Ground Fault Protection Nuisance Trip

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Nic Tesla

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There is a 2000A GE PBII breaker tripping for Ground Fault Protection (nuisance tripping) everytime a 400HP is started via Allen Bradley SMC soft start. GF protection is set at 1200A longest time delay. If the rating plug is removed the motor can start and when putting plug back the breaker does not trip.
Does anyone experience something like that before?.............please advise
 
The first think that I would look at is the neutral current sensor. You didn't say anything about the distribution system so attempting to provide an answer is an educated guess.
The following is one thing that I would eliminate first.
I would guess is that it's a 480Y/277. The second thing that I would guess is that you do have some 277v 1ph loads which are line to neutral loads.
Since you did say that the GF sensitivity was set at the highest, 1200a, and that the time delay was set at its longest, I would find it interesting to see the breaker tripping on ground fault.
I am to assume that you are sure that the breaker is in fact tripping on ground fault because your are getting a GF trip indication on the breaker.
The first thing that I would zero in on is in the neutral sensor (again, I'm assuming that this is a 480Y/277 system). Again, assuming that the neutral sensor (CT) is a separate device and that there are wires running from it to the breaker, get the instructions to the breaker and assure that it is wired correctly to the breaker. Check the wiring carefully.
It is not uncommon for a neutral sensor to not be installed correctly. If the neutral sensor is not sending neutral current information to the breaker the neutral the breaker will assume that any neutral current is GF current. However, it will not trip because the neutral current is not high enough and may never cause neusance tripping.
There may be something which is occurring with the soft start that may add enough to the neutral current already present to trip the breaker on ground fault.
 
There are some cases of harmonics from drives or soft starts causing the ground fault trip to activate.
Don
 
.Does this circuit breaker have separate GFP (large window type CT) or is the GFP residual with the phase CT's built in the CB (or possible separate) and a separate neutral CT (If a neutral is utilized).

I am assuming there is a GFP indicator on the CB or is this a separate relay?
(I did not do a web search for the CB).

This list is not prioritized just wrote as it came to me.

1. Test the GFP relay or the CB. If separate CT's during the test test each CT.
2. Even though the motor has been on line I would megger the motor circuit (soft start disconnected).
3. What is the start time?
4. Do you have a large enough clamp-on with min-max function to encompass all conductors to the motor during start up. If so measure current during start up, it should be "0" or close to "0" amps.
5. Have you contacted the manufacture of the soft start?
6. Measure current at the neutral ground bond, before during and after start up.

For starters.

Did a search and this CB has a Micro Versa trip unit, I would primary and secondary test the device. In addition (assuming no neutral) there may be a connection for a neutral CT if this is not properly configured it may result in a trip (I do not have access to the data at home but will check Monday morning)
 
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The reason that I addressed the GF trip issue as I did is that if the GF sensor is not installed correctly the breaker only compares the line conductors for an imbalance and assumes that unbalance current that flow through the neutral are in fact ground fault current. However, the unbalance current may not be great enough to exceed the 1000-1200a GF pick-up of the breaker.
Now add harmonic currents that flow through the neutral on top of that and it may raise the neutral current above the GF trip setting.
I didn't run into this type of problem exactly but I did run into a problem similar to this at a new warehouse type food store where the switchboard builder wired the neutral CT incorrectly by the switchboard builder. With the right combination a switching the 277v lighting loads the breaker would trip of ground fault.
It baffled the electrical contractor for weeks that assumed that the breaker was defective claiming that it was a warranty problem.
Simply speaking don't overload the obvious. A breaker with the GF pick-up set at 1000a is either seeing a 1000a GF, being tricked into seeing it or is defective.
For those that don't understand the theory of GF sensing the simplest thing to do is to conclude that the breaker is defective. They replace the breaker and discover that they still have the same problem. Now they have spent a great deal of money that they have to explain away.
Assuming that the GF has been installed correctly and is in fact functioning one must direct their attention to the motor. An internal L-G arcing fault may be the issue.
 
Some GE Micro Versa Trip units can have false GF trips on high inrush currents, especially if harmonics are involved. It usually occurs if the trip unit is wired for a 3-phase, 3-wire system, like it would be for a motor feeder with no neutral wire. The trip unit still has a neutral CT input on the circuit board, even though there is no connection to it. The open input picks up noise during high inrush currents and fools the GF circuit into tripping.

In our case, GE field service tested the units and declared they were good, twice. We finally brought in 50kA test sets from a NETA company and cranked high phase-phase current through the breakers proving the trip units would false trip on high currents. We found the problem on multiple breakers, vintage 2002.

More likely, your problem is related to the harmonics. A change to a fancier trip unit may be needed to get a true RMS sensing device. Or defeat the GF on the breaker, add a shunt trip, donut CT and an external GF relay.
 
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