Shielding should be considered for non-metallic covered cables operating at a circuit voltage above 2000 volts for single conductor cables and 5000 volts for assembled conductors with a common overall jacket.
Definition of shielding
Shielding of an electric power cable is the practice of confining the electric 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
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, but the following features should be considered as a working basis for the use 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 either a metallic conduit, a damp non-metallic conduit or a metallic binding tape or rings on an aerial cable, a loose metallic sheath, etc. 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 nonshielded nonmetallic jacketed cables are used in underground ducts containing several circuits which must be worked on independently, the external field if sufficiently intense can cause shocks to those who handle or contact energized cable. In cases of this kind, it may be advisable to use shielded 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 cables may be handled by personnel who may not be acquainted with the hazards involved.
When installing shielded cable, metallic shielding must be solidly grounded. Where conductors are individually shielded, each must have its shielding grounded and the shielding of each conductor should be carried across every joint to assure positive continuity of a shielding from one end of the cable to the other. Where grounding conductors are part of the cable assembly, they must be connected with the shielding at both ends of the cable.
For safe and effective operation, the shielding should be grounded at each end of the cable and at each splice. For short lengths or where special bonding arrangements are used, grounding at one point only may be satisfactory.
All grounding connections should be made to the cable shield in such a way as to provide a permanent low resistance bond. Soldering the connection to the cable shield in usually preferable to a mechanical clamp, as there is less danger of a poor connection, loosening, or injury to the cable. 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 nut and bolt, may be used to fasten the ends of the connection together. This combination of a soldered and mechanical connection provides permanent low resistance which will maintain contact even though the solder melts.
The wire or strap used to connect the cable shield ground connection to the permanent ground must be of ample size to carry fault currents.
Effect of Grounding Metallic Shield
The metallic coverings of cables must be grounded to provide satisfactory operating and safety conditions. As the method of grounding may affect the current carrying capacity, formulas for calculating losses and correcting the current carrying capacity for those losses may be found on pages 19 and 20 of Okonite's Engineering HandBook.
Installations of shielded single conductor cables must be studied to determine the best method of grounding. This is necessary as voltage is induced in the shield of a single conductor cable carrying alternating current due to the mutual induction between its shield and any other conductors in its vicinity. This induced voltage can result in two conditions:
1. Metal shields bonded or grounded at more than one point have circulating currents flowing in them, the magnitude of which depends on the mutual inductance to the other cables, the current in these conductors, and the resistance of the shield. This circulating current does not depend on the length of the cables nor the number of bonds, providing there are bonds at each end. The only effect of this circulating current is to heat the shield and thereby reduce the effective current carrying capacity of the cable. If the shield loss exceeds 5 percent or the copper loss, the current carrying capacity should be reduced.
2. Shields bonded or grounded at only one point will have a voltage built up along the shield. The magnitude depends on the mutual inductance to other cables, the current in all the conductors, and the distance to the grounded point. This voltage may cause discharge or create an unsafe condition for workmen. The usual safe potential is about 25 volts for cables having nonmetallic covering over the shield.
If operating conditions permit, it is desirable to bond and ground cable shields at more than one point, to improve the reliability and safety of the circuit. This decreases the reactance to fault currents and increases the human safety factor.
Some general recommendations may be made, but it must be remembered that variations in insulation thickness, conductivity of sheath, spacing of conductors, and the current being carried all affect these recommendations. It is impossible to cover all these variations.
The following single conductor cables carrying alternating currents may, in general, be operated with multisheath grounds.
1. Shielded cables up to and including 250 kcmil with phases in separate ducts.
Cables in ac circuits should not be installed with each phase in separate magnetic conduits under any circumstances due to the high inductance under such conditions. Cables in a-c circuits should not be installed with each phase in separate metallic non-magnetic conduit when their size exceeds 4/0 unless the conduit is insulated to prevent circulating currents.
2. Shielded cables installed with all three phases in the same duct.
3. Cables of any size may be installed with multi-shield grounds, provided allowance is made for heating due to current induced in the shield. Cables carrying direct current may always be solidly grounded at more than one point, except where insulating joints are required to isolate earth currents or to permit cathodic protection.
Shields Grounded at One Point
Shields of single conductor cable carrying alternating current will have a potential buildup if grounded at only one point. The table below gives the maximum lengths which should be allowed between insulating joints in order to keep this potential below the maximum safe value of 25 volts.
They apply to cables operating at any 60 Hz a-c voltage. Many conditions will permit longer lengths between insulating joints, as for example, where cables are operating at less than full load.
The lengths given are from the grounded point to the insulating joint. If the mid-point of the section is grounded, the total length between insulating joints may be twice the length given.
Induced Shield Voltages, Currents and Losses
The Okonite Engineering Handbook gives formulas for calculating the induced voltage and shield loss for single conductor cables. These formulas neglect proximity loss, but are accurate enough for practical purposes.
It is assumed that the cables are carrying balanced currents.
For cables installed three per conduit use arrangement II. The spacing S in this case will be equal to the outside diameter of the cable increased by 20 percent to allow for random spacing in the conduit.
Maximum lengths for single conductor cables with shields insulated at joints and terminals and grounded at end of each section only.
These lengths are based on cables spaced on 7.5 centers operating at 75% load factor with ampacities given here for 15kV rated cables for 1/C per duct and for ampacities given here for 3 x 1/C cables per duct.
ENGINEERING TECHNICAL CENTER