5 kV PT wire in a 15 kV ATS 1

[cslater]

We had a fault in an ATS last week. Upon inspection, we found that a wire in one of the cabinets had burned through and arced in a few places.

This is a 12.47 kV ATS (nominal 15 kV). The wires feeding the loads are large 15 kV rated cables. There are also smaller (#4) wires that feed the PT's. It looks like these are only rated at 5 kV.

So one of those cables was laying against the cabinet and after about a year of service, the insulation burned through.

I am being told by the manufacturer that these wires are allowed to be 5 kV rated and that the problem is faulty installation - not keeping the wires dressed so that they stay away from the frame.

I don't understand how it can possibly be acceptable to put 5 kV rated cable in a 15 kV compartment. Isn't that a code violation?

 

[dpc]

Not sure how you could use 5 kV insulation for PTs that are operating at 7200 V to ground.

Also, unshielded cable is used for medium-voltage wiring within switchgear enclosures. This is OK, but the cable has to be kept away from grounded surfaces. If the unshielded cable is resting on the surface of the metal enclosure, it can fail, even if it is 15 kV.



David Castor

http://www.cvoes.com

 

[living2learn]

I just saw this for the first time myself the other day inside a transformer. 12.47kv D:480 Y and as soon as the conductors came inside the gear they terminated and transitioned from 15kV sheilded cable to 5kV cables. I assumed this was OK because it was a UL listed piece of gear, but was completely asstonished by it.

I looked pretty hard and could not find a standard allowing this - has anybody else?

 

[scottf]

I think that's pretty common where the wire isn't supposed to be against a grounded surface.

 

[cslater]

Doesn't that violate NEC 300.3 c 1 ?

 

[jghrist]

If this was factory installed internal wiring, I would think it would be up to the factory to install it in a manner that kept it away from grounded surfaces.

 

[cslater]

The factory provided the wiring, but it was landed in the field. They provide instructions requiring 5" clearance.

Isn't that like me putting in a 100A panel with 200A of load and telling the home owner to never run everything at once?

 

[LionelHutz]

You can use bare conductors if you have sufficient air, gas or oil insulating the wires from each other and from ground.