United Wire & Cable Inc. -

Summary

Single conductor cables - always shielded
Multiple conductor, non-armoured - always shielded
Multiple conductor, armoured - shielded recommended/unshielded allowed

Shielding of an electric power cable is the practice of confining the dielectric field of the cable to the insulation of the conductors. It is accomplished by means of conductor and Insulation shields.

Conductor Strand Shielding

A conductor shield is employed to preclude excessive voltage stress between conductor and insulation. Conductor strand shielding, which consists of carbon loaded semi-conducting compound extruded over the conductor strands, remain a definite necessity at 3kV and higher voltages levels

Insulation Shield

Basically, individual phase conductor insulation shield consists of carbon loaded semi-conducting compound extruded intimately onto the insulation surface followed by an auxiliary lower resistance metallic path to ground such as a copper tape. Under normal system operating conditions, the voltage levels are concentrically contained and fully dampened from the conductor interface to the insulation outer surface.

Why Insulation Shield

In the event of a cable system surge or impulse, instantaneous voltages can be imposed beyond the insulation outer surface. The ground plane in the form of an insulation shield would convert this energy level to current thus preventing ionization/corona burning from occurring at the insulation surface. This shield eliminates a voltage build up which would cause arcing to ground or to a surface at a lower potential. For this reason, it is imperative to always effectively shield individual phase conductor insulation surfaces for system voltages 4160 volts and higher.

Non-Insulation Shielding Precautions

Through basic theory and experience, the wire and cable industry has realized that if a 5kV cable system is protected from electrical impulsing and if the three phase conductors are placed in a close trefoil formation covered by an overall metallic medium classified as a Class "C" shield, individual phase insulation shields are not mandatory at this stress level. However, there are concerns and precautions which must be taken into consideration with such an unshielded cable, of which some are:

Trefoil Departure - It is important to electrically treat each insulated phase conductor as a bare conductor when departing from a close trefoil formation and Class "C" shield at terminations.

Contamination/Moisture - Caution must be exercised in highly contaminated and moisture prone areas, in order to prevent any possible insulation surface tracking.

Ultra Violet Rays - If unshielded XLPE insulated cables are exposed to Ultra violet rays, it is necessary to use a carbon filled XLPE compound to counteract possible insulation cracking as a result of these damaging rays. When adding such a filler into the virgin XLPE insulating compound, the insulation dielectric properties are reduced, resulting in cable with higher electrical power losses.

Electrical Testing - In field testing or locating faults in cable, frequent DC-HiPot testing can cause insulation failure through space charge build up. For this reason Time Domain Reflection (TDR-radar) testing is becoming more widely used. Unfortunately, TDR testing, which can identify a cable problem location almost exactly, cannot be performed on unshielded cable due to the lack of an insulation surface electrical ground plane.

Although unshielded cables are preferred by installers because they do not require shield terminations, undoubtedly, the unshielded cables are less reliable than a shielded cable design for achieving long-term operation.

Comparative Criteria

A brief comparison between individual phase insulation shielded and non-shielded overall armoured 4160-volt cables is as follows:

Key Criteria	Non-Shielded	Shielded
Stress Relief Terminations	Not required	Required
Ground Plane Clearance Requirements Near Terminations	Critical	Not required
Cable Testing	DC-HiPot only	TDR or DC-HiPot
Exact Fault Location using TDR (radar method)	No	Yes
Moisture & Contaminant Surface Tracking	Critical	No problem with proper termination
Electrical System Impulsing/Surging	Critical	No problem
Insulation Dielectrics	Lower	Higher
Finished Product Cost	Lower	Higher
Overall Long-term Reliability	Lower	Higher
Cable Normal Electrical Power Losses	Higher	Lower

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