ICEA P Short Circuit Performance of Metallic Shields and Sheaths on Insulated Cables. standard by Insulated Cable Engineers. ICEA P SHORT CIRCUIT PERFORMANCE. OF. METALLIC SHIELDS AND SHEATHS. ON. INSULATED CABLES. Approval by. AMERICAN. TEST METHOD FOR MEASUREMENT OF HOT CREEP OF POLYMERIC INSULATIONS. ICEA S ICEA S standard for power cables.
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Voltage regulation is often the limiting factor in the choice of either conductor or type p-45-428 insulation. While the heat loss in the cable determines the maximum current it can safely carry without excessive deterioration, many circuits will be limited to currents lower than this in order to keep the voltage drop within permissible values. In this connection it should be remembered that the high voltage circuit should be carried as far as possible so that the secondary runs, where most of the voltage drop occurs, will be small.
While these cables are applicable to the great majority of cable installations that are on grounded systems, they may also be used on other systems iccea which the application of cables is acceptable, provided the above clearing requirements are met in completely de-energizing the faulted section.
Cables in this category may be applied in situations where the clearing time requirements of the percent level category cannot be met, and iccea there is adequate assurance that the faulted section will p-45–482 de-energized in a time not exceeding one hour. Also, they may be used when additional insulation strength over the percent level category is desirable.
Their use ocea recommended also for resonant grounded systems. Consult the cable manufacturer for insulation thicknesses. In common with other electrical equipment, the use of cable is not recommended on systems where the ratio of the zero to positive sequence reactance of the system at the point of cable application lies between -1 and since excessively high voltages may be encountered in the case of ground faults.
For three-phase circuits, use voltage to neutral and resistance and reactance of each conductor to neutral. This p-45-48 will give the voltage drop to neutral.
To obtain the voltage drop line-to-line, multiply the voltage drop by 3. The percent voltage drop is the same between conductors as from conductor to ground and should not be multiplied by 3.
The following equation can be utilized to find the reactance of a given configuration by using the concept of geometric mean radius. This diagram can be used to determine the reactance of any solid or concentric stranded conductor. It covers spacing encountered for conduit wiring as well as for open wire circuits. Various modifications are necessary for use under special conditions is covered in notes on the nomogram.
The reactances shown are for Hertz operation. Where regulation is an important consideration several factors should be kept in mind in order to obtain the best operating conditions. Open wire lines have a high reactance. This may be improved by using parallel circuits but is much further reduced by using insulated cable. Three conductors in the same conduit have a lower reactance than conductors in separate conduits.
Single conductors should not be installed in individual magnetic conduit because of the excessive reactance. Three conductors in magnetic conduit will have a somewhat higher reactance than cables in non-magnetic conduit. The following table lists equations commonly used for determining various parameters of an electrical system where: Definition of shielding Shielding of an electric power cable is the practice of confining the dielectric field of the cable to the insulation of the conductor or conductors.
It is accomplished by means of strand and insulation shields. Functions of Shielding A strand shield is employed to preclude excessive voltage stress on voids between conductor and insulation. To be effective, it must adhere to or remain in intimate contact with the insulation under all conditions. An insulation shield has a number of functions: To confine the dielectric field within the cable. To obtain symmetrical radial distribution of voltage stress within the dielectric, thereby minimizing the possibility of surface discharges by precluding excessive tangential and longitudinal stresses.
To protect cable connected to overhead lines or otherwise subject to induced potentials. To limit radio interference. To reduce the hazard of shock. This advantage is obtained only if the shield is grounded. If not grounded, the hazard of shock may be increased. Use of Insulation Shielding The use of shielding involves consideration of installation and operating conditions. Definite rules cannot be established on a practical basis for all cases. However, shielding should be considered for nonmetallic covered cables operating at a circuit voltage over where any of the following conditions exist: Connection to aerial lines.
Transition from conducting to nonconducting environment.
Transition from moist to dry earth. Dry soil, such as in the desert. Where the surface of cable collects conducting materials. Where electrostatic discharges are of low enough intensity not to damage cable but are sufficient in magnitude to interfere with radio or television reception. Where safety to personnel is involved. More specific information on requirements for shielding by cable type is provided in the following table.
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Single conductor including assemblies of single conductors a. With metallic sheath or armor Multiple conductors with common covering a. Importance of Shielding Where there is no metallic covering or shield over the insulation, the electric field will be partly in the insulation and partly in whatever lies between the insulation and ground.
The external field, if sufficiently intense in air, will generate surface discharge and convert atmospheric oxygen into ozone, which may be destructive to rubber insulations and to protective jackets.
If the surface of the cable is separated from ground by a thin layer of air and the air gap is subjected to a voltage stress, which exceeds the dielectric strength of air, a discharge will occur, causing ozone formation. The ground may be a metallic conduit, a damp non-metallic conduit or a metallic binding tape or rings on an aerial cable, a Industrial Drive Lexington, South Carolina Phone: Likewise, damage to non-shielded cable may result when the surface of the cable is moist, or covered with soot, soapy grease or other conducting film and the external field is partly confined by such conducting film so that the charging current is carried by the film to some spot where it can discharge to ground.
The resultant intensity of discharge may be sufficient to cause burning of the insulation or jacket. Where non-shielded non-metallic jacketed cables are used in underground ducts containing several circuits, which must be worked on independently, the external field if sufficiently intense can cause shock to those who handle or contact energized cable.
Shielding used to reduce hazards of shock should have a resistance low enough to operate protective equipment in case of fault.
In some cases, the efficiency of protective equipment may require proper size ground wires as a supplement to shielding. The same considerations apply to exposed installations where personnel iicea may not be acquainted with the hazards involved handle cables. Grounding of the Insulation Shield The insulation shield must be grounded at least at one end and preferably at two or more locations.
This decreases the reactance to fault currents and increases human safety factor. It is recommended that the shield be grounded at cable terminations and at splices and taps. Stress cones should be made at all shield terminations. The shield should operate at or near ground potential at all times.
Frequent grounding of shields reduces the possibility of open sections on nonmetallic covered cable. Multiple grounding of shields is desirable in order to improve the reliability and safety of the circuit. All grounding connections should be made to the shield in such a way as to provide a permanent low resistance bond. Using a mechanical clamp such as a constant tension spring or a hose clamp is usually adequate to secure the connection.
The area of contact should be ample to prevent the current from heating the connection and melting the solder. For additional security, a mechanical device, such as a clamp, may be used to fasten the ends of the connection together. This combination will ensure a permanent low resistance, lcea will maintain contact even if the solder icfa. The wire or strap used to p4-5-482 the cable shield ground connection to the permanent ground must be of adequate size to carry the fault current. Shielding which does not have adequate ground connection due to discontinuity of the shield or to improper termination may be more dangerous than non-shielded non-metallic cable and hazardous to life.
Shield Materials Two icae types of materials are employed in constructing cable shields: Nonmetallic shields – consist of either a conducting tape or a layer ices conducting compound. The tape may be conducting compound, fibrous tape faced or filled with conducting compound, or conducting fibrous tape. Metallic shield – should be nonmagnetic and may consist of tape, braid, concentric serving of wires, or a sheath. Auxiliary nonmetallic conducting shields may adhere to the insulation, so that the use of a grit aluminum oxide cloth or similar material may be required to assure removal of the conducting material from the insulation surface See pages 28 – 30 for generalized discussion of cable preparation for splicing and terminating.
Short-Circuit Currents for Insulated Cable Conductors Steady increases in kVA capacity of power distribution systems have resulted in possible short circuit currents of such magnitude that the resulting high conductor temperature may seriously damage the conductor insulation. As a guide in preventing such serious damage, icae allowable short circuit temperatures that damage the insulation to a slight extent only, Industrial Drive Lexington, South Carolina Phone: The system short circuit capacity, the conductor cross-sectional area icra the circuit breaker opening time should be such that these maximum allowable short circuit currents are not exceeded.
The maximum time that a given shortcircuit current can flow in a given shield or sheath, or 2. The maximum short-circuit current that can flow in a given shield or sheath for a icew time, or 3. The effective cross-sectional area of a shield or sheath needed to withstand a given short-circuit current for a given time. The material in contact with the shield or sheath shall limit the temperature of the shield or sheath.
Wires applied either helically, as a braid or serving; ;-45-482 longitudinally with corrugations. Helically applied tape not overlapped. Helically applied flat tape, overlapped See note 3. Corrugated tape, longitudinally applied.
The effective area of composite shields is the sum of the effective areas of the components. Lcea example, the effective area of a composite shield consisting of a helically applied tape l-45-482 a wire serving would be the sum of the areas calculated from Formula 2 or 3 and Formula 1. The effective area of thin, helically applied overlapped tapes depends, also, upon the degree ice electrical contact resistance of the overlaps.
Formula 3 may be used to calculate the effective cross-sectional area of the shield for new cable. An increase in contact resistance may occur after cable installation, during service exposed to moisture and ice. Under these conditions the contact resistance may approach infinity, where Formula 2 could apply. P-4-482 capacitance of a one conductor shielded cable is given by the following formula: