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Onan 7.5JB Separating T2 From Ground


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  #21  
Old 10-09-2018, 01:07:08 PM
Vanman Vanman is offline
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Default Re: Onan 7.5JB Separating T2 From Ground

If the neutral conductor between Leon's generator and house were to fail open, the generator chassis would become energized, assuming no other low impedance paths exist (like a metal gas pipe).

This is identical to the old code that allowed ranges and dryers to use the ground as the neutral. It *almost* never created a problem.

Truth be told, it isn't something I would worry about either. It is not technically correct. Though it may have been at the time of installation.
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Old 10-09-2018, 01:11:42 PM
len k len k is online now
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Default Re: Onan 7.5JB Separating T2 From Ground

Simple fact is today's code says new installations can only have ONE neutral/ground bond, if you want install to be legal. Not being legal has potential non-payment insurance problems.

Don't know if insurance will refuse to pay if gen non-code issue has nothing to do with your claim. But just like health care insurance used to be when BIG bills came insurance companies started looking for loop holes to not pay. They used to refuse to pay big bills like cancer using excuse you didn't disclose you had colds, your tonsil's or wisdom teeth removed as a kid, and policy application required you to disclose ALL previous issues. Not reasonable ,but that's what they did.

Didn't change until gov said on medical insurance once they issued policy they are stuck with you, ( gov said they should have done the research BEFORE they accepted your money). Have not heard anything like that for building insurance.
.

Last edited by len k; 10-09-2018 at 01:48:46 PM.
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Old 10-09-2018, 01:12:31 PM
Bill Aggenbach Bill Aggenbach is offline
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Default Re: Onan 7.5JB Separating T2 From Ground

I believe that the NEC provides a way to leave the neutral and ground bonded at the generator. This would involve using a transfer switch that also opens the neutral the same time as the two hots. This is a more expensive transfer switch and is not something that a lot of the common Breaker interlock panels or Gen Tran Systems Provide. By switch the neutral when you are on Utility power your electrical system does not see the Bond at the generator. When you switch the Transfer Switch to Generator the then you have disconnected the utility neutral and you are acting like your own utility. I believe that unlike the utility where they only have three wires going to your house you are still required to run a ground wire between transfer switch and generator.
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  #24  
Old 10-09-2018, 01:40:21 PM
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Jim McIntyre Jim McIntyre is offline
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Default Re: Onan 7.5JB Separating T2 From Ground

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Originally Posted by Leon N. View Post
I would like to hear how a single fault is going to make my generator chassis hot wrt earth ground.
Vanman nailed it. I can draw a picture if you still don't understand...
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Old 10-09-2018, 02:19:35 PM
Leon N. Leon N. is offline
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Default Re: Onan 7.5JB Separating T2 From Ground

In my situation a fourth ground wire is not going to add anything WRT safety from electric shock.

You see, my T2 neutral is connected solidly not only at the gen chassis but solidly to the utility neutral at the point where it is also connected to an earth ground. So how does adding a fourth wire create a ground loop? In my case a fourth wire is redundant.

One way a ground loop is created is if two earth grounds are employed. Just adding a fourth wire between two electrically equivalent points is not going to create a ground loop.
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Old 10-09-2018, 03:24:56 PM
Zephyr7 Zephyr7 is offline
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Default Re: Onan 7.5JB Separating T2 From Ground

Quote:
Originally Posted by Bill Aggenbach View Post
I believe that the NEC provides a way to leave the neutral and ground bonded at the generator. This would involve using a transfer switch that also opens the neutral the same time as the two hots. This is a more expensive transfer switch and is not something that a lot of the common Breaker interlock panels or Gen Tran Systems Provide.
Iíve never seen a residential installation where the neutral goes over with the transfer to generator. Setups with the neutral being transferred ARE common in commericial setups and I spec them frequently. The usual need is for services over 1,000 amps on 480v systems where ground fault protection is required at the main, and in cases with either multiple mains or multiple service entrance locations and a shared generator.

My read of the code for residential systems is that a conflict exists in the code. The ground/neutral bond is required to be made at the main disconnect location. The main disconnect is required to be the first circuit breaker where the service enters the building. This is why service-rated transfer switches have both a main breaker and a ground/neutral bond.

The conflict is that when the load is transferred to generator, and if the generator has a breaker (which I think pretty much all of them do), then the main disconnect is the breaker within the generator so the ground/neutral bond needs to be made at the generator.

The problems arise in the case of a fault. If there is no ground/neutral bond at the generator, and if a fault occurs shorting any of the phase conductors to ground, the breaker wonít open to clear the fault. Admittedly, in a permanently installed system the chances of that type of fault occurring should be pretty low. For mobile units with camlocks the chances can be high.

In my opinion, keeping a bond at the generator is safer than having it at the transfer switch only. This does, however, technically violate the ďone bond onlyĒ rule in the code. There are many ways to make it work electrically but when you try to think of safety and all the possible fault scenarios and meeting codes things get more complex.

Bill
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Old 10-09-2018, 05:11:48 PM
Leon N. Leon N. is offline
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Default Re: Onan 7.5JB Separating T2 From Ground

If the neutral on either side opens it is a serious problem. Think that’s bad, how about when the utility pole mounted secondary neutral is open and 7.9kv enters the house seeking a ground path.

We have had cases like that around here when a tree takes down the secondaries which includes the 120/240 neutral and not the 7.9 kv primary.

The neutral wire(s) and the connection points are very carefully designed to prevent this.

Thinking about what Vanman said, I do not see how the generator frame becomes energized if the neutral I run from T2 to the house should open. There are no other electrical paths to/from the generator and the stator windings are not shorted to the frame. As I see it the 120 volt loads are now in series with the 120 volt loads from the other stator winding.

There have been cases as I mentioned earlier where by the utility hi line voltage energizing the pole mounted transformer primary was completed through the secondary neutral to ground instead of the 3-phase neutral due to down. wires.

Last edited by Leon N.; 10-09-2018 at 05:36:25 PM.
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Old 10-09-2018, 07:00:44 PM
Zephyr7 Zephyr7 is offline
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Default Re: Onan 7.5JB Separating T2 From Ground

Quote:
Originally Posted by Leon N. View Post
If the neutral on either side opens it is a serious problem. Think thatís bad, how about when the utility pole mounted secondary neutral is open and 7.9kv enters the house seeking a ground path.

Snip 8<

There have been cases as I mentioned earlier where by the utility hi line voltage energizing the pole mounted transformer primary was completed through the secondary neutral to ground instead of the 3-phase neutral due to down. wires.
If the primary neutral opens, the return path for the HV should be through the ground. Typical utility construction has the neutral grounded at every pole, and every comm (communication cable) attachment bonded to that ground. The ground is generally a #6 solid copper wire running down the side of the pole and coiled or stapled to the bottom of the pole underground.

I could see a very bad situation if the primary neutral opened and the HV completed its circuit through the ground at some house. I know I see shared current on my ground at home since all the grounds in the houses are paralleled with the utilities ground.

The only way I can see your generators frame becoming energized is if something comes loose inside. Iíve seen this happen when lugs vibrate loose and the wires pop out (surprisingly common and why I prefer compression lugs in generator housings) and Iíve see debris abrade the enamel insulation on windings (very rare).

Bill
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Old 10-09-2018, 07:33:02 PM
Leon N. Leon N. is offline
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Default Re: Onan 7.5JB Separating T2 From Ground

Bill

I agree with you. The only thing that would happen is the 120 volt circuits would be exposed to either higher or lower voltage depending upon the load resistance.

The T2 ground in my JB is not about to come loose. I am very confident in my electrical wiring. Very slim chance it will ever come loose. My JB is in a very benign environment.

You are correct about the #6 AWG running down the pole. Best I can tell it is from the primary side lightning arrester. I cannot see if it is attached to the secondary neutral. When I get a chance I plan to look up at some other pole mounted secondary step down transformers to verify.
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Old 10-09-2018, 08:18:12 PM
Zephyr7 Zephyr7 is offline
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Default Re: Onan 7.5JB Separating T2 From Ground

Quote:
Originally Posted by Leon N. View Post
The T2 ground in my JB is not about to come loose. I am very confident in my electrical wiring. Very slim chance it will ever come loose. My JB is in a very benign environment.

You are correct about the #6 AWG running down the pole. Best I can tell it is from the primary side lightning arrester. I cannot see if it is attached to the secondary neutral. When I get a chance I plan to look up at some other pole mounted secondary step down transformers to verify.
I’ll bet the T2 lead, and the others, probably have ring terminals crimped on too a ring terminal on a stud with a lockwasher is pretty secure.

The #6 ground wire on the pole should be attached to the neutral. Usually there is a small jumper off of it close to the attachment point on the pole. I can’t get more specific because I don’t know the type of pole construction you are looking at. The utilities don’t distinguish between ground and neutral on the primary, they are the same — only one wire. If the ground is above the phases, such as on a transmission tower (and there is no insulator for that wire), then it’s known as a “static shield”. All the lightning arrestors connect between phase and ground. Transformers run between phase and phase, or between phase and ground. If you do find the ground bond is missing on your service pole, report it — that’s a safety issue and your utility will want to know so that they can come fix it.

At my work, if we don’t bond the steel support strand for our fiber optic cables to the pole ground at every pole they can fine us or just remove all the cable we installed from their poles. They actually take grounding very seriously.

Utilities are different from in-building wiring covered by the electric code. Utility standards, in the US anyway, are mostly covered by the NERC and REA standards. Utilities use a lot of auto transformers too, which are practically prohibited from use in indoor systems. The autotransformer is common for residential services and will have one HV insulator (a bushing, really), and then usually three output connections for the low voltage. The “neutral” on the LV side is also the ground for the HV. If you are fed from 7.2kv, that 7.2kv line is likely one of the three phases of a 12,470v wye on the primary. That’s a standard primary voltage. Others are 4.8/8.3Y and 7.9/13.8Y. In my area they use 7.6/13.2Y, which is much less common nationally than those other two voltages. They also have some legacy ungrounded delta 4.8kv stuff which can’t use an auto transformer so the transformers all have two bushings on top (for single phase transformers).

I’ve probably been zapped a few too many times so I like this stuff

Bill
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