MV Power cable Shield Grounding

[joan271273]

For MV power cables you can ground the cable shield as follows:

+ Single Point.
+At Both Ends
+Transposition

For single point grounding I have noticed that certain vendors refers to a safe value of 25V for the open end side of the cable circuit. So the side were shield is grounded the potential between the shield and the ground is 0 Volts, but at the other end ( open end) you have a value different from 0 Volts but cannot exceed 25V . Can someone please advise were does the 25V threshold come from????.,

 

[DanDel]

MV power cables should have the shields at both ends grounded.

 

[alehman]

That's the voltage the manufacturer feel comfortable that the jacket can withstand and that will be safe to personnel. Normally both ends are grounded, except for very long runs where circulating shield current can reduce cable ampacity.

 

[ausphil]

Measure the load current of the cable, and the open circuit voltage at the end of the shield. You can then scale up the value of the open circuit voltage to the rated load current of the cable. This is the figure that needs to be less than 25V.

This value will determine whether or not under fault conditions (ie high fault current flowing in conductor producing high shield open circuit voltages) the cable sheath will puncture to earth from this high voltage (for a short period of time until the protection operates hopefully).

Safety wise, generally the open end of the shield should not be exposed to personnel when in service as it will produce a potentially lethal voltage for that short time.