Why do we shield power cables when voltages are over 4KV? I understand why we shield communications cables - to keep electrical noise out. But what is the objective for shielding power cables?

 

EMI Sheilding for devices and wiring not shielded

Physical protection of the conductors

As a Grounding Conductor

 

Responding to Ed Battle's Nov 19, 2:08am query and Anonymous' Nov 20, 4:05pm reply... the most important reason is based upon safety:

1) Shielding is used to confine a cable's dielectric field to the inside of the cable's insulation. It consists of surrounding the insulation or assembly with a grounded (earthed) conducting media, often referred to as a "shield!" This protects anyone contacting the insulation from exposure to the voltage-gradient within the insulation.

Without the shield there are two capacitors in series having different dielectric constants: one representing insulation; and the other air. Using 4-kV (3-phase) as an example, then the gradient between conductor and insulation surface, if grounded, is 4/sqrt3. But, any part of the cable not in contiguous contact with ground experiences a higher voltage caused by the insulation and air-media capacitors in series. Because of the voltage-divider effect the touch-potential between ground and any part of the cable that doesn't touch ground could introduce a significant shock hazard.

2) Anonymous mentioned some of the other purposes for the shield. Here are some additional pros and cons:

a) Smooths electric-field gradients within the insulation. Also reduces electrical stresses associated with the surface of the insulation.

b) Ameliorates surge potentials by increasing surge-impedance.

c) "Shields" as ground return-paths are usually applied in limited ground-fault systems.

d) "Shields" must be designed so they don't introduce significant losses.

e) Finally, shields aren't necessarily metallic. They can also be provided by semi-conductor materials.

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

Thanks Phil,

Ed Battle, PE (Pittsburgh, PA)
([email protected])