Ground-fault protection issues

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kbsparky

kbsparky

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Delmarva, USA
On one of my projects, I noticed that there was a large bus-duct with several disconnects plugged in.

This was a 3 phase 480 Volt busway, fed from a bank of (3)- 500 kVA transformers. There were several 225 Amp disconnects which also had a junction box mounted above them. Inside that box was a CT device connected to a shunt-trip mechanism in the disconnecting breakers. It appeared to be some sort of Ground-fault protective device for equipment.

Is this required? I thought that the ground-fault protection for equipment was only required on larger (over 1000 Amp) feeders and services? According to my calculations, that transformer bank would produce just over 600 Amps at full capacity.....
 
It'll also give you opportunity for better coordination with upstream devices for ground faults, as long as the devices are set to coordinate.
That way a ground fault at the busway deosn't trip the feeder breaker, so less equipment is affected by an outage.
John M
 
There appeared to be some sort of interconnection between at least 2 of them. A small ?" EMT with a couple of wires in it?

Would there even be a requirement for GFEP on a bus duct fed from a bank of 500 kVA transformers?
 
zog said:
Not required unless you want protection from ground faults. Some people do more than the minimum requirements.
Click to expand...

A standard breaker or fuse Installed in a grounded system provides ground fault protection.

If you want more sensitive ground fault protection that is another issue.
 
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Experience has shown that too much protection can result in unwanted outages. For example:

I was working on fixing an exit sign, on a 277 volt circuit. The wire was nicked in the connector, and shorted out while I had the sign open.

This resulted in the branch circuit breaker (20 Amp) tripping out. No problem in most cases, except that it ALSO tripped out a 1200 Amp main back in the electric meter room, knocking out power to a dozen other stores as well! Seems that the GFEP sensor tripped out the mains.

I'm all for proper protection, but you can overdo things and in the case of providing emergency power, I'd rather a small branch circuit breaker trip out if there's a problem on that circuit while the mains remain on.
 
kbsparky said:
Experience has shown that too much protection can result in unwanted outages. For example:

I was working on fixing an exit sign, on a 277 volt circuit. The wire was nicked in the connector, and shorted out while I had the sign open.

This resulted in the branch circuit breaker (20 Amp) tripping out. No problem in most cases, except that it ALSO tripped out a 1200 Amp main back in the electric meter room, knocking out power to a dozen other stores as well! Seems that the GFEP sensor tripped out the mains.

I'm all for proper protection, but you can overdo things and in the case of providing emergency power, I'd rather a small branch circuit breaker trip out if there's a problem on that circuit while the mains remain on.
Click to expand...


That is a sign that no coordination study has been done and the GFP setting are incorrect.
 
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