Cable shields and metallic sheath/armor should be solidly grounded at one or more points so that they operate at or near ground voltage at all times. For additional information see IEEE Std 575-1988.
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 500 kcmil copper or smaller installed together in a common duct
c) Triplexed or three-conductor individually shielded cables containing 500 kcmil copper or smaller
d) Single-conductor lead sheathed cables containing 250 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.
A nonmagnetic metallic material applied over the insulation of the conductor or conductors to confine the electric field of the cable to the insulation of the conductor or conductors.
Single conductor cables rated above 2 kV and multiconductor cables with a common overall discharge resisting jacket rated above 5 kV should be shielded, except for special applications or cable designs.
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.
The following shielding recommendations contained in the NEMA standards publications for the type of insulation being utilized should be followed: NEMA WC 3-1980, NEMA WC 5-1973, NEMA WC 7-1988, and NEMA WC 8-1976.
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.
Multiconductor cable shielding should be considered in the 2 kV to 5 kV range where any of the following conditions exist:
a) Transition from conducting to nonconducting environment
b) Transition from moist to dry environment
c) Dry soil, such as in a desert
d) Damp conduits
e) Connections to overhead lines
f) Locations where the cable surface collects conducting materials, such as soot or salt deposits
g) Electrostatic discharges are sufficient in magnitude to interfere with control and instrumentation circuit functions
h) Safety to personnel is involved
i) Long underground cables
j) Direct earth burial