGrath reference [B86]. Circulating currents will flow in cable shields when they are grounded at both ends.

This is accounted for by the factor Ysc, calculated using Equation C. See Neher and McGrath [B86] for other situations. This heats the conduit and raises the conductor temperature.

When all three phases are in a conduit, the magnetic field is significantly reduced due to phase cancellation. For a single conductor cable, there is no cancellation and the heating is significant so this situation should be avoided. Loss factor may be calculated using Equation C. For metric units 6. For a three-phase circuit, the per-phase reactance is given by Equation C. For a two-wire, single- phase circuit, the reactance will be twice that given by Equation C.

Typically load current and power factor are required. Consideration should be given to whether the type of load is constant current, constant power or constant impedance. The characteristics of the different load types are summarized in Table C.

It is recommended that current be determined for the desired load voltage. If the current is available only for a specific voltage, then the current may be estimated using the formula in Table C. The short- circuit rating of an insulated conductor is based on the maximum allowable conductor temperature and insulation temperature. Conductor temperature is dependent on the current magnitude and duration. Dielectric losses, resistance to flame propagation, and gas generation when burned are the most common performance considerations.

The selected voltage rating should result in a cable insulation system that maintains the energized conductor voltage, without installation breakdown under normal operating conditions. In some cable installations, specifications may call for safe operation under high-temperature conditions. The cable manufacturer should be consulted for recommendations for specific chemical requirements to which the cable may be exposed.

All thermoset and thermoplastic jacket covering materials shall be selected suitable for the conductor insulation temperature rating and the environment in which they are to be installed.

In the past, lead sheaths were commonly used, but are being phased out due to the adverse effects of lead in the environment. Attenuation is measured in decibels per unit length and indicates the loss of signal in the cable. Capacitance is measured in picofarads per unit length. High capacitance of communication cables slows down the signals. High capacitance of long control cables, 60 m and more ft , may lead to transient overvoltages over circuit elements relay coils, contacts etc.

This information is usually obtained from the hardware or device manufacturer. This information may be available from company policy documents or specific engineering or design standards. This information is often obtained through other departments e. Generally one protective device is required per circuit. There may be general utility specifications and design criteria based upon experience and regional design criteria.

Raceway systems consist primarily of cable tray and conduit. Conduit fill is based on the following Equation E. If the fill limitations and cable area are known, the raceway area can be calculated and an adequate size can be selected.

If they are installed direct buried in soil, consideration should be given to the zinc coating having a limited life, and corrosion may be rapid after the zinc coating is consumed or damaged. When used where excessive alkaline conditions exist, the conduit should be protected by a coat of bituminous paint or similar material. PVC-coated steel conduit may be used in corrosive environments.

Plugs should be used to seal spare conduits in wet locations. Coefficient of expansion should also be considered for outdoor applications. Flammability of such conduits is of particular concern in indoor exposed locations.

Burning or excessive heating of PVC in the presence of moisture may result in the formation of hydrochloric acid which can attack reinforcing steel, deposit chlorides on stainless steel surfaces, or attack electrical contact surfaces.

The use of exposed PVC conduit indoors should generally be avoided, but may be considered for limited use in corrosive environments.

Guidance in the determination of hazardous areas is given in the NEC [B]. Aluminum conduit may be exposed in wet and dry locations. Aluminum conduit should not be embedded in concrete or direct buried in soil unless coated bitumastic compound, etc. Aluminum conduit may be used, exposed or concealed, where a strong magnetic field exists; however, conduit supports should not form a magnetic circuit around the conduit if all the cables of the electrical circuit are not in the same conduit.

Protection should be provided against attack by insects, rodents, or other indigenous animals. Troughs constructed of concrete or other material may be laid on the grade. Cost savings may be realized when comparing above-grade trays, conduit, and troughs to similar below-grade systems. The judicious location of these boxes may result in considerable savings. Liquid-tight flexible conduit is commonly used for this application. Flexible conduit length should be as short as practical, but consistent with its own minimum bending radius, the minimum bending radius of the cable to be installed, and the relative motion expected between connection points.

A separate ground wire should be installed if the flexible conduit is not part of the grounding and bonding system. See the NEC [B] for additional guidance. Drain fittings and air vents in the equipment enclosure should also be considered.

Expansion couplings should be installed in the conduit run or at the enclosure to prevent damage caused by frost heaving or expansion. Reinforcing steel in the manhole walls should not form closed loops around individual nonmetallic conduit entering the manhole.

Nonmetallic spacers should be used. When this is not practical, lean concrete or porous fill can be used between the frost line and the duct bank. See the NEC [B] for additional information. Each threaded joint should be cleaned to remove all of the cutting oil before the compound is applied.

The compound should be applied only to the male conduit threads to prevent obstruction. IEEE Std IEEE Guide for the Design and Installation of Cable Systems in Substations e Large radius bends should be used to reduce the cable sidewall pressure during cable installation and in conduit runs when the bending radius of the cable to be contained in the conduit exceeds the radius of standard bends.

The connections should be of minimum lengths and should employ at least the minimum bending radii established by the cable manufacturer. Loading calculations should include the static weight of cables and a concentrated load of N lb at midspan. The tray load factor safety factor should be at least 1. For horizontal elbows, rung spacing should be maintained at the center line. A minimum clearance of 23 cm 9 in should be maintained between the top of a tray and beams, piping, etc.

In selecting material for trays, the following should be considered: a A galvanized tray installed outdoors will corrode in locations such as near the ocean or immediately adjacent to a cooling tower where the tray is continuously wetted by chemically treated water. If an aluminum tray is used for such applications, a corrosive-resistant type should be specified. Special coatings for a steel tray may also serve as satisfactory protection against corrosion.

The use of a nonmetallic tray should also be considered for such applications. Ice, snow, and wind loadings should be added to loads described in item a of E. Aluminum alloys T6, T6, and M34 are acceptable, with careful recognition of the differences in strength. Mill-galvanized steel should normally be used only for indoor applications in noncorrosive environments.

Hot-dipped galvanized-after-fabrication steel should be used for outdoor and damp locations. Tray load capacity is defined as the allowable weight of wires and cables carried by the tray. This value is independent of the dead load of the tray system. In addition to and concurrent with the tray load capacity and the dead load of the tray system, any tray should neither fail nor be permanently distorted by a concentrated load of N lb at midspan at the center line of the tray or on either side rail.

This results in cable crossing and void areas, which take up much of the tray cross-sectional area. This will result in a tray loading in which no cables will be installed above the top of the side rails of the cable tray, except as necessary at intersections and where cables enter or exit the cable tray systems. This restraint is generally applicable to instrumentation cables, but may also apply to power and control cables.

When conduit is rigidly clamped, consideration should be given to the forces at the connection during dynamic seismic loading of the tray and conduit system. Conduit connections through the tray bottom or side rail should be avoided. When cable trays are used as raceways for solidly grounded or low-impedance grounded power systems, consideration should be given to the tray system ampacity as a conductor. Inadequate ampacity or discontinuities in the tray system may require that a ground conductor be attached to and run parallel with the tray, or that a ground strap be added across the discontinuities or expansion fittings.

The ground conductor may be bare, coated, or insulated, depending upon metallic compatibility. Tray sections should be supported near section ends and at fittings such as tees, crosses, and elbows. Wireways are for exposed installations only and should not be used in hazardous areas. Consideration should be given to the wireway material where corrosive vapors exist.

In outdoor locations, wireways should be of raintight construction. Taps from wireways should be made with rigid, intermediate metal, electrical metallic tubing, flexible-metal conduit, or armored cable. The excavation is then backfilled. Care should be exercised in backfilling to avoid large or sharp rocks, cinders, slag, or other harmful materials. A warning system to prevent accidental damage during excavation is advisable. Several methods used are treated wood planks, a thin layer of colored lean concrete, a layer of sand, strips of plastic, and markers above ground.

Untreated wood planks may attract termites, and overtreatment may result in leaching of chemicals harmful to the cables. Spare cables or ducts may be installed before backfilling. This system has low initial cost, but does not lend itself to changes or additions, and provides limited protection against the environment. Damage to cables is more difficult to locate and repair in a direct burial system than in a permanent trench system.

This system has the advantages of minimum interference to traffic and drainage, good physical protection, ease of adding cables, shielding effect of the ground mat, and the capacity for a large number of cables.

Disadvantages include high initial cost and danger that fire could propagate between cable trays and along the length of the tunnel. Fire hazards may be reduced by providing fire stops. Typical trench configurations are shown in Figure E. Figure E. Trenches may be made of cast-in-place concrete, fiber pipes coated with bitumastic, or precast material.

Where trenches interfere with traffic in the substation, vehicle crossovers—permanent or temporary—may be provided as needed. Warning posts or signs should be used to warn vehicular traffic of the presence of trenches. The trenches may interfere with surface drainage and can be sloped to storm sewers, sump pits, or French drains.

Open-bottom trenches may dissipate drainage water but are vulnerable to rodents. A layer of sand applied around the cables in the trench may protect the cables from damage by rodents. Trenches at cable entrances into control buildings should be sloped away from the building for drainage purposes. The trenches also should have a barrier to prevent fire or rodents from entering the control building.

The tops of the trench walls may be used to support hangers for grounded shield conductors. The covers of trenches may be used for walkways. Consideration should be given to grounding metal walkways and also to providing safety clearance above raised walkways. Added concern should be given to the flammability of wood. Removable covers may be made of metal, plywood, or other materials.

Nonmetallic cover materials should be fire retardant. Trenches cast into concrete floors should be covered. It should be noted that metal covers in the rear of switchboards present a handling hazard, and nonmetallic, fire-retardant material should be used. Where cables pass through holes cut in covers, for example, in rear or inside of switchboards, the edges should be covered to prevent cable damage from sharp edges.

Entrance from the outside into the raised floor system may be made at any point along the control house wall. Use of a fire protection system under the floor should be considered. When stacking cable trays, the primary and backup systems should not be stacked over each other in order to minimize the possibility of a cable fire damaging both systems. Physical separation between transient source and control cables is an effective means of transient control. Because mutual capacitance and mutual inductance are greatly influenced by circuit spacing, small increases in distance may produce substantial decreases in interaction between circuits Dietrich et al.

When control cables must be run parallel to EHV busses, maximum practical separation should be maintained between the cables and the busses Dietrich et al. NOTE—Tests indicate that in some cases, nonshielded control cables may be used without paralleling ground cables when they are parallel and are located at a distance greater than 15 m 50 ft from or are perpendicular to a typical kV bus Garton and Stolt [B22].

When practical, control cables may be installed below the main ground grid. High-voltage cables should not be in duct runs or trenches with control cables Dietrich et al. This affects the proximity routing of trenches and the use of radial raceways rather than a grouped raceway. IEEE Std IEEE Guide for the Design and Installation of Cable Systems in Substations Annex G normative Transient protection of instrumentation, control, and power cable This annex provides information on the origin of transients in substations, and guidance for cable shielding and shield grounding for medium-voltage power, instrumentation, control, coaxial, and triaxial cable systems.

Typically during this type of switching, intense and repeated sparkovers occur across the gap between the moving arms. At each sparkover, oscillatory transient currents, A to A crest, circulate in buses, in the ground grid, in bushing capacitances, in CVTs, and in other apparatus with significant capacitances to ground. The number of individual transients in an opening or closing operation can vary from 5 to 10 Gavazza and Wiggins [B23].

The transients are coupled to the low-voltage wiring by three basic modes. These are as follows: a Radiated magnetic or electric field coupling b Conducted coupling through stray capacitances such as those associated with bushings, CTs, and CVTs c Conductive voltage gradients across ground grid conductors.

In many instances, design requirements dictate installation of several banks in parallel. These high- energy transients typically couple to cables through the overhead bus and the ground grid conductors. In many respects these switching transients are similar to those generated by an air break switch energizing or de-energizing a section of bus. These transients differ from the other transients in regards to the magnitude of the transient current and its associated frequencies.

IEEE Std IEEE Guide for the Design and Installation of Cable Systems in Substations c Low-frequency oscillations occurring between the capacitor banks and the power-frequency source contain the frequency range of Hz to Hz these frequencies are dominant in the case of a bank switched against the bus d 50 Hz or 60 Hz source frequency The modes by which the voltage and current transients are coupled to the cables are basically the same as those listed in G.

In general, lightning is a high-energy unidirectional surge with a steep wave front. In the frequency domain, a broad frequency band represents this type of surge. The frequency range covered by this spectrum is from dc to megahertz. The following are some ways lightning can cause over-voltages on cables: a Direct strike to the mast or overhead shield wire in the substation b Lightning entering the substation through overhead transmission or distribution lines c Induced lightning transients due to strikes in the vicinity of the substation The surge current flows into earth via ground grid conductors and through the multi-grounded shield and neutral network.

There are two primary modes of coupling to the cables. The inductive coupling is due to voltage and current waves traveling in the overhead shield wires, in the buses, and in the ground grid conductors. The conductive coupling consists of voltage gradients along the ground grid conductors due to flow of transient current. In a substation, a transient grid potential rise TGPR with respect to a remote ground may also exist. This transient voltage most likely will couple to telecommunication lines entering the substation from remote locations.

If proper isolation is not provided, this voltage may cause damage to the telecommunication equipment in the substation. The magnitude of TGPR is proportional to the peak magnitude and rate of rise of the stroke current and the surge impedance of the grounding system. Incidences involving erroneous operation of relay circuits are known to occur under these conditions. There are two basic modes of coupling, which exist when a phase-to-ground fault occurs in a substation.

The induced voltage on the cable due to the fault current flowing in ground conductors is one mode of coupling.

More dominant coupling, however, is the conductive voltage gradient along the ground grid conductors resulting from the current flow. Coupling due to GPR with respect to remote ground may exist on telecommunication circuits entering the substation.

Sometimes, the telecommunication circuit leaving the substation parallels the power line. In this case the total coupling would be a net result of GPR and the induced voltage due to fault current flowing in that power line. Normally, the maximum voltage will exist at the instant of interruption.

The surge voltage magnitude is proportional to the impedance of the supply circuit and the speed of interruption. Voltages in excess of 10 kV have been observed across a V coil in laboratory tests, but 2.

Once produced, these powerful, fast rising, high-voltage pulses are conducted throughout the supply circuit and can affect adjacent circuits where capacitive coupling exists. The extensive use of surge capacitors on solid-state equipment and the longer control cable runs associated with EHV stations have substantially increased the capacitance between control wiring and ground. Inadvertent momentary grounds on control wiring cause a discharge or a redistribution of charge on this capacitance.

Although this seldom causes failure, the equipment may malfunction. Saturation of CTs by high-magnitude fault currents, including the dc offset, can result in the induction of high voltages in the secondary windings. This phenomenon is repeated for each transition from saturation in one direction to saturation in the other. At this point, it is important to visualize two loop areas enclosed by cable pair including its terminal equipment.

The loop area enclosed between the conductors of a pair is relatively small and typically links a fraction of disturbing field. The voltage so developed across the conductors is called differential mode voltage. In general, the differential mode voltages are too small to cause any equipment damage.

However, the loop currents that result from these voltages sometimes are responsible for erroneous operations of protective devices. Using a twisted pair cable may eliminate this problem altogether. Responsible for most damages are the common mode voltages at the terminals.

The common mode voltage results due to the loop formed between the pair and ground grid conductors. A strong coupling from disturbing fields usually exists due to the large area enclosed by this loop. The common mode voltage is defined as the voltage between the cable conductors and the ground. The main objective of conductive shields is to minimize or preferably eliminate these voltages and resulting currents. Common and differential mode voltages at cable terminations cannot be completely eliminated, but can be limited in magnitude.

Since transient voltages are coupled to the cables due to their exposure in the substation yard, the responsibility of providing protection to reduce these coupled transients rests with utility engineers.

Discussion on terminal protection is beyond the scope of this guide. This is effectively accomplished by: — Installing the cable pairs running to the same apparatus in one trench or conduit — Avoiding the loop formed due to cables running from one apparatus to another apparatus and returning by different route — Running the circuits in a tree fashion with a separate branch to each equipment such as breaker, transformer, etc.

The trench or conduit carrying the cables should not run parallel to the overhead HV buses. In cases where this is unavoidable, provide as much separation distance as practically feasible to reduce the capacitive coupling from the buses. A substation may have underground HV circuit running across the yard. A power-frequency fault current in the HV cable may cause a transient in control cables laid in parallel and in proximity due to magnetic coupling.

Avoiding the parallel run or providing a larger separation distance can reduce the transient overvoltage. This protection is a result of eddy currents set up by the external magnetic field in the coaxial shield. The eddy currents in the shield then produce the opposing field reducing the field coupled to the signal conductors.

Due to its high conductivity and immunity from saturation, a nonmagnetic nonferrous material is typically used for shielding purpose. A typical nonmagnetic material used for shielding purpose may include copper, aluminum, bronze, or lead. Table G. For example, an ungrounded shield cannot protect the cable from capacitively coupled voltages. This can amount to a common mode voltage of several thousand volts.

With the shield grounded at one end, the capacitively-coupled electric field is prevented from terminating on the cable resulting in virtually no differential or common mode voltage. Grounding the shield at one end effectively protects the equipment at that end but equipment connected at the ungrounded end remains unprotected. In some instances, shield-to-ground and conductor-to-ground voltages may even increase at the ungrounded end Dietrich et al. For providing protection at both ends of the cable, the shield should be grounded at both ends Garton and Stolt [B22].

Grounding the shield at both ends links a minimum external field due to reduced loop area enclosed by the cable pairs and shield conductor. The shield conductors are not rated to carry power-frequency fault currents.

For this reason, one or more ground conductors should be installed in the proximity of the cable circuits where shield conductors are grounded at both ends. In the case of an unbalanced circuit equipment circuit is not grounded in the electrical middle , a differential voltage across the pair develops if the impedance on each side of the signal ground in the terminal equipment is different.

This differential voltage will be proportional to the current due to the common mode voltage during the transient. Depending on the unbalance at the terminal, grounding the shield at both ends may increase this differential voltage.

For a given transient, this differential voltage can be reduced by grounding the signal circuit nearly in the electrical middle IEEE Std [B65]. It is necessary to keep the shield in a cable intact, as a broken or separated shield can greatly reduce the shield efficiency. Also, in a substation where there may at times be large fault currents, a problem arises if the shield is grounded at two widely separated locations.

The power-frequency potential difference on the ground grid may cause enough current to flow in the shield to cause damage. The ground grid, even when designed with a very low resistance, cannot be considered as an equal-voltage surface. Substantial grid voltage differences may exist particularly in a large substation yard. Several factors influence voltage gradients across the ground grid conductors.

Since it is impractical to eliminate voltage gradients along ground grid conductors, additional measures are necessary to reduce their influence on the cables.

Typically this measure consists of installing low- impedance ground conductors in proximity and parallel to the affected circuits. These conductors carry currents proportional to voltage gradients along the grid conductors and serve several purposes. The flow of currents in these conductors induces a counter voltage in the control circuits and also reduces the conductive voltage difference between the two terminals.

In the case of a power-frequency fault, these ground conductors carry most of the fault currents protecting the shield conductors grounded at both ends. IEEE Std IEEE Guide for the Design and Installation of Cable Systems in Substations The following are some guidelines to maximize protection from parallel ground conductors: a Ground conductors in trenches 1 Install conductors with sufficient conductivity to carry maximum available fault current in the substation and having adequate mechanical strength.

If required, additional ground conductors can be placed outside but in proximity of the trench. Ground conductors can also be placed in conduits provided that they intercept radiated fields. This ground bus provides a convenient means of grounding individual cable shields if required. However, unshielded cables receive more benefit from the parallel ground conductors.

To be most effective, the ground conductors should be as close to the cables as possible. Provisions should be made for replacement with shield conductors should the unused conductors later be used for active circuits. A parallel ground conductor should accompany the cable if a spare pair is grounded at both ends. The clause also provides shielding guidelines for high-voltage power cables, coaxial and triaxial cables, and the cables carrying low magnitude signals.

The source of transients in many of such cases is the capacitive current interruption by an air break switch. The surge impedances of the ground leads connecting the CVT bases to local ground grid are primarily responsible for developing these high transient voltages. Measuring CTs are normally located in breaker bushings.

The bushing capacitances generate the voltage transients on breaker casings in the same manner as the CVT devices. These transients then can be coupled to CT secondary circuits or any low-voltage circuit or equipment residing in the breaker cabinet. This can be accomplished by mounting the CVT or breaker cabinets as close to the ground as permitted by clearance standards and by providing multiple low-resistance conductors between the cabinets for three standalone cabinets and between the cabinets and the station ground grid.

The secondary circuits exiting the cabinets should run in the vicinity of the ground leads. Additionally, the secondary cables should be laid out radially and as close to the ground grid conductors as possible. If ground grid conductors in the proximity are not available, dedicated ground conductors should be installed.

Using shielded cables for secondary circuits can provide additional immunity. In such a case, the shield should be grounded at both ends. Making the ground connection at the relay or control building has the following advantages: a Voltage rise is minimized near the relay equipment.

A pre-insertion resistor or current limiting reactor inserted between the banks can substantially reduce the switching transient in back-to-back switching. This will reduce the probability of personnel simultaneously contacting both structures and being in series with the potential difference between the peninsula and the rest of the grid during capacitor switching, or during a fault.

Also, ground all cable shields grounded in this manhole at their remote ends. During capacitor switching and faults, the potential of the peninsula ground grid and the area around the first manhole may be quite high. A high voltage could exist between cables if some shields are not grounded, and between the ends of the shields if both ends are not grounded.

IEEE Std IEEE Guide for the Design and Installation of Cable Systems in Substations h High-voltage shunt capacitor banks of a given voltage should have the neutrals from individual banks connected together and then connected to the station ground grid at only one point.

To facilitate single point grounding, all capacitor banks of a given voltage should be at one location. High susceptibility circuits are those carrying low level voltage and current signals. A thermocouple circuit carrying analog signals in millivolt range is one good example of this type of circuit.

The protection measures described in this section may not be necessary if interference due to steady-state noise is not a concern even for high susceptibility circuits. Users should follow the general shielding and grounding practices described in G. Using cables with twisted pair conductors and individually insulated shields over each pair is also effective in minimizing crosstalk in communication circuits.

If the receiver is receiving the signal from a grounded voltage source, a thermocouple, for example, the receiver input should be capable of high common-mode rejection. This can be accomplished by either isolating the receiver from the ground or installing a differential amplifier with isolated guard at the receiver input terminals.

Isolating the circuits from ground effectively opens the ground common-mode voltage path in the signal circuit. If a single-ended amplifier already exists at the input terminal of the receiver, the low side of the signal circuit is not broken and should be considered grounded at the terminal. In this case, the same isolation procedure as indicated above should be followed.

When an ungrounded transducer is used, the receiver may not need isolation. In such a case, a single-ended amplifier can be installed at the input terminal if required.

If the shield is grounded at some point other than where the signal equipment is grounded, charging currents may flow in the shield because of the difference in voltages between signal and shield ground locations. Similarly if the shield is grounded at more than one point, voltage gradients along the ground conductors may drive current through the shield.

In either case, the common mode noise current in the shield can induce differential mode noise in the signal leads. Depending on the unbalance in the signal circuit, noise voltages of sufficient magnitudes may be developed to reduce the accuracy of the signal sensing equipment. A preferred practice, in such a case, is to isolate the cable shield from the amplifier guard shield and to ground the shield only at the transducer end. This shield grounding practice minimizes the shield-induced common-mode current while permitting the amplifier to operate at maximum common- mode rejection capability.

To provide immunity from transient overvoltages, the nongrounded end of the shield may be grounded through a suitable capacitor or a surge suppressor varistor. These types are as follows: insulated single conductor, coaxial cable, and triaxial cable.

It can also be used as the interconnecting lead for short bypasses. Bare conductors and coaxial cables should be avoided for these applications, since either one can introduce excessive leakage currents or excessive stray capacitance.

Since a single conductor is at a high impedance point when connected between a coupling capacitor and a line tuner, stray capacitance-to-ground and leakage currents can affect the coupling circuit performance.

The stray capacitance can cause a reduction in bandwidth, and the leakage currents can cause a loss in carrier power. To reduce stray capacitance and leakage currents, either of the following methods may be used: a An insulated single conductor should be run as directly as possible between its required terminations. It should be mounted on insulators and fed through bushings at each end. The conductor insulation should be unbroken between its ends to maintain low leakage.

The insulated single conductor should be isolated from the flexible metal conduit with nonconductive washers spaced about mm 6 in apart. If the conductor has a significant portion of its length outside the flexible metal conduit, it should be mounted on insulators and fed through bushings at its ends as in item a. A typical insulated carrier lead, 12 mm 0.

If both shield ends are grounded, large surge currents can flow under certain conditions, causing saturation of the impedance- matching transformer and resulting in an inoperative carrier channel. This cable provides an additional heavy shield, which does not carry signal currents.

The outer shield is capable of carrying large induced surge currents under fault conditions and is grounded at both ends. This arrangement provides an effective shielding against both magnetic and electrostatic induction.

Because mutual capacitance and mutual inductance are greatly influenced by circuit spacing, small increases in distance may produce substantial decreases in interaction between circuits.

Table H. Communication circuits should be in a dedicated duct, or sub-duct, whenever possible. Duct banks All types, segregated as necessary into individual ducts Trench All types. Barrier required for power circuits greater than V ac. Communication circuits should be enclosed in a sub-duct within the trench. They may be systems where personnel safety is involved, such as fire pumps, or systems provided with redundancy because of the severity of economic consequences of equipment damage or system reliability.

Primary and backup relaying and normal and backup station service supplies are practical examples of redundant cable systems. The degree and type of separation required varies with the potential hazards to the cable systems in particular areas of the substation. Raceway fill, maximum sidewall pressure, jam ratio, and minimum bending radius are design limits which should be examined in designing a proper cable pull.

These design limits are prerequisites needed in designing a cable raceway system. Once these limits are determined for a particular cable, the raceway system can then be designed. If the system has already been designed, modifications may be required in order to pull the cable without damage. Conduit and duct system design should consider the maximum pulling lengths of cable to be installed. The maximum pulling length of a cable or cables is determined by the maximum allowable pulling tension and sidewall pressure.

The pulling length will be limited by one of these factors. Pull points or manholes should be installed wherever calculations show that expected pulling tensions exceed either maximum allowable pulling tension or sidewall pressure. The cable manufacturer should be consulted when tensions exceeding these limits are expected. When pulling by basket grip over a nonleaded jacketed cable, the pulling tension should not exceed 4.

When using a basket-weave type pulling grip applied over a lead-sheathed cable, the force should not exceed 6. Jam ratio is defined for three cables of equal diameter as the ratio of the conduit inside diameter D to the cable outside diameter d.

The jam ratio is a concern because jamming in the conduit could cause damage to one or more of the cables. The possibility of jamming is greater when the cables change direction. Therefore, the inside diameter of the conduit at the bend is used in determining the jam ratio. Guidance for minimum bending radii can be obtained from the NEC [B] or the cable manufacturer.

Table J. It is very accurate where the incoming tension at a bend is equal to or greater than 10 times the product of cable weight per meter foot times the bend radius r expressed in meters feet.

Cases in which the exact equations may become necessary are where light tensions enter large radii bends. Usually Equation J. The typical duct run used for the calculations is shown in Figure J. The completed weight of this cable is Plastic conduit suitable for direct burial Type DB is to be used for this example installation. Using Equation E. The jam ratio, as discussed in J. Assuming field bending of the conduit 1.

Also, where triplexed cable is used, jamming is not a problem since jamming is the wedging of cables in a conduit when three cables lie side by side in the same plane. The cabling factor for three conductors triplexed is 2. Pulling from A towards H Since pulling down the vertical section A-B and around the curve B-C would require a negligible tension, the calculations are started at C.

The weight correction factor c for three single cables in a triangular configuration is calculated using Equation J. The maximum sidewall pressure for this pull will occur at curve F-G and is calculated using Equation J. Therefore, a cable pull from H to A should not be permitted. The cable should be pulled from A to H. The let-off reel should be at the riser pole and the cable should be pulled toward the manhole, in order not to exceed the maximum allowable pulling tension or sidewall pressure.

During storage, the ends of the cables should be sealed against the entrance of moisture or contamination. Reels should be stored on solid ground to prevent the flanges from sinking into the earth. NOTE—When stored outside for long periods of time longer than typical installation staging periods , the cable will require protection from sunlight UV radiation. It is preferable to store the cable inside if UV protection cannot be provided. If necessary, cables should be protected by fire-resistant material.

Fiber-optic cable is addressed separately in 6. Handling or pulling cables in extremely low temperatures can cause damage to the cable sheathing, jacketing or insulation. To prevent damage of this nature, store cables in a heated building at least 24 h prior to installation. The lubricant should not set up so as to prevent the cable from being pulled out of the conduit at a later time.

Cable lubricants should not support combustion. Use of truck bumpers is not recommended for longer pulls due to risk of unsteady pull. Turning the reel and feeding slack cable to the duct entrance will reduce the pulling tension. The radius of the feeder device should not be less than the minimum bending radius of the cable.

If a feeder device is not used, the cable should be hand-guided into the raceway. Low-voltage power cables are designed to supply power to utilization devices of the substation auxiliary systems rated V or less.

Control cables are applied at relatively low current levels or used for intermittent operation to change the operating status of a utilization device of the substation auxiliary system. NOTE — As used in this document, leads from current and voltage transformers are considered control cables since in most cases they are used in relay protection circuits. However, when current transformer leads are in a primary voltage area exceeding volts they should be protected as required by the NESC, Rule As used in this document, instrumentation cables consist of cables for Supervisory Controls and Data Acquisition SCADA systems or event recorders, and thermocouple and resistance temperature detector cables.

Instrumentation cables are used for transmitting variable current or voltage signals analog or transmitting coded information digital. High-voltage power cables should be segregated from all other cables.

Cables installed in stacked cable trays should be arranged by descending voltage levels, with the higher voltages at the top. Methods for achieving this segregation are a Installation of high-voltage cables in raceways that are separated from low-voltage power and control cables and from instrumentation cables. Installation of different voltage classes of high-voltage power cables in separate raceways is also suggested.

Consideration should be given to insulation deformation when cable diameters differ greatly. When cable classifications are mixed, the power cable ampacity is calculated as if all the cables were power cables. Methods for achieving segregation are a Installations that provide physical separation between the instrumentation cables and any electrical noise source [B15], [B38].

Shielded voice communications cable without power supply conductors may be included in raceways with analog signal cables. The most common optical cable jacket materials are polyethylene all types , PVC, and polyurethane. The placement of optical cable in conduit is quite common. Conduit offers protection from crushing, ground disruption, rodents, and other environmental abuse. In addition, the cable is easier to replace or upgrade in the future.

Several methods and types of conduit systems are used. The inner ducts can be smooth walled or corrugated either longitudinally or horizontally. One method of installation involves a composite optical overhead ground wire OPGW on a transmission line to link substations together.

The OPGW is usually terminated in a standard splice case at a substation structure. At this splice case, it is interfaced with the substation optical cable. The substation optical cable should be installed in conduit from the splice case into the substation cable duct or trench system. The optical cable should be installed in conduit from the substation cable duct or trench system to the control house where it is terminated on a fiber termination panel.

There are important differences to be considered in the handling and installation of fiber optic cable, as compared to metallic cable. In ladder type cable tray, optical cable may be subjected to stress due to the weight of other cables which can induce microbending into the optical cable.

Therefore, it may be a better practice to place the optical cable in a separate duct installed in the tray. Optical cables in substations should be installed in the same manner as metallic conductor cables. This practice requires robust optical cables that can withstand normal construction handling and still protect the fibers inside.

Separation of redundant cable systems This clause provides guidance for the separation of redundant cable systems. They may be systems where personnel safety is involved, such as fire pumps, or systems provided with redundancy because of the severity of economic consequences of equipment damage or system reliability.

Cables meeting the requirements of IEEE Std should be considered for application for these functions. NOTE — Primary and backup relaying are examples that may utilize redundant cable systems. The degree of separation required varies with the potential hazards to the cable systems in particular areas of the substation. Shielding and shield grounding This clause provides information on the origin of transients in substations and guidance for shielding and shield grounding of high-voltage power, instrumentation, control, coaxial, and triaxial cable systems.

Opening or closing a switching device to de-energize or energize a section of substation bus is generally accompanied by arcing and will initiate a high-frequency transient. The frequency will be determined by the self-inductance and shunt capacitance of the high-voltage conductors involved.

The resulting overvoltages can exceed two per unit. Both electric and magnetic coupling between high-voltage and low-voltage conductors can result in high-level transients in the low-voltage system. Switching a capacitor bank causes a current transient which is a function of the bank size and the circuit constants back to the source.

If other capacitors are already connected nearby to the same line or bus, they lower the impedance seen by the switched capacitor, increasing the magnitude and frequency of the transient. Energy stored in the nearby bank may contribute further to the severity. The circuit between banks is likely to ring at a high frequency because of the low inductance in the short line connecting the banks and the reduced effective capacitance considering the banks in series [B39].

This phenomenon further enhances the tendency of the transient to interfere with nearby circuits. This phenomenon is similar to capacitor bank switching, with the difference being the distributed nature of the inductance and capacitance of the line. The magnitude of the line charging current tends to be substantially less than that for capacitor bank switching. The frequency of the transient current or voltage is inversely proportional to the line length [B38]. The capacitors in these devices, in conjunction with inductances of the power system conductors, constitute a resonant circuit whose frequency can be in the megahertz range.

Unless the base of the CCVT has a low-surge impedance to the substation ground grid, a high voltage can appear between the CCVT secondary terminals and the grid. The high voltage will be generated primarily during air-break switching operations. GVR is the voltage rise proportional to the magnitude of the ground current and to the ground resistance. Under normal conditions, the grounded electrical equipment operates at essentially zero ground voltage within the substation yard.

During a fault, the portion of fault current which is conducted by a ground electrode in the earth causes a rise of the electrode voltage with respect to remote earth see IEEE Std and [B26]. Both electromagnetic coupling and conduction can contribute to substantial ground voltage rise differences, particularly at the higher frequencies typical of many transients occurring on a high-voltage power system. Even well designed grounding grids that extend over the large areas needed for high-voltage switchyards have sufficient inductance to cause high voltage differences.

Electromagnetic coupling to the ground grid is directly proportional to the rate of change of flux and the length and orientation of the current-carrying conductor and inversely proportional to the height of the conductor above the ground grid.

Conduction of power system transients to the ground grid is typically provided through metallic grounding of transformer neutrals and capacitive paths, such as bushings, coupling capacitors, and CCVTs. These are low-impedance high-energy paths that can induce common-mode voltages on control circuits see IEEE Std Other phenomena that generate transients occur in power systems.

Some examples are undesirable time spans between the closing of the poles of a circuit breaker, fault occurrence, fault clearing, load tap-changing, line reactor de-energizing, series capacitor gap flashing, arcing ground faults, failing equipment, lightning, GIS surges, and capacitor reinsertion.

Normally, the magnitudes of such transients are less than those of other phenomena described herein. The selection of the shield grounding locations and the effects of single and multiple grounds are points to be considered for the proper installation of shielded cable. Multiconductor cable applications in the operating range of 2 kV to 5 kV require careful judgment, and each installation should be evaluated based on the existing and anticipated conditions.

Shielding can be used to monitor or test cable installation for additional assurance of insulation integrity. A shield screen material is applied directly to the insulation and in contact with the metallic shield. It can be semiconducting material or, in the case of at least one manufacturer, a stress control material. At the high voltages associated with shielded cable applications, a voltage gradient would exist across any air gap between the insulation and shield.

The voltage gradient may be sufficient to ionize the air, causing small electric arcs or partial discharge. These small electric arcs burn the insulation and eventually cause the cable to fail. The semiconducting screen allows application of a conducting material over the insulation to eliminate air gaps between insulation and ground plane.

Various shield screen material systems include: a Extruded semiconducting thermoplastic or thermosetting polymer b Semiconducting woven fabric tape c Semiconducting coating paint used with semiconducting woven fabric tape d Extruded high-dielectric-constant thermoplastic or thermosetting polymer, referred to as a stress control layer NEMA and AEIC standards require shielded power cable to be partial-discharge or corona tested.

This test evaluates the effectiveness of the conductor and shield insulation screen materials and application, and verifies the absence of voids within the insulation. A cyclic aging test is required by AEIC as a qualification of the cable design to ensure gaps do not develop between tape layers as a result of expansion and contraction cycles.

If all elements of the shield are not removed, excessive leakage current with tracking or flashover may result. Accidental removal of the shield ground can cause a cable failure and a hazard to personnel. The length of cable run should be limited by the acceptable voltage rise of the shield if the shield is grounded at only one point. The derating of ampacity due to multiple-point short circuited shields has a negligible effect in the following cases for three-phase circuits: a Three-conductor cables encased by a common shield or metallic sheath b Single-conductor shielded cables containing kcmil copper or smaller installed together in a common duct c Triplexed or three-conductor individually shielded cables containing kcmil copper or smaller d Single-conductor lead sheathed cables containing kcmil copper or smaller installed together in a common duct Because of the frequent use of window type or zero-sequence current transformers for ground overcurrent protection, care must be taken in the termination of cable shield wires at the source.

If the shield wire is passed through the window-type current transformer, it should be brought back through this current trans- former before connecting to ground in order to give correct relay operation.

Table can be used to calculate the induced shield voltage. A maximum voltage of 25 V, under normal operating conditions, is a commonly accepted limit. For cables installed three per conduit, use arrangement II in table. All three phases of a circuit should be installed in the same conduit. When it is necessary to run only one phase per conduit, then nonmetallic or nonmagnetic metallic conduit should be used. If nonmagnetic metallic conduits are used, the reduction of cable ampacity due to conduit heating should be considered.

Also, no magnetic metal, such as clamps or reinforcing bar, should form a closed ring around the conduit. Table 2 gives the maximum lengths of single conductor cable with shields grounded at one point to stay within the 25 V maximum for the conditions stated.

Other conditions will permit different lengths. For example, cables operated at less than rated ampacity will allow longer lengths. Direct-buried cables operating at their rated ampacity, with all other conditions being the same, will require shorter lengths to stay below the 25 V maximum. The length listed is the duct length. For further details on shielding and grounding of instrumentation and control circuits, see IEEE Std Common-mode noise may be caused by one or more of the following: a Electrostatic induction.

With equal capacitance between the signal wires and the surroundings, the noise voltage developed will be the same on both signal wires. With the magnetic field linking the signal wires equally, the noise voltage developed will be the same on both signal wires.

NOTE — When electromagnetic shielding is intended, the term electromagnetic is usually included to indicate the difference in shielding requirement as well as material.

To be effective at power system frequencies, electromagnetic shields would have to be made of high-permeability steel. Such shielding material is expensive and is not normally applied. Other less expensive means for reducing low-frequency electromagnetic induction, as described herein, are preferred.

Isolating the circuits from ground effectively opens the ground common-mode voltage path through the signal circuit. If an intervening amplifier is a single-ended amplifier, the low side of the signal circuit is not broken and is grounded at the terminal.

Therefore, the situation is not changed, so the same procedure should be followed with the terminal as indicated above. A guarded isolated differential amplifier provides isolation of both input terminals from the chassis or ground and from the output. This amplifier is capable of high-common-mode rejection and provides the input-output isolation so that the output ground will not affect the input circuit.

Typically, the common-mode rejection ratio of an isolated differential amplifier used in instrumentation systems is about dB and is the ratio of common-mode voltage applied to the amount of normal-mode voltage developed in the process. When an ungrounded transducer is used, it may be possible to obtain satisfactory results by leaving the transducer circuit ungrounded, connecting the cable shield to the amplifier guard shield, and grounding the shield at either the transducer end or the amplifier end.

However, it is considered that connecting the cable shield to the amplifier guard shield and grounding both transducer cable shield and circuit at the transducer will result in a less noisy, more stable system.

See 6. A properly grounded shield will greatly reduce the capacitance between the signal conductors and external sources of electrostatic noise so that very little noise voltage can be coupled in the signal circuit.

By alternately presenting each conductor to the same electromagnetic field, voltages of equal magnitude and opposite polarity are induced in each conductor with respect to ground. The frequency of twisting lay affects noise reduction ability and, therefore, should be considered in specifying twisted pair cable. The materials normally used for shielding of instrumentation cable are nonferrous and cannot shield against power frequency electromagnetic fields.

The steels normally used in conduit or tray are not of high enough permeability to provide very effective shielding at power frequencies. However, some benefit may accrue from the use of rigid steel conduit or steel trays with solid bottoms and tightly fitting solid steel covers. However, physical separation in itself, unless carefully analyzed, may not achieve the desired degree of immunity. Cables should be run in accordance with clause An exception to this is when the shield is used for the excitation of a neutralizing transformer.

If the shield is grounded at some point other than where the signal equipment is grounded, charging currents may flow in the shield because of differences in voltage between signal and shield ground locations. If the shield is grounded at more than one point, differences in ground voltage will drive current through the shield.

In either case, shield current can induce common- mode noise current into the signal leads, and by conversion to normal-mode noise, voltage proportional to signal circuit resistance unbalance can reduce accuracy of signal sensing. In a system with grounded transducer and isolated- input differential amplifier, the cable shield should connect to the amplifier guard shield, but grounding the shield at the amplifier will reduce the amplifier's common-mode rejection capability.

Grounding the shield only at the transducer will maintain the shield at the same ground voltage as the transducer, which will minimize shield-induced common-mode current while permitting the amplifier to operate at maximum common-mode rejection capability.

Also see 6. They can also be used in a combined isolating-drainage transformer configuration. They neutralize extraneous longitudinal voltages resulting from ground voltage rise or longitudinal induction, or both, while simultaneously allowing ac or dc metallic signals to pass. For noncomputer type applications, such as annunciators, shielding may not be required. Separation of digital input cables and digital output cables from each other and from power cables may be required.

Where digital inputs originate in proximity to each other, twisted pair multiple conductor cables with overall shield should be used or multiple conductor cable with common return may be permitted, and overall shielding may not be required.

Digital output cables of similar constructions may also be permitted. Individual twisted and shielded pairs should be considered for pulse-type circuits. When two lengths of shielded cable are connected together at a terminal block, an insulated point on the terminal block should be used for connecting the shields. Furthermore, the use of shielded, twisted pairs into balanced terminations greatly improves transient suppression.

It is never acceptable to use a common line return both for a low-voltage signal and a power circuit [B13]. A separate ungrounded power supply should be furnished for the group of RTDs installed in each piece of equipment.

Normally, the maximum voltage will exist at the instant of interruption. Magnitude is very dependent on supply circuit impedance. If impedance is high, voltage will be proportionally high. The higher the speed of interruption, the higher the surge voltage generated. Voltages in excess of 10 kV have been observed across a V coil in laboratory tests, but 2. DC circuit energization has an effect on adjacent circuits where capacitive coupling exists.

Full battery voltage appears initially across the impedance of the adjacent circuit and then decays exponentially in accordance with the resistance- capacitance RC time constant of the circuit [B38]. The extensive use of surge capacitors on solid-state equipment and the longer cable runs associated with extra-high voltage EHV stations have substantially increased the capacitance between control wiring and ground. Inadvertent momentary grounds on control wiring cause a discharge or a redistribution of charge on this capacitance.

Although this seldom causes failure, the equipment may malfunction. Saturation of current transformers by high-magnitude fault currents, including the dc offset, can result in the induction of very high voltages in the secondary windings. This phenomenon is repeated for each transition from saturation in one direction to saturation in the other.

The voltage appearing in the secondary consists of high-magnitude spikes with alternating polarity persisting for an interval of a few milliseconds every half cycle [B38]. The most significant interference comes from voltages or currents, or both, induced in the circuits as a result of exposure to nearby conductors in which transient currents or voltages appear as a result of switching or faults. Although voltages used in the transmission of power have been increasing over the years, the level of control voltages and signal power has had a tendency to remain constant or even decrease.

Since induced interference increases with the use of higher voltages and increased fault current levels, the ratio of unwanted signal noise to useful signal will be increased if precautions are not taken to protect the signal circuits. Transient voltages on cables cannot be completely eliminated, but can be limited in magnitude. In the interest of compatibility with solid-state relaying systems, one suggested limit is the peak of the surge withstand capability SWC test described in IEEE Std C Many different things can be done separately or in combination to reduce the magnitude of the transients, depending upon economics and equipment configuration.

The following methods are primarily confined to control cable installation. Because mutual capacitance and mutual inductance are greatly influenced by circuit spacing, small increases in distance may produce substantial decreases in interaction between circuits [B8].

Where possible, control cables should be routed perpendicular to high-voltage buses [B17], [B38]. When control cables must be run parallel to high-voltage buses, maximum practical separation should be maintained between the cables and the buses [B8]. NOTE — Tests indicate that in some cases, nonshielded control cables may be used without paralleling ground cables when they are parallel and are located at a distance greater than 50 ft Great care should be exercised in routing cables through areas of potentially high ground grid current either 60 Hz or high-frequency currents [B17].

When practical, control cables may be installed below the main ground grid. All cables from the same equipment should be close together, particularly to the first manhole or equivalent in the switchyard [B17]. High-voltage cables should not be in duct runs or trenches with control cables [B8], [B17], [B38].

Radial arrangement of control circuitry will reduce transient voltages. Circuits routed into the switchyard from the control house must not be looped from one piece of apparatus to another in the switchyard with the return conductor in another cable. All supply and return conductors must be in a common cable to avoid the large electromagnetic induction possible because of the very large flux-linking-loop that the loop arrangement provides [B8], [B38].

The ground grid, even when designed with a very low resistance, cannot be considered as an equal-voltage surface. Substantial grid voltage differences may occur that will be directly influenced by a number of factors, for example, grid resistance, grid geometry and distribution of ground currents see IEEE Std , earth resistivity see [B51] and IEEE Std , and frequency of the transient [B14]. Since it is impractical to eliminate grid voltage differences, their effects must be neutralized.

Neutralization can be accomplished by a low-resistance shield conductor parallel to, and in proximity to, the affected control circuits. Such a conductor may be the shield of a shielded control cable, unused conductors of an unshielded control cable, or a separate shield conductor.

These conductors will carry currents proportional to the grid voltage differences and induce a counter voltage in the control circuits, thus effecting neutralization. Grounding, neutralizing, and shielding methods that have been found to be effective are as follows: a In trench systems, shield conductors which are connected to the substation grid as necessary should be attached to the top sides of the trench. This places the shield conductors between the transient source and the control cables [B51].

These shield conductors should have sufficient conductivity to carry fault currents without damage and have adequate mechanical strength. This ground bus provides a convenient means of grounding individual cable shields.

For equivalent conductivity, several smaller shield conductors are more effective than a single large conductor. Provisions should be made for replacement with shield conductors should the unused conductors later be used for active circuits.

To be most effective, shield conductors must be in the closest possible proximity to the control cables, particularly where unshielded cables are used. Making the ground connection at the relay or control building has the following advantages: 1 Voltage rise is minimized near the relay equipment. To facilitate one point grounding, all capacitor banks of a given voltage should be at one location. For a concentric shield, this ratio should be one. For an adjacent shield wire, the ratio must always be less than one.

If the resistance of the shield is considered, then the cancelling voltage generated by the shield current is reduced by the ratio of the self-inductive reactance of the shield to the total complex self-impedance of the shield. The resistance becomes significant at low frequencies where the inductive reactance of the shield is low and can generally be neglected at high frequencies see IEEE Std Protection may take the form of surrounding the sensitive circuits with an equal-voltage surface to prevent capacitive coupling to high-voltage conductors and magnetic shielding to mitigate the effect of strong magnetic fields.

When shielded control cable is used, grounding the shield at both ends is recommended [B13]. Care should be exerted in keeping the shield intact, as a broken or separated shield can greatly reduce the shield efficiency. If only one end of the shield is grounded, large transient shield-to-ground and conductor-to-ground voltages may be present at the ungrounded end [B8], [B35].

Grounding a shield at both ends allows shield current to flow. The shield current resulting from magnetic induction creates a counter-flux which will tend to cancel the flux that created the shield current.

The net effect of the shield on the lead is to reduce the noise level. An exception to this is that the current flowing in shields not produced by flux linking the lead will cause the surge or noise voltage on the lead to be higher than it would be if there were no shield [B38], [B13], [B51]. The lower the shield impedance, the greater is the amount of transient voltage cancellation because of greater current flow.

Generally, a lower surge impedance permits larger induced transient currents to flow in the shield [B35]. A grounding conductor may be run parallel to the shielded cable to help protect the shield from being damaged when fault currents are present [B51]. If electrostatic shields are required, they should be within the outer shield [B35]. Auxiliary power and yard lighting circuits should not be installed without adequate shielding near shunt capacitor banks [B8].

Experience has shown that in high-voltage substations, steps should be taken to reduce the transients in auxiliary power cables, lighting cables, etc. Three specific types of conductors are normally used: insulated single conductor, coaxial cable, and triaxial cable.

It can also be used as the interconnecting lead for short bypasses. Bare conductors and coaxial cables should be avoided for these applications, since either one can introduce excessive leakage currents or excessive stray capacitance.

The stray capacitance can cause a reduction in bandwidth, and the leakage currents can cause a loss in carrier power. To reduce stray capacitance and leakage currents, either of the following methods may be used: a An insulated single conductor should be run as directly as possible between its required terminations. It should be mounted on insulators and fed through bushings at each end. The conductor insulation should be unbroken between its ends to maintain low leakage. The insulated single conductor should be isolated from the flexible metal conduit with non- conductive washers spaced about 6 in mm apart.

If the conductor has a significant portion of its length outside the flexible metal conduit, it should be mounted on insulators and fed through bushings at its ends as in item a. A typical insulated carrier lead, 0. If both shield ends are grounded, large surge currents can flow under certain conditions, causing saturation of the impedance-matching transformer and resulting in an inoperative carrier channel. This cable provides an additional heavy shield which does not carry signal currents.

The outer shield is capable of carrying large induced surge currents under fault conditions and is grounded at both ends. This arrangement provides very effective shielding against both magnetic and electrostatic induction so that surges induced in the signal leads are small. Chlorosulfonated polyethylene and silicone rubber compounds are examples of materials that have been used in high-temperature cables or where cable fire propagation is a consideration.

This voltage can be reduced by lowering the surge impedance achieved by mounting the CCVTs as close to the ground as permitted by clearance standards and by providing multiple low-resistance conductors between the CCVT base and the station ground grid, and between phases. All secondary circuits from the CCVTs should be radial and contained within a single shielded cable to provide cancellation of the differences in ground grid voltage [B8].

The secondary cables should follow the ground conductor as closely as possible. Cable penetration fire stops, fire breaks, system enclosures, and cable coatings This clause provides guidance for the selection and application of cable penetration fire stops, cable fire breaks, cable system enclosures cocoons , and coatings for cable systems.

NOTE — Several types of fire stops, cable system enclosures, fire barriers, and coatings are made from materials that are thermal insulators.

Their use can result in significant cable derating, which should be considered in sizing cables. NOTE — To be effective, fire barriers must have sufficient fire resistance to withstand the effects of the most severe fire that may be expected to occur in the area adjacent to the fire barrier, and must provide a complete barrier to the spread of fire.

Modifications or additions of cables through the fire stop should not compromise the integrity of the fire stop. A special example of this method is using a solid section of tray which is then filled with sealant. The sealant or compound should be compatible with the cable outer surface material.

The void around the cable should be filled with a fire-resistive seal. Cable penetration fire stops should be used when sleeve or tray penetrations are used beneath control boards or other panels. Fire detection systems This clause provides guidance or information for the selection of fire detection systems for cable systems. Automatic fire detection systems may be installed in areas of high cable concentration.

The zone of influence is determined by extending lines from the bottom of the side rails of the lowest cable tray at a 30 degree angle from vertical see figure 1. Fire detection systems may also be considered in areas of lesser cable concentration that provide vital service, or areas where, because of its location, a cable fire may go unnoticed for a relatively long period of time.

Fire-extinguishing systems This clause provides guidance for the selection and application of fire-extinguishing systems protecting cable systems. Fire-extinguishing systems may be utilized for the protection of cable systems. Installation and handling This clause provides guidance for the construction methods, materials, and precautions in handling and installing cable systems. Optical cable is addressed separately in this clause.

During storage, the ends of the cables should be sealed against the entrance of moisture or contamination. Reels should be stored on solid ground to prevent the flanges from sinking into the earth. Handling or pulling cables in extremely low temperatures can cause damage to the cable sheathing, jacketing or insulation.

To prevent damage of this nature, store cables in a heated building at least 24 h prior to installation. The lubricant should not set up so as to prevent the cable from being pulled out of the conduit at a later time. Cable lubricants should not support combustion. Turning the reel and feeding slack cable to the duct entrance will reduce the pulling tension.

Whenever a choice is possible, the cable should be pulled so that the bend or bends are closest to the reel.

The worst condition possible is to pull out of a bend at or near the end of the run. Cable splices should not be placed directly on racks or hangers. It is desirable that the system be designed so that additions and changes can be made with ease, economy, and minimum outages. If necessary, cables should be protected by fire-resistant material. The radius of the feeder device should not be less than the minimum bending radius of the cable.

If a feeder device is not used, the cable should be hand-guided into the raceway. The ends of all other cables should be properly sealed during and after installation in wet locations. Cables such as aluminum, mineral-insulated, paper, and varnished cambric should be resealed after pulling, regardless of location. If water has entered the cable, a vacuum should be pulled on the cable or the cable should be purged with nitrogen to extract the water.

Projections and sharp edges on pulling hardware should be taped or otherwise covered to protect against snagging at conduit joints and to prevent damage to the conduit. A mandrel should be pulled through all underground ducts prior to cable pulling.

Any abrasions or sharp edges that might damage the cable should be removed. Minimum bending radius should never be less than that recommended by the manufacturer. To prevent damage by deformation due to excessive bearing pressure or cable tension, vertically run cables should be supported by holding devices in the tray, in the ends of the conduit, or in boxes inserted at intervals in the conduit system. Raceway fill, maximum sidewall pressure, jam ratio, and minimum bending radius are design limits which should be examined in designing a proper cable pull.

These design limits are prerequisites needed in designing a cable raceway system. Once these limits are determined for a particular cable, the raceway system can then be designed. If the system has already been designed, modifications may be required in order to pull the cable without damage. Conduit and duct system design should consider the maximum pulling lengths of cable to be installed.

The maximum pulling length of a cable or cables is determined by the maximum allowable pulling tension and sidewall pressure. The pulling length will be limited by one of these factors. Pull points or manholes should be installed wherever calculations show that expected pulling tensions exceed either maximum allowable pulling tension or sidewall pressure. A sample calculation for determining cable pulling tensions is shown in the annex. If the fill limitations and cable area are known, the raceway area can be calculated and an adequate size can be selected.

The cable manufacturer should be consulted when tensions exceeding these limits are expected. When pulling by basket grip over a nonleaded jacketed cable, the pulling tension should not exceed lb 4. When using a basket-weave type pulling grip applied over a lead-sheathed cable, the force should not exceed lb 6. The maximum allowable sidewall pressure is lb per ft of radius for multiconductor power and control cables and single-conductor power cables 6 AWG and larger, subject to verification by the cable manufacturer.

The recommended maximum allowable sidewall pressure for single-conductor power cable 8 AWG and smaller is lb per ft of radius subject to verification by the cable manufacturer.

Jam ratio is defined for three cables of equal diameter as the ratio of the conduit inside diameter D to the cable outside diameter d. The jam ratio is a concern because jamming in the conduit could cause damage to one or more of the cables. The possibility of jamming is greater when the cables change direction. Therefore, the inside diameter of the conduit at the bend is used in determining the jam ratio. The values given are usually stated as a multiple of cable diameter and are a function of the cable diameter, and whether the cable is nonshielded, shielded, armored, or single or multiple conductor.

It is very accurate where the incoming tension at a bend is equal to or greater than 10 times the product of cable weight per foot times the bend radius 10 w r expressed in feet. If the tension into a bend is less than 10 w r, the exact equations can be found in reference [B41].

Cases in which the exact equations may become necessary are where light tensions enter large radii bends. Usually equation 19 is precise enough for normal installations. The glass fibers are usually well protected by buffer tubes inside the cable itself. Even though the glass in the fiber is actually stronger higher tensile strength per unit area than a metal conductor, there is very little cross-sectional area in a fiber available for strength and support.

For this reason, most optical cables have other components to provide the strength for cable support during pulling, handing, etc. The maximum allowable pulling tension on optical cable can vary from as low as 50 lb force The maximum tension for a particular optical cable should be obtained from the cable manufacturer. This maximum recommended pulling tension should be noted on any drawings, installation instruction, etc. The theory of pulling tension is the same for optical cable as it is metallic conductor cable.

Pulling tension can be calculated based on cable weight, conduit system design, and lubricated coefficient of friction. Probably the most common installation mistake is making tight bends in the cable. Tight bends, kinks, knots, etc. Minimum bending radius in traditional optical cable is usually in the range of 20 times the cable diameter, considerably higher than electrical cable; however, new fiber technologies are lowering this minimum bending radius.

This bending radius should be considered by the engineer when specifying conduit bends and pull box openings or sizing guide pulleys, sheaves, mid-assist capstans, etc. Optical cables are often pulled for much longer distances than electrical cables. Continuous fiber pulls of over ft m are not uncommon. These long pulls minimize the number of splices in optical cable which is desirable for fiber performance.

The light weight of the cable makes these long pulls possible, although proper lubrication and a good conduit installation are also necessities. Pulling lubricants with some unique features are required by the special nature of optical cable pulling, i. Lightweight optical cable rubs on all sides of the conduit through the natural undulation of long straight runs. Many common lubricants flow to the bottom of the raceway and lose effectiveness in this type of pulling.

As with electrical cable, specific coefficients of friction depend on cable jacket type, conduit type, and the lubricant as well. One of the types of conduit used for buried optical cable is the continuous-reeled type. Such continuous duct is popular because it is inexpensive and offers enough protection to allow the use of the less expensive cable constructions. While these undulations may look minor, they can result in hundreds of degrees of bend per thousand foot of pull, and vastly increase pulling tensions even with an extremely low friction coefficient.

Short-length optical cable pulls may not require lubricant; however, for long or complex fiber pulls, lubricant is critical to making an efficient, high quality installation. Some of the requirements for optical cable pulling lubricant are: a Compatibility with polyethylene no stress cracking and other types of cable jacket b Complete and even coating on the cable for friction reduction at all friction points c Consistent low coefficient of friction over time Acceptance testing of installed cables This clause provides guidance for the testing of cables after installation, but before their connection to equipment, and includes cable terminations, connectors, and splices.

It should be noted, however, that these tests may not detect damage that may eventually lead to cable failure in service, e. Cable ends should be properly cleaned of all conducting material. Cable test results, environmental conditions, and data should be recorded and filed for maintenance reference. These cables may be tested as part of the system checkout. Unshielded high-voltage cables should not be subjected to high-voltage dc tests; insulation resistance tests are suggested. Raceways This clause provides guidance for both a means of supporting cable runs between electrical equipment and physical protection to the cables.

Raceway systems consist primarily of cable tray and conduit. If they are installed direct buried in soil, consideration should be given to the zinc coating having a limited life, and corrosion may be rapid after the zinc coating is consumed or damaged.

When used in cinder fills, the conduit should be protected by non-cinder concrete at least 2 in thick. When used where excessive alkaline conditions exist, the conduit should be protected by a coat of bituminous paint or similar material. PVC-coated steel conduit may be used in corrosive environments. Plugs should be used to seal spare conduits in wet locations.

Since ABS and PVC conduit may have different properties, a review should be made of their brittleness and impact strength characteristics. Coefficient of expansion should also be considered for outdoor applications. Flammability of such conduits is of particular concern in indoor exposed locations. Burning or excessive heating of PVC in the presence of moisture may result in the formation of hydrochloric acid which can attack reinforcing steel, deposit chlorides on stainless steel surfaces, or attack electrical contact surfaces.

The use of exposed PVC conduit indoors should generally be avoided, but may be considered for limited use in corrosive environments. Aluminum conduit may be exposed in wet and dry locations. Aluminum conduit should not be embedded in concrete or direct buried in soil unless coated bitumastic compound, etc. Aluminum conduit may be used, exposed or concealed, where a strong magnetic field exists; however, conduit supports should not form a magnetic circuit around the conduit if all the cables of the electrical circuit are not in the same conduit.

Protection should be provided against attack by insects, rodents, or other indigenous animals. Cost savings may be realized when comparing above grade trays, conduit, and troughs to similar below-grade systems. Care should be taken in routing above grade systems to minimize interference with traffic and equipment access, and to avoid a reduction in minimum electrical clearances.

These systems are more vulnerable to fires, mechanical damage, environmental elements, and seismic forces, and offer greater susceptibility to electrostatic and electromagnetic coupling than if the cables were below grade. The judicious location of these boxes may result in considerable savings.

Liquid-tight flexible conduit is commonly used for this application. Flexible conduit length should be as short as practical, but consistent with its own minimum bending radius, the minimum bending radius of the cable to be installed, and the relative motion expected between connection points. A separate ground wire should be installed if the flexible conduit is not part of the grounding and bonding system.

Drain fittings and air vents in the equipment enclosure should also be considered. Expansion couplings should be installed in the conduit run or at the enclosure to prevent damage caused by frost heaving or expansion. Reinforcing steel in the manhole walls should not form closed loops around individual nonmetallic conduit entering the manhole. Nonmetallic spacers should be used. When this is not practical, lean concrete or porous fill can be used between the frost line and the duct bank.

Each threaded joint should be cleaned to remove all of the cutting oil before the compound is applied. The compound should be applied only to the male conduit threads to prevent obstruction.