Ground Fault mis-operation
Ground Fault mis-operation
(OP)
I installed a main circuit breaker 1600A on the secondary of an existing 1000kVA 12.47kV delta:480Y277V with ground fault protection. I received a call stating the circuit breaker failed the ground fault test because it did not pass the no trip test. It was set at 320A and was tripping at 160A. The testing firm is reputable and the breaker is brand new. No testing reports were supplied with the new breaker unfortunately. I asked if the impedance across the neutral disconnect link was measured and was told yes. Unfortunately I didn't ask what it was because I looked at the test reports and found out it was .1 ohm's with the link removed.
The contractor has ordered a new circuit breaker and has concluded this one is broken. I believe we have parallel paths because the neutral is obviously bonded more than once which the circuit breaker would see the current twice.
Is my conclusion correct that if the neutral was bonded more than once we would have parallel paths and the current would be seen twice?
Thoughts?
The contractor has ordered a new circuit breaker and has concluded this one is broken. I believe we have parallel paths because the neutral is obviously bonded more than once which the circuit breaker would see the current twice.
Is my conclusion correct that if the neutral was bonded more than once we would have parallel paths and the current would be seen twice?
Thoughts?






RE: Ground Fault mis-operation
The no trip test circulates current through two phases of the breaker or through one phase and the neutral. Circulating current of the proper polarity (In "A", Out "B" or In "A" Out "N") should not trip at high currents. It simulates a large load.
If the neutral CT is wired backwards or the test current polarity is wrong, the signals add instead of cancel and cause a trip at 50% of setpoint.
If the test current was run through the breaker, and not the neutral, an internal phase CT may be backwards.
RE: Ground Fault mis-operation
So my thoughts about a multi-bonded system are not accurate?
RE: Ground Fault mis-operation
Test set output is low voltage, high current and is usually ungrounded and isolated. Test set current only flows between the two test lead connections. There is no parallel path for current flow with the isolated test set output. If the test connections did have a parallel path, the tripping point would be higher than setpoint due to the shunting of the parallel path.
Your results still don't make sense unless there is a CT or rating plug error. Pushing current through just one breaker phase should cause a GF trip right at setpoint, not 50%.
RE: Ground Fault mis-operation
I agree completly with all your comments! Thanks again - I have been scratching my head pretty hard on this one and all I could come up was multiple N-G bonds recirculating.
RE: Ground Fault mis-operation