This brief Technical Newsletter will outline the need for and benefits of conductor and insulation shields used on all types of polymeric insulated power cable.

No conductor or insulation shields are required for cables rated up to 2 kV.

In this newsletter we will be referring to semi-conducting materials. In these cases a semi-conducting material means a material that is somewhere between a conductor and an insulator. Basically they are plastic or polymeric materials that have a high carbon black content. This makes them a poor insulator and a poor conductor, but at higher voltages they look like a smooth conducting surface.

At 2kV and above a conductor shield is required by standards such as U.L., C.S.A., and I.C.E.A. Modern conductor shields are mostly an extruded semi-conducting material that in effect smoothes out the conductor to make it look like it has a solid smooth surface. This lowers the electrical stress at the conductor / insulation interface which allows for a thinner insulation than would otherwise be required. Lower electrical stress also increases cable life. There are a few exceptions where a conductor shield is not required for cables rated above 2kV but power cables above 2kV always have them. They may not be obvious if the insulation is black but they are there.

At 5kV an insulation shield may or may not be required depending on the cable type and it’s application. Above the 5kV level an insulation shield is required by virtually all recognized standards. The semi-conducting component of an insulation shield, like the conductor shield, is an extrusion of semi-conducting material over the outer surface of the insulation. This shield controls the electrical stress within the insulation and prevents objects from disturbing the stress at the insulation’s outer surface. An insulation shield is always made up of two components, an extruded semi-conducting layer over the insulation and a metallic component over that.

The extruded semi-conducting material used for insulation shields is slightly different than that used for conductor shielding. Conductor shields bond to the insulation and are inseparable, where the insulation shield adheres to the insulation but must be removed at all splices and terminations. In the case of power cables above 35 kV some extruded insulation shields are bonded to improve electrical properties but these shields require special tools and procedures for removal.

Basically the metallic component of an insulation shield is there to bridge or lower the resistance of the semi-conducting layer. As outlined above the semi-conducting material is somewhere between a conductor and insulator so in order to prevent electrical problems some form of metallic layer must be used. This metallic layer carries the insulation’s leakage current, any possible short circuit current, and in the case of concentric neutral cables the system neutral current. The metallic layer must be grounded at least at one location in the cable’s run. In the case of single conductor cable care should be taken with regard to ampacity when grounding at more than one points. If a single conductor cable is grounded at only one point a standing voltage will be generated at the ungrounded end. This standing voltage is generally small under most conditions but increases with increasing cable length, higher current and during current surges or fault conditions.

The metallic component can take many forms, some of which are detailed below.

If a cable has an insulation shield it must have some form of stress relief at every splice and termination. This can be stress cones, moulded devices, heat shrink or cold shrink kits.  All kits must be suitable for the voltage class and cable size.

Copper Tape - helically applied copper tape(s).

• may be applied with gaps or more commonly overlapped.
• low short circuit capacity.
• provides good electrostatic shield.
• if grounded both ends, conductor current derating is small.

Wire Shield - helically applied small ( #22 to #18 AWG ) wires.

• same applications as copper tape.
• same short circuit and ampacity derating as copper tape.
• very uncommon construction.

Concentric Neutral - helically applied wires ( #6 to #9 AWG ).

• used by utilities on underground distribution cables.
• 1/3 or 33% conductance used on 3 phase systems.
• full or 100% conductance used on single phase systems.
• higher short circuit levels.
• conductor current derating can be high if grounded both ends.
• carries neutral return current of distribution system.

L.C. Shield - longitudinally applied corrugated copper tape.

• used on large single conductor sized utility and industrial cables.
• provides good electrostatic shield.
• provides a moisture barrier when longitudinal overlap is sealed.
• short circuit level and conductor current derating is between copper tape shield and concentric neutral cables.

Metal Sheaths - lead or aluminum sheath or covering.

• good electrostatic shield and moisture / chemical barrier
• high short circuit level and conductor current derating if grounded both ends.
• sometimes difficult to splice or terminate.

Cable Types Where Conductor and Insulation Shields are Used