Cable termination earthing 1

[wowski]

I have been asked to looked at the termination requirements for two new 12MVA 33/11kV transformers. The specific question I have is to do with the earthing of the lead on the single core cables at the termination points. Is it better to only earth one end to prevent any circulating currents in the lead, or to earth both ends in the event of the lead ever becoming discontinuous. We have a mixture of both techniques on our existing plant, and nobody can really give me a definite answer.

 

[redtrumpet]

I believe it is a matter of preference. Shield/sheath grounded at both ends will allow a circulating current which may require ampacity derating of the cable. Shield grounded at one end is susceptible to voltage buildup at the ungrounded end, from which personnel may have to be protected.

Canadian Electrical Code requires derating to 70% of ampacity or in accordance with the manufacturer's recommendation where sheath currents exist.

My experience has been that the majority of installations are grounded at one end only and have the shield/sheath isolated from ground at all other points. On long runs, sometimes the sheath is broken into sections and each section grounded at one end and otherwise isolated from the other sections and ground. This limits the voltage buildup at the open end.

 

[busbar]

Do a search at this site on "shield grounding." Lots of hits...

 

[jbartos]

Suggestion: Normally, the shield or sheath is gounded on both ends when it comes to low voltage cables by means of connectors at the ends (there is not much of choice). The medium and high voltage cables "connectors" may vary. The sending end is usually grounded. If both ends can be sending then the both ends are grounded.

 

[busbar]

Although the original poster’s voltages imply it is other than an ANSI installation, there is an interesting but nebulous mandate shoehorned into the NEC of note. I could not find specific numbers or further guidance, but:

99NEC 310-60(c){1) Grounded Shields. Ampacities shown in Tables 310-69, 310-70, 310-81, and 310-82 are for cable with shields grounded at one point only. Where shields are grounded at more than one point, ampacities shall be adjusted to take into consideration the heating due to shield currents.

Apparently single-grounded shields are clearly permitted by the 99NEC. The ‘more than one shield ground’ ampacity adjustment handily complicates MV-cable specification. Any rough idea what sort of correction factors are needed for this?