Louisiana Emergency Generators

[superdave02]

We are installing many generators from 56kw to 2meg. We are finding grounding electrode conductors with amperage. The generator conducotrs run to the building and typically connect at a main breaker or main panel. The grounding electrode is at the generator. We have one right now with 6 amps present on the grounding electrode conductor. The generator netrual and ground are bonded and yes the building first disconnect or OCP is bonded. The MQ generators are typically on metal skids. Why the amperage on ground?

 

[Dsg319]

If you have both bonded generator and bonded transfer switch that will cause objectionable parallel current on all grounded/metallic paths between the two.

And in your case it seems to be going even through the dirt via the Grounding electrode.

Need to have either floating gen-sets or float in the main disconnect.

Any oppositions ?

 

[superdave02]

If you have both bonded generator and bonded transfer switch that will cause objectionable parallel current on all grounded/metallic paths between the two.

And in your case it seems to be going even through the dirt via the Grounding electrode.

Need to have either floating gen-sets or float in the main disconnect.

Any oppositions ?

can I call you? you call me? 704-919-7841

 

[don_resqcapt19]

What are you using for a grounding electrode? Are you measuring this current on a GEC to a grounding electrode that you have installed at the generator? What have you connected the GEC at the generator to? Is there a bond between the generator neutral and the GEC at the generator?

 

[hillbilly1]

Generally, on a temp generator like that, the grounding electrode connection is made to the existing service grounding electrode at the service itself. Never seen anyone make their own for a temporary generator where there is an existing electrode.

 

[tom baker]

Your post title mentions emergency generator, are these Art 700 Emergency Generators?
(I ask as with generators, the type is important and that determines building wiring, signage, etc.)
Are they used with any building electrical systems that have ground fault protection?
(GFP almost always requires 4 pole transfer switches)
Perhaps these are generators installed as POCO power is not available....So in a way its an emergency generator

 

[superdave02]

If you have both bonded generator and bonded transfer switch that will cause objectionable parallel current on all grounded/metallic paths between the two.

And in your case it seems to be going even through the dirt via the Grounding electrode.

Need to have either floating gen-sets or float in the main disconnect.

Any oppositions ?

Thanks! It was of course the rod at the generator.

 

[don_resqcapt19]

Thanks! It was of course the rod at the generator.

It would be pretty rare for a driven rod to be able to flow 6 amps into the earth. Yes, it is a parallel path to the neutral between the generator and the building, but with 6 amps, I would be doing a close check on the neutral conductor connections. I would expect the difference of the impeadace between the neutral conductor and that of the driven rod to be substantial...with the earth having and impedance 1000s of times greater than the actual neutral conductor. Even if the difference is only a factor of 100, (and that would be almost impossible unless there is a neutral connection problem) that would indicate a 600 amp load on the generator neutral.

 

[Dsg319]

It would be pretty rare for a driven rod to be able to flow 6 amps into the earth. Yes, it is a parallel path to the neutral between the generator and the building, but with 6 amps, I would be doing a close check on the neutral conductor connections. I would expect the difference of the impeadace between the neutral conductor and that of the driven rod to be substantial...with the earth having and impedance 1000s of times greater than the actual neutral conductor. Even if the difference is only a factor of 100, (and that would be almost impossible unless there is a neutral connection problem) that would indicate a 600 amp load on the generator neutral.

I would second that of checking the neutral connections.

 

[superdave02]

Generally, on a temp generator like that, the grounding electrode connection is made to the existing service grounding electrode at the service itself. Never seen anyone make their own for a temporary generator where there is an existing electrode.

We have installed some using the existing facility ground rod as directed. Other installations have been installed with it's own ground rod at the generator. The ones that have their own ground rod with amperage present I understand why now. We installs these generators temporarily until untily power is on. The QA QC guys and us are unclear on what to do since no transfere switches are unsed and we know the first disconnecting means has is bonded.

 

[hillbilly1]

We have installed some using the existing facility ground rod as directed. Other installations have been installed with it's own ground rod at the generator. The ones that have their own ground rod with amperage present I understand why now. We installs these generators temporarily until untily power is on. The QA QC guys and us are unclear on what to do since no transfere switches are unsed and we know the first disconnecting means has is bonded.

You are better off using the existing grounding electrode, as it usually a much better system than a single or even two ground rods. Most, but not all of our customers have a “Kirk” key system, where utility and generator supplies both cannot energize the buss. The others we lock and tag the main off, and hit the buss past the main. We never pull the service conductors off unless the utility pulls the fuses out of the transformer. Basically you have to treat the generator as a second utility transformer, and don’t worry about the existing bond in the gear. If this was a permanent installation, this would be an entirely different situation.

 

[don_resqcapt19]

We have installed some using the existing facility ground rod as directed. Other installations have been installed with it's own ground rod at the generator. The ones that have their own ground rod with amperage present I understand why now. We installs these generators temporarily until untily power is on. The QA QC guys and us are unclear on what to do since no transfere switches are unsed and we know the first disconnecting means has is bonded.

I don't. The earth path via a ground rod or two should not be flowing that much current even with two neutral to earth bonding points.

 

[ptonsparky]

Are you reducing the neutral size considerably?

 

[dkidd]

There must only be one neutral to ground bond. Do not bond the generator neutral if the normal system has a neutral to ground bond.
The attached graphic (slightly modified) illustrates the problem.
Just imagine a ground rod at the generator. Neutral current will flow through the earth.

 

[kwired]

There must only be one neutral to ground bond. Do not bond the generator neutral if the normal system has a neutral to ground bond.
The attached graphic (slightly modified) illustrates the problem. View attachment 2557792
Just imagine a ground rod at the generator. Neutral current will flow through the earth.

But how likely is one to have low enough resistance at the rod to be able to have 6 amps of current flowing?

Especially if you already have a neutral conductor that is in good working condition? Open that neutral and rely on earth as the return path and it maybe is more likely to have that kind of current.

 

[don_resqcapt19]

There must only be one neutral to ground bond. Do not bond the generator neutral if the normal system has a neutral to ground bond.
The attached graphic (slightly modified) illustrates the problem. View attachment 2557792
Just imagine a ground rod at the generator. Neutral current will flow through the earth.

That is true, but the path via the earth will have an impedance that should be 1000s of times greater than the path via the actual neutral conductor. This parallel circuit is a current divider with the current on the paths being in inverse proportion to the impedance's of the paths. Even if the impedance only differs by a factor of 100, which would be highly unlikely, there would be have to be at least 600 amps of current on the neutral to have 6 amps on the grounding electrode conductor.

It remains my opinion that the neutral conductor is compromised if you have 6 amps of current going to a ground rod at a remote generator.

 

[dkidd]

It would be helpful if OP posts what the current through the neutral is.

 

[Hv&Lv]

It would be helpful if OP posts what the current through the neutral is.

He did in post #1
6 amps

 

[dkidd]

He did in post #1
6 amps

Did not.

"We have one right now with 6 amps present on the grounding electrode conductor."

GEC

 

[synchro]

It could be useful to measure the voltage between the GEC and a screwdriver or other metal object stuck in the ground 50 feet away or so. A significant voltage difference would need to be present to get 6 amps to flow through the earth because of its relatively high resistivity as Don has mentioned.

The only other possibility I can think of is that the ground rod has contacted an underground water pipe or other metal object that's connected to the neutral somewhere. This could provide a parallel path for neutral current and therefore allow a larger current to flow through the ground rod. It's not a likely situation but it's still possible.