Equipment Ground Conductor (EGC) on delta system with a grounding transformer

[rockman7892]

On an industrial or renewable site that is served from the Delta side of an interconnection transformer and has a grounding transformer (wye -delta) for sourcing ground fault current and limiting overvoltage, is an equipment ground conductor (EGC) as required by NEC still appliable to this system to carry fault current from equipment back to the neutral of the grounding transformer?

Similarly to how the NEC requires and EGC on typical wye systems to carry fault current would this still be applicable on a delta system with a grounding transformer? I would think yes since the intent would still be to provide a path for fault current back to its source.

For example is there was a L-G fault in the delta winding in the transformer, and EGC would be needed to carry fault current from fault location, to transformer ground, along the EGC back to the source at the grounding transformer. Am i looking at this correct?

 

[waross]

Yes.
However you may be overlooking the safety aspect of equipment grounding conductors.
A fault to an ungrounded equipment enclosure such as a motor frame or a junction box will typically energize the enclosure to about1/1.73 of line to line voltage.
The source is the capacity to ground of the system.
While there will seldom be enough current to do serious damage, there will often be enough current to be lethal.
The Equipment Grounding Conductor limits the current on the surface of equipment.
Don't look for a loop-hole on EGCs.
There isn't any.
Even an ungrounded system (delta or single phase) and also high resistance grounded systems must have EGCs.

 

[rockman7892]

Thanks Waross

I'm trying to determine if the shield of an MV cable can be used to satisfy that EGC or if I must add a separate EGC cable or change cable to concentric neutral type. From what I understand the NEC doesn't allow shield to be used as an EGC to carry GF current of significant magnitude however a concentric neutral may be used?

The cable from the delta side of transformer goes underground before transitioning to overhead between two poles that have a old neutral (old neutral before insertion of transformer, with neutral no longer being continuous to utility). wire on it before transitioning back underground to get to grounding transformer. Would this EGC or concentric neutral just connect to that neutral running between poles to continue EGC over to other riser poles where cables transition back underground?

 

[waross]

I'm trying to determine if the shield of an MV cable can be used to satisfy that EGC

A couple of issues here;
The code specifies the ampacity of the ECG.
It is doubtful if the shield meets the ampacity requirement.
A shield on a single conductor cable will have a voltage induced.
If the shield is grounded at both ends, a current will flow.
The code generally requires the cable ampacity to be derated to allow for the extra heat from the shield current.

 

[davidbeach]

I agree with Bill about the shield. A concentric neutral, might, maybe, be used as the ECG. Admittedly, it's been 20 years since I worked on the NEC side of the meter, but at that point there weren't any concentric neutral cables NTRL listed as 2 conductor assemblies. There were single conductor shielded cables listed as a single conductor assembly. Without that listing, there was no using concentric neutral cable in installations subject to the requirements of the NEC and AHJ inspection, so it was (still is as far as I know) just a utility product and not a customer side of the meter product. Might be some value in looking into it, but don't hold your breath, and make sure the AHJ fully buys into anything that they aren't absolutely familiar with.

 

[rockman7892]

Good points on shield.

I'll have to look into concentric neutral as use as 2 conductor assembly but I do recall seeing recently that NEC permitted its use as an EGC. If that is the case is it still cheaper to run a separate EGC than use a concentric neutral cable just for the purpose of shielding or EGC? I have seen stated elsewhere on this forum that a concentric neutral cable used for anything other than carrying neutral current is a waste of money?

If using a concentric neutral as an EGC and bonded at both ends i'm guessing it's still subjected to circulating currents? Does the cable have to have ampacity de-rated in similar fashion as tape shield grounded at both ends for circulating current or is the impedance of concentric neutral much less than tape shield and doesn't have as significant as heating impact?

On 2nd thought after typing above paragraph I would suspect that you don't need to de-rate cable for concentric neutral grounded at both ends since the cable is designed to carry normal neutral current without any de-rating? Am I correct in that a cable with concentric neutral carrying neutral current does not need any further de-rating?

 

[waross]

Am I correct in that a cable with concentric neutral carrying neutral current does not need any further de-rating?

In low voltage cables, 300 Volt rated or 600/1000 Volt rated, a concentric neutral would be used with a three conductor cable for single phase, (two hots and the neutral, or a four conductor cable for three phase, three hots and the neutral.
As the neutral carries only the unbalanced current, no derating is needed.
I am not familiar with the cable constructions available for higher voltages.

Ampacity ratings:
Cables are not normally rated in ampacity.
The AWG or metric size is given and then the code is consulted for the allowable ampacity depending on the insulation temperature rating and on the conditions of installation.