A client of ours has been having problems with a main breaker ground fault tripping. I have been doing research on this and other sites and sources to determine causes and troubleshooting techniques. It occurred to me that this circuit breaker is in a outdoor switchboard that was added to accommodate a major expansion of this facility, a middle school, and that it probably backfed the old service. In such a case, and this seems obvious but I'll ask, wouldn't you have to remove the bonding jumper in the old service disconnect? Couldn't this be a potential source of multiple ground fault current paths in the system?
Yes it could be the source of the nuisance trips, but something as major as leaving the old bonding jumper in place would be more likely to cause an immediate trip or inability to reset.
You write as if multiple fault current paths would be a bad thing. It would not be.
The problem would be the possibility for normal neutral return current to take a path intended for fault clearing only.
Two things come to minds:
1. A single load that has enough leakage to ground that under some conditions it draws enough fault current to trip the breaker. (What is the trip setting, and is it adjustable?)
2. A constant fault path that allows some fraction of the neutral current to leave the neutral. But it only trips the breaker when load imbalance raises the neutral current above the critical level.
A key step in troubleshooting would be to know the apparent ground fault current as a function of time.
