Troubleshooting ground fault tripping breaker

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mull982

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I am trouble shooting the attached circuit that keeps tripping both of the breakers in the circuit (coordinatin issue) from what appears to be a ground fault. Both circuit breakers keep tripping intermitently. The circuit shown is a 480V L-L single phase circuit. The load will sometimes run for hours before tripping only to be rest and then trip out right away. After waiting some time the breakers can be reset and the circuit continues to operate.

I put a power meter on the circuit to see what is going on when the breakers trip. During normal operation I see current on both legs of the circuit at about 170A. However right as breaker trips I see the one leg as indicated on the sketch jump up to about 700A or so. I thought this was weird that only one leg would go high and not the other so I suspected there may be a ground fault. I set up another zero sequence CT around both of the legs as shown and find that when the breaker trips the meter sees the 700A on the one leg as shown as well as the zero sequence CT. Zero sequence CT reads 0 during normal operatioin. Seeing this I am suspecting there is a ground fault on the primary of the circuit somewhere on the one phase as indicated? Do you agree?

We went through and meggered both the cables and the primary of the transformer as shown in the circuit and everything is clear. We cannot seem to find eveidence of a short anywhere.

My thought is that it has to be a ground fault on the primary of the circuit. If there is a ground fault or any other type of fault on the secondary of the circuit then this would only appear as an overload on the primary of the transformer correct? In other words I there was a ground fault in the secondary of the transformer wouldn't this appear as an overload on the priamry with current seen in both primary legs as opposed to just 1?

Do you agree that what I am seeing must be a ground fault on the primary of the transformer circuit?

The only thing I'm not sure about is the SCR. Could something be going on with this to see what I am seeing?
 
Sounds like it must be a ground fault. Can you swap the two conductors at various points (one at a time, of course) and see at which point the excess current does and does not change wires?
 
I had a weird one were the 1200 amp GE breaker kept tripping on long time, The built in meter would read 1300 amps on C phase, 480, 500 on A and B, The actual load on C was around 520. Replaced the rating plug, didn't make any difference. The breaker had been replaced about 3 months earlier and the original was only three years old. Replaced it again and so far everything is working properly, the meter is reading correctly and has not tripped since kinda odd having two bad breakers in a row, but the second one was sold to us as new, but looked to be a reman. Went directly to GE for the third one, because they said they would warrant the first one, even though it was out of warranty. (We already had an independent lab verify the first one was bad.)
 
hillbilly1 said:
I had a weird one were the 1200 amp GE breaker kept tripping on long time, The built in meter would read 1300 amps on C phase, 480, 500 on A and B, The actual load on C was around 520.
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Sounds like a bad or incorrect CT.

hillbilly1 said:
Replaced the rating plug, didn't make any difference. The breaker had been replaced about 3 months earlier and the original was only three years old. Replaced it again and so far everything is working properly, the meter is reading correctly and has not tripped since kinda odd having two bad breakers in a row, but the second one was sold to us as new, but looked to be a reman.
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Only GE or an authorized distributor can sell that as "New". Didn't the 2nd one have a waranty?

hillbilly1 said:
Went directly to GE for the third one, because they said they would warrant the first one, even though it was out of warranty. (We already had an independent lab verify the first one was bad.)
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What did they say was "bad"? What type breaker? Not many things would warrant replacing the untire breaker if it just tested "bad"
 
zog said:
Sounds like a bad or incorrect CT.

Only GE or an authorized distributor can sell that as "New". Didn't the 2nd one have a waranty?

What did they say was "bad"? What type breaker? Not many things would warrant replacing the untire breaker if it just tested "bad"
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1. CT built into breaker, if incorrect, factory screwup.
2. They claim to buy from the manufacture in bulk, and claimed it was new, they would warrant it, but GE would not. Decided not to take a second chance that they would send another crappy breaker, also if GE sent a crappy breaker, we could hold their feet to the fire much easier. I had a feeling this was a production issue that they knew about, and had been fixed, but the supplier we bought from had one that had been bought before the defect was found.
3. The testing facility did not mention what was bad with it, just that it failed.
 
hillbilly1 said:
1. CT built into breaker, if incorrect, factory screwup.
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Very possible, which is why acceptance training is important. These do nto get tested at the factory either.

hillbilly1 said:
2. They claim to buy from the manufacture in bulk, and claimed it was new, they would warrant it, but GE would not. Decided not to take a second chance that they would send another crappy breaker, also if GE sent a crappy breaker, we could hold their feet to the fire much easier. I had a feeling this was a production issue that they knew about, and had been fixed, but the supplier we bought from had one that had been bought before the defect was found.
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?????, you had to buy another breaker.

hillbilly1 said:
3. The testing facility did not mention what was bad with it, just that it failed.
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I find that hard to believe, I test breakers everyday and always provide a full test report and failure analysis, as do most testing companies. If you went to your doctor and he said you were going to die would you ask why?

Find a real testing facility.
 
I would look closely at the mounting of the SCR and the forward bias diode, if on a heat sink look to see if it is properly insulated in the mounting of either the heat sink of the SCR to it if it is an isolated SCR, if the SCR is not electrically isolate from the heat sink then make sure the heat sink is electrically isolated from any grounding, flash over under the SCR mounting is possible?
 
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