Medium Voltage Cable Shields 2

[nickoliver]

I can't seem to find any information on using the shield (tape, wire, uni-shield etc.) in medium voltage cable as the equipment grounding conductor. In installations I have been involved with we always pulled an equipment grounding conductor along with the feeders and grounded the shields at every termination and splice. This was engineered on the drawings. I have seen older installations where PVC was used and no separate equipment grounding conductor pulled. Can someone give me a run down on what is allowed and what is not? Can shields be used to meet the requirements of article 250 of the NEC? Thanks

 

[busbar]

Related discussion at
thread238-94974
thread238-90515
thread238-99064

One thing that sometimes gets overlooked is arrestor termination in IEEE Std C62.22.1 …Connection of Surge Arresters…

Also IEEE Std 1215 …Application of Separable Insulated Connectors

 

[nickoliver]

Thanks for the links. However they don't quite cover what I am after. (I did a search for more threads as well) Basically, in an installation that is covered by the NEC does the shield (what ever type it may be) qualify as an equipment grounding conductor per 250.118 of the '02 NEC? Section X (grounding of systems and circuits of 1KV and over) states that this section shall comply with all preceding sections of article 250. It is also geared more toward system grounding rather than equipment grounding. Based on everything I have seen I would say no but it doesn't qualify but I can't come up with a specific article that prohibits it. I do realize that the shield is commonly used in non NEC applications where the protective relaying has been coordinated to not exceed the cables short circuit capacity but what I am really after is weather it is NEC compliant or not?

 

[busbar]

 
There is manufacturer boilerplate..
   

http://www.kerite.com/catalog/catalogfiles/short_circuit_limitations.htm

   

http://www.okonite.com/engineering/shielding-currents.html

Code-enforcement officials may consider this data as part of a listed cable’s installation instructions, concerning adequacy of cable shields (alone) for fault withstand.

 

[ppaya]

Tape shileded MV cables are present at many old distribution transformer stations, however, they are obsolite nowdays (it's not easy to ground tape shiled to TS grid at the source side - TS). My experience is that new station construction design practise call for concentric neutral (cooper rather then aluminum 27.6 KV usually 1000MCM cable). Tape shield unlike concentric neutral is not return neutral system current path back to Tx star point (usually zig-zag secondary) and is neither meant to be equipment grounding path. For equipment grounding therefore you have usually 4/0 cooper cable connected from TS grid to particular equipment on secondary MV side. System neutral (if you have MV cables tape shielded) has to be seperate, isolated with PVC(distribution 27,6 kV level - you need 250-300MCM separate neutral per feeder or per feeder combination, 2-3 feeders one seperate neutral, depending where your pole lines go).



ppaya

 

[DanDel]

Given that NEC 250.118 does not specifically list a cable shield, and Table 250.112 lists the minimum sizes of Equipment Grounding Conductors(with which your shield probably does not comply), along with common sense and a knowledge of how an inspector would interpret the NEC, I would say that a MV cable shield could not be used.
Do not design a system based on what the NEC doesn't directly prohibit, rather, it is up to you to find an article allowing what you want to design (especially if it isn't in common usage).

 

[peebee]

Cable shields do NOT have sufficient ampacity or low enough impedance to be considered an equipment grounding conductor.