CURRENT GOING DOWN GROUND ROD HELP!!!!

[5 S electric]

We recently installed a new 225 amp 3 phase 208/120 service to an existing building. It also has a back up generator and transfer switch. This building is on a college campus. It also has another 400 amp 3 phase 208/120 service that was existing. At the service disconnect the neutral coming from the building has 11 amps on it. This is the only place we bond the neutral to the GEC. Leaving the service disconnect going to the transformer the neutral has 7 amps on it and the GEC has the remaining 3 amps going down it. I would appreciate anybody who has an idea why all of the current isn't following the neutral back to the transformer. The back up generator has a ct monitoring the grounding conductor. The generator trips and shuts off saying it has a ground fault. We didn't start noticing this till they added more server racks on this service. We have 2 separate transformers feeding the building. The 400 amp transformer has 4 amps going down the GEC. We opened all sub panels and they are not bonded, but it is bonded in the gutter box that the service entrance conductors enter the building. (There is no MDP for the 400 amp service just a gutter box with 5 disconnects. They are just tapping off of the 500 mcm feeding the disconnects.) The service neutral just has a ground wire kernyed to it i guess that is how they bonded this service. I know it sounds like a mess and it is a mess. Back in the day that was pretty popular around here. Instead of putting an MDP they just tapped and put disconnects.

Any help would be appreciated!

Thanks,
Scott

 

[cowboyjwc]

Well try removing the ground wire where it's tied to the neutral and see what that does.

I just kind of breezed through what you wrote so I may have missed something.

 

[Smart $]

Are the transformers' primary ground or grounded conductor bonded to the secondary neutrals? If so, you have a parallel path for neutral current of both systems. I'm thinking this is true for the generator even if the transfer switches the neutral.

 

[templdl]

Could it be that the current is originating from an outside source and seaking your neutral via your ground rod?

 

[Joe Villani]

I think you may have multiple seperate issues here.

I think the priority issue is the generator tripping.

Based on the original post the generator is equipped with ground fault protection. Correct?

How is the transfer switch configured? 3 pole or 4 pole switch?

Thanks

Joe

 

[macmikeman]

Could it be that the current is originating from an outside source and seaking your neutral via your ground rod?

Bravo, I was thinking the same thing, but there is no thank's button to hit so I will use this-

 

[5 S electric]

Are the transformers' primary ground or grounded conductor bonded to the secondary neutrals? If so, you have a parallel path for neutral current of both systems. I'm thinking this is true for the generator even if the transfer switches the neutral.

Yes the transformers are bonded to neutral and we are sharing the same GEC with the transformer and service disconnect. So we are bonded in two places. But i don't think that i can break the bond at the service disconnect wouldn't that be a code violation? The transformer has a factory bond built on the transformer. Primary is 12470 volt. The transfer switch is only a 3 pole. The neutral doesn't get switch.

 

[Smart $]

Yes the transformers are bonded to neutral and we are sharing the same GEC with the transformer and service disconnect. So we are bonded in two places. But i don't think that i can break the bond at the service disconnect wouldn't that be a code violation? The transformer has a factory bond built on the transformer. Primary is 12470 volt. The transfer switch is only a 3 pole. The neutral doesn't get switch.

Well the question was whether the primary ground is connected to the secondary neutral (?)... but 12470V supply should be line to line (but there is a chance it is a 12470/7200V wye primary). Per your description of secondary grounding, the GES is a parallel path for neutral current...

Are these truly service transformers or separately derived systems? The grounding requirements are slightly different and have varied over the years... so what Code cycle were these installed under?

If the neutral is not switched, there should be no generator neutral to ground bond. But that would defeat generator ground fault detection... assuming it is on the generator side of the transfer switch. As such, the neutral should be switched.

 

[kwired]

I would appreciate anybody who has an idea why all of the current isn't following the neutral back to the transformer. The back up generator has a ct monitoring the grounding conductor. The generator trips and shuts off saying it has a ground fault. We didn't start noticing this till they added more server racks on this service. We have 2 separate transformers feeding the building. The 400 amp transformer has 4 amps going down the GEC. We opened all sub panels and they are not bonded, but it is bonded in the gutter box that the service entrance conductors enter the building. (There is no MDP for the 400 amp service just a gutter box with 5 disconnects. They are just tapping off of the 500 mcm feeding the disconnects.) The service neutral just has a ground wire kernyed to it i guess that is how they bonded this service. I know it sounds like a mess and it is a mess. Back in the day that was pretty popular around here. Instead of putting an MDP they just tapped and put disconnects.

Any help would be appreciated!

Thanks,
Scott

You have neutral current flowing in all available paths - is reason why it is not all flowing in your neutral conductor.

If the CT monitor for the generator equipment is on the supply side of the main/system bonding jumper, it will only read what current is passing through the conductor it is monitoring, you need this CT on the load side of the bonding jumper, or eliminate what ever bond that may be there that shouldn't be so this monitors what you want it to monitor.


Up to six disconnecting means would have been acceptable on the original service. Without more details I do question whether it is NEC compliant to add your second service, but if it is compliant it can have up to six disconnecting means as well plus additional allowed exceptions like say a fire pump. Usually though the second service needs to have different characteristics then the first one or be larger capacity then typically supplied by the POCO. - for three phase services you often are looking at least 2000 amps or more for that to happen.

 

[cowboyjwc]

Well the generator should have it's own ground rod as it's a separately derived system.

Also, as has happened here, the utility it bonding their neutrals now and you could just be getting it from them as someone else mentioned.

 

[Smart $]

Well the generator should have it's own ground rod as it's a separately derived system.

...

If the neutral is not switched, it is not a separately derived system. But with separate GFP, the neutral should be switched... and then it would be an SDS.

Any grounding electrode considerations hinge a little bit on what Code cycle the installation is under.

 

[jtinge]

Are these POCO transformers or customer owned transformers?


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[5 S electric]

Well the question was whether the primary ground is connected to the secondary neutral (?)... but 12470V supply should be line to line (but there is a chance it is a 12470/7200V wye primary). Per your description of secondary grounding, the GES is a parallel path for neutral current...

Are these truly service transformers or separately derived systems? The grounding requirements are slightly different and have varied over the years... so what Code cycle were these installed under?

If the neutral is not switched, there should be no generator neutral to ground bond. But that would defeat generator ground fault detection... assuming it is on the generator side of the transfer switch. As such, the neutral should be switched.

Just talked to our electric coop. Yes the primary ground is bonded to the neutral. They are service transformers. There is no bond at the generator. My question is can I safely remove the bond at my service disconnect while energized without getting hurt or hurting the equipment on this load?

 

[5 S electric]

Are these POCO transformers or customer owned transformers?


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They use to be POCO owned but we took over all high volt about 4 years ago. So now they are customer owned. We sub the POCO to do all of the high volt work.

 

[5 S electric]

You have neutral current flowing in all available paths - is reason why it is not all flowing in your neutral conductor.

If the CT monitor for the generator equipment is on the supply side of the main/system bonding jumper, it will only read what current is passing through the conductor it is monitoring, you need this CT on the load side of the bonding jumper, or eliminate what ever bond that may be there that shouldn't be so this monitors what you want it to monitor.


Up to six disconnecting means would have been acceptable on the original service. Without more details I do question whether it is NEC compliant to add your second service, but if it is compliant it can have up to six disconnecting means as well plus additional allowed exceptions like say a fire pump. Usually though the second service needs to have different characteristics then the first one or be larger capacity then typically supplied by the POCO. - for three phase services you often are looking at least 2000 amps or more for that to happen.

There is no bonding jumper at the generator where this CT is. Cant bond it here because the neutral is not switched. Yeah about the six disconnects i agree, just a poor way of doing things. I cant isolate the service wires to make any changes. I really don't like this method of wiring.

 

[5 S electric]

If the neutral is not switched, it is not a separately derived system. But with separate GFP, the neutral should be switched... and then it would be an SDS.

Any grounding electrode considerations hinge a little bit on what Code cycle the installation is under.

Yeah I wished they would have let me order the generator and transfer switch. The customers purchased these on there own and now we have to deal with what they ordered.

 

[jtinge]

Since transformer ownership changed 4yrs ago, it's not clear to me where your service point is now.

If it switched to the supply side of the transformers, your new equipment installation may not be a service.

Grounding and bonding requirements for feeders and SDS for whatever your current code cycle is would be more appropriate than those for a service.

We don't let anyone do work on energized equipment (other than troubleshooting, voltage testing or non-contact inspections with appropriate PPE) If if the work requires use of a tool on energized conductors or circuit parts, we shut it down first.



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[5 S electric]

Since transformer ownership changed 4yrs ago, it's not clear to me where your service point is now.

If it switched to the supply side of the transformers, your new equipment installation may not be a service.

Grounding and bonding requirements for feeders and SDS for whatever your current code cycle is would be more appropriate than those for a service.

We don't let anyone do work on energized equipment (other than troubleshooting, voltage testing or non-contact inspections with appropriate PPE) If if the work requires use of a tool on energized conductors or circuit parts, we shut it down first.



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I understand just hard to shut this building down servers run a majority of campus. But they might have to take the outage if they want it fixed.

 

[jtinge]

We have the same battle but we use an outage coordinator to schedule required outages during periods of least impact to the facility. Still non urgent activities requiring an outage can take a while to schedule.


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[Smart $]

Just talked to our electric coop. Yes the primary ground is bonded to the neutral. They are service transformers. There is no bond at the generator. My question is can I safely remove the bond at my service disconnect while energized without getting hurt or hurting the equipment on this load?

You cannot remove the bonding jumper at the service disconnects. You can remove the building GEC connection to neutral at the transformers' secondary... but you will have to install a grounding electrode (usually a single rod nearby) connected to the transformers' neutral, or enclosure if neutral to enclosure bonding jumper is of adequate size under NEC... with only one rod, it should be, as only #6 is required.

Shouldn't present any problem with proper PPE, but I am not suggesting you do this energized without appropriate substantiation. With everything prepped to the max, it'd probably take less than five minutes for each transformer, to do the actual lifting and isolating the existing GEC, and landing the new GEC. And then there is the possibility they already have the local electrode and GEC... you may just have to lift and isolate the building GEC.

You may want to take care of the generator problem first, so you can shut down the service. However, I fail to see how CT monitoring the grounding conductor at the generator has any effect on GFP if the neutral is not bonded to ground. Are there any grounding electrodes at the transformer? If so, what are they connected to, enclosure or neutral?

Does the standby system include a UPS backup that can withstand say fifteen minutes without power? Regardless, you may want to consider connecting a recording monitor to the generator, including the grounding conductor, so you can review what happens when you shutdown the service.