Grounding conductor for high voltage systems

[CrossfitSparky]

I have a project in which an engineer is requesting we run a 25kV cable as the equipment grounding conductor. Has anyone ever heard of this? It is for a bunch of circuits from a 24.9kV metal enclosed switch to a VFI, and from the VFI to various transformers. I asked if I could use an appropriately sized 600V conductor for the ground and he tried explaining something about the conductor being rated for the voltage during a fault condition. I'm skeptical of his theory, but want to make sure I do my homework before I inform a very good customer that his consultant is about to spend a lot of his money unnecessarily. I've been doing this a long time and have never heard of a voltage rating for the insulation of an equipment grounding conductor. Any input?

 

[ceb58]

Not sure about your situation but take a look at 250.118 (1)

 

[Smart $]

...Any input?

If it's not even required to be insulated, I don't see how he can justify his position. Size of the conductor is what is important.

 

[Rockyd]

Agree with Smart. See if the engineer can offer up some kind of theory to explain his logic? All the 13.8, and 33, that we do is just a bleed, and copper wire.

 

[ohmhead]

Well most HV cable insulated will and can after shut down re charge itself like after shut down if it is not grounded to bleed off it will by capacitance of insulation to conductor now have voltage on it again with power off .

It must be grounded so i fail to see how a engineer would want a insulated ground on any high voltage system .

 

[dbuckley]

Its not at all unusual in certain specific circumstances.

A way to look at this is to imagine you have a bolted fault at a panel, with the panel having a 25KV feeder, with a ground wire back to the supply. What is the voltage with respect to true ground (not the ground wire) at the point of the bolt?

The accurate answer is to work out the impedences of the supply and ground cables, and using potential divider math, work out the voltage. The answer will generally be about half the supply voltage, or greater, if the ground conductor is of smaller cross sectional area than the phase conductor.

If the supply is stiff and holds up, that is 12.5KV at the point of the bolt, which is sufficient to cause arcing with a bare wire, and puncturing of insulation with subsequent arcing with 600V wire.

 

[erickench]

Take a look at NEC 300.3(C)(2).

 

[Smart $]

Take a look at NEC 300.3(C)(2).

It contrasts "Conductors of circuits over 600 volts..."conductors of circuits rated 600 volts, nominal, or less..."

The EGC is not a circuit conductor.

 

[roger]

There is no reason to use an MV counductor for a solidly grounded system, see the attached from Soares which is the same as the NEC.

If we were talking about an impedance grounded system an MV conductor should be used to the primary side of the resistor.

Roger