Current on EGC

[HotConductor]

Last week I was called in to help another ec troubleshoot an intermittent main breaker tripping issue. He already had the store on a portable genny with three hot cables, one neutral and one ground(120/208/3 ph). There was an mdp with an 800 amp main and eleven breakers feeding panel boards and rooftop equipment. I found between 5-10 amps of current on each egc from the panel boards back to the mdp.

After the store was put back on utility power, the ground currents went back close to 0. Does this have something to do with the genny frame being bonded to the neutral?

 

[hurk27]

Last week I was called in to help another ec troubleshoot an intermittent main breaker tripping issue. He already had the store on a portable genny with three hot cables, one neutral and one ground(120/208/3 ph). There was an mdp with an 800 amp main and eleven breakers feeding panel boards and rooftop equipment. I found between 5-10 amps of current on each egc from the panel boards back to the mdp.

After the store was put back on utility power, the ground currents went back close to 0. Does this have something to do with the genny frame being bonded to the neutral?

Yes.
Most likely you had parallel neutral currents on the grounding.

 

[Pierre C Belarge]

It probably would not be a bad idea to check the panels/disconnects for neutral/ground bonds.

 

[hurk27]

It probably would not be a bad idea to check the panels/disconnects for neutral/ground bonds.

it sounds like they tied the generator feeds to the neutral bar and generator grounding conductor to the panel grounding bar without taking off the grounds or neutral from the MDP, this would have placed the neutral in parallel with the grounding since they were bonded together at the MDP and the generator, after removing the generator all was fine.

 

[Smart $]

it sounds like they tied the generator feeds to the neutral bar and generator grounding conductor to the panel grounding bar without taking off the grounds or neutral from the MDP, this would have placed the neutral in parallel with the grounding since they were bonded together at the MDP and the generator, after removing the generator all was fine.

I can see that causing current through the grounding conductor to the gennie... but how would this cause current on the grounding conductors between MDP and subpanels if there was no intentional EGC to neutral bonding downstream....???

 

[hurk27]

I can see that causing current through the grounding conductor to the gennie... but how would this cause current on the grounding conductors between MDP and subpanels if there was no intentional EGC to neutral bonding downstream....???

Well when I read the OP, I see he said that they had a problem with a tripping main, but as I read on he made the point that he also had HVAC equipment still powered from 11 panels that are supplied from a MDP with an 800 amp main, so this told me the first main wasn't a main but a feeder breaker in the MDP maybe feeding a panel for the store, so I presumed that the first electricians hook up the portable generator to this sub panel for the store, without unhooking the feeder neutral which would have opened the parallel path.
Now since you have current on your grounding trying to reach back to the MBJ in the MDP, it will take all paths to this point, including each and every other connection in the system, this includes a common gas pipe the might also be connected to the roof top units and something in the store.

Lets say there is a gas water heater in the store that has 120 volts for a exhaust blower, now this EGC has a path to the gas pipe, now the roof top units have gas pipes to them, so each rooftop unit will have current flowing in its EGC back through its sub panel to the MDP.

 

[Pierre C Belarge]

it sounds like they tied the generator feeds to the neutral bar and generator grounding conductor to the panel grounding bar without taking off the grounds or neutral from the MDP, this would have placed the neutral in parallel with the grounding since they were bonded together at the MDP and the generator, after removing the generator all was fine.

Wayne
My comment was in regards to this sentence in his OP.

"After the store was put back on utility power, the ground currents went back close to 0."

Without knowing what "went back close to 0." actually means, it would not be a bad idea to check for neutral bonding. I see neutral bonding occuring more than I care to say.

 

[HotConductor]

There were no obvious neutral/ground bonds in any of the panel boards. The first ec lugged the genny cables directly to the main buss bars in the mdp. The "0" readings were taken at each panel board on each egc running with the hots and neutral.

Thanks for all the responses.

 

[Smart $]

Well when I read the OP, I see he said that they had a problem with a tripping main, but as I read on he made the point that he also had HVAC equipment still powered from 11 panels that are supplied from a MDP with an 800 amp main, so this told me the first main wasn't a main but a feeder breaker in the MDP maybe feeding a panel for the store, so I presumed that the first electricians hook up the portable generator to this sub panel for the store, without unhooking the feeder neutral which would have opened the parallel path.
Now since you have current on your grounding trying to reach back to the MBJ in the MDP, it will take all paths to this point, including each and every other connection in the system, this includes a common gas pipe the might also be connected to the roof top units and something in the store.

Lets say there is a gas water heater in the store that has 120 volts for a exhaust blower, now this EGC has a path to the gas pipe, now the roof top units have gas pipes to them, so each rooftop unit will have current flowing in its EGC back through its sub panel to the MDP.

I understand multiple paths... However, in order for there to be neutral current on grounding conductors, there has to be a continuity between the neutral and EGC at no less than two points. We assume two for certain: one at the gennie and one at the main. If the gennie is connected at the main (confirmed by OP'er in his post after yours), the only assumed parallel path is on the EGC of the gennie because of contuinuity points through GES bonding at the main, and EGC-neutral bonding at the gennie. All other paths should be "circular" paths, i.e. the path starts and ends at the same location, and thus no current... so something out of the ordinary occurred on this system.

Now if there was an electrode connected to the gennie, that would change things...

 

[hurk27]

Wayne
My comment was in regards to this sentence in his OP.

"After the store was put back on utility power, the ground currents went back close to 0."

Without knowing what "went back close to 0." actually means, it would not be a bad idea to check for neutral bonding. I see neutral bonding occuring more than I care to say.

I agree as I have too, but the OP wasn't clear on where the generator was tied into the system at, and the paths I mapped out in my head was all I could think of that made any sense, now after his last post nothing makes sense?

I understand multiple paths... However, in order for there to be neutral current on grounding conductors, there has to be a continuity between the neutral and EGC at no less than two points. We assume two for certain: one at the gennie and one at the main. If the gennie is connected at the main (confirmed by OP'er in his post after yours), the only assumed parallel path is on the EGC of the gennie because of contuinuity points through GES bonding at the main, and EGC-neutral bonding at the gennie. All other paths should be "circular" paths, i.e. the path starts and ends at the same location, and thus no current... so something out of the ordinary occurred on this system.

Now if there was an electrode connected to the gennie, that would change things...

I was trying to map the path if the generator was tapped in at a sub panel, but as we see it wasn't so like you said, current on the grounding doesn't make sense to me if the generator was at the MDP?

Ground rod? wouldn't think that much parallel current could flow? and still shouldn't put current on the sub panel grounding the path is not there?

 

[Smart $]

I agree as I have too, but the OP wasn't clear on where the generator was tied into the system at, and the paths I mapped out in my head was all I could think of that made any sense, now after his last post nothing makes sense?



I was trying to map the path if the generator was tapped in at a sub panel, but as we see it wasn't so like you said, current on the grounding doesn't make sense to me if the generator was at the MDP?

Ground rod? wouldn't think that much parallel current could flow? and still shouldn't put current on the sub panel grounding the path is not there?

All we can do is speculate...