what do I do with the concentric neutral

[JLuc]

"ground or not" the concentric neutral of MV cable.

I read a lot in different threads and found opposite answers, it probably depends on the setup you have.

In my situation: it is a 4160V resistance grounded (10A)system. The tranformer is outdoor and the cable run is about 300 feet. I was told to ground the source side and isolate the load side (indoor switchgear).

I would like to have a confirmation on the best practice to use, based on your experience.

Thanks

JL

 

[slavag]

Hi Jluc.
This is a Q!!!.
I don't know, possible found answers or not.
From my experience ( sorry not from my, from others):
300feet it's about 100m, I'm hope that I'm correct.
For this distance you don't need connect concentric conductor to ground on both sides. Your solution is right.
But wait to other ideas.
Best Regards.
Slava

 

[davidbeach]

A concentric neutral and a shield are quite different animals and if you are buying concentric neutral and not connecting it at both end, it is a conductor intended to carry current, you are wasting your money. Concentric neutral is strands of conductor spiraled around the main conductor, with or without a tape shield. If all you have is the tape shield, you don't have a concentric neutral. I wouldn't use concentric neutral on an impedance grounded system as it would be too easy to make additional ground connections.

With a shield, or when using a concentric neutral as a shield, you do have the question of when/where to ground. Grounding the shield at both ends minimizes the voltages that may be present on the shield away from the ground point, so that is a positive of grounding at both ends. The negative of grounding at both ends is you now have a path for circulating currents. Those circulating currents add heat to the cable, reducing the ampacity of the conductor. There are various cross-bonding schemes that provide many of the benefits of both, but at the cost of needing access to the cables at intermediate points.

At 300 feet, if you have a good low impedance ground path between the two points, I'd ground at both ends, but if the shield would be the "low impedance" ground path between the two ends I'd only bond at one end.

 

[lansford]

It would not be common in the USA to not ground both ends of the shield especially since it is 10 amp grounding and you want the optimum path for ground fault return.
If the length of a primary cable was 30,000 feet then there may be "shield break" or two in the run but that is a different case for diffent reasons.
JIM