New Service and Bonding of Old Service

[hbendillo]

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?

 

[GoldDigger]

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.

 

[hbendillo]

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.

1. The main is a 2500 amp circuit breaker. Yes it is adjustable. Both the time delay and the ground fault pickup setting on the main circuit breaker is set to maximum. I need to do some checking to see exactly what time and amperage those settings represent.
2. The breaker is tripping randomly. Middle of the night and middle of school day, weekends too I think. I guess putting a load measuring instrument on the service for a few weeks may yield some information for us. What would be an example of a constant fault path?

A fault and device coordination study is among some of the suggestions I am going to make.

 

[GoldDigger]

What would be an example of a constant fault path?

The first one that occurs to me would be an inadvertent or legacy ground to neutral bond at the far end of a branch circuit somewhere (grandfathered range circuit, or bootlegged ground for example). Since it is at the end of a long wire path, it might not carry more than a small fraction of the neutral current when a more direct path is provided by the regular neutral. Depends on how the resistances in the wiring network work out.
It might be made worse if there were a slightly high resistance neutral or intermittently high resistance neutral somewhere in that part of the system too.