# vehicle mounted generator bonding and grounding



## SparkyInCanadaEh (Mar 7, 2008)

Maybe some of you have come across this before. I am to install a 85kw 3 phase 120/208 generator (diesel powered) on a truck (24 ft straight truck). I will be installing a loadcenter 100A 3 phase 4 wire for distribution. If the generator neutral is internally bonded to the frame of the generator than there should be no neutral bond at the loadcenter. Here is the question: Is this a sufficient ground? The way I look at it is: this "ground" is not "really" a ground in the normal sense. My concern would be if the gennie is running and there was a short or poor neutral connection than there is an opprortunity for current to flow to a person touching the truck and the earth at the same time. And grounding the frame of the genereator to the frame of the truck could further complicate this? Sorry if it's confusing, I'm struggling with getting this out.


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## RePhase277 (Feb 5, 2008)

SparkyInCanadaEh said:


> Maybe some of you have come across this before. I am to install a 85kw 3 phase 120/208 generator (diesel powered) on a truck (24 ft straight truck). I will be installing a loadcenter 100A 3 phase 4 wire for distribution. If the generator neutral is internally bonded to the frame of the generator than there should be no neutral bond at the loadcenter. Here is the question: Is this a sufficient ground? The way I look at it is: this "ground" is not "really" a ground in the normal sense. My concern would be if the gennie is running and there was a short or poor neutral connection than there is an opprortunity for current to flow to a person touching the truck and the earth at the same time. And grounding the frame of the genereator to the frame of the truck could further complicate this? Sorry if it's confusing, I'm struggling with getting this out.


You are confused about the purpose of the Earth ground. Current cannot flow to the earth unless there is a connection to the earth at some point. If the generator is mounted to the frame of a truck and the truck is isolated from the ground on rubber tires, then there is no circuit path for "ground" current.

Run five wires to the load center and keep the neutrals and grounds separated. Solidly bond the generator frame to the truck frame.

InPhase277


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## SparkyInCanadaEh (Mar 7, 2008)

not confused. If there was an individual touching the truck ie. to get on or off, current could flow thru the person to earth if the truck frame became energized due to a fault in the system.


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## RePhase277 (Feb 5, 2008)

SparkyInCanadaEh said:


> not confused. If there was an individual touching the truck ie. to get on or off, current could flow thru the person to earth if the truck frame became energized due to a fault in the system.


No it would not. Electricity flows in complete circuits only. The neutral of the truck would have to be connected via a grounding electrode to the Earth before a current would flow from hot to Earth. If the truck is sitting up on tires, insulated from the ground, there is no reason a current would flow. There is no complete circuit. 

Think about it like this: If you removed the Earth ground at your house and at the utility transformer, so that there was no Earth connection whatsoever in your home's electrical system, what would happen if you touched a hot wire while standing bare footed on moist Earth?

InPhase277


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## SparkyInCanadaEh (Mar 7, 2008)

What I am saying is that if the truck frame becomes energized from the generator than there is no path for current to flow because of the rubber tires... although if you introduced a ground, from the truck to earth, via the persons conductivity, there would be a potential difference.


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## RePhase277 (Feb 5, 2008)

SparkyInCanadaEh said:


> What I am saying is that if the truck frame becomes energized from the generator than there is no path for current to flow because of the rubber tires... although if you introduced a ground, from the truck to earth, via the persons conductivity, there would be a potential difference.


No. Let's look at the path. Energized truck frame---> person---> Earth---> back to where? How does the current get from the person to the Earth back to the truck to complete the circuit? The Earth is not naturally going to draw current.

You are missing the point of the Earth ground. In a normal system, the Earth ground is not a point of current return. It serves no purpose for the normal operation of the electrical system. We have a "ground" wire that we run with our circuits, but this ground is connected to the neutral at the service. This allows for a path that fault current can return to the service neutral on and trip a circuit breaker. The Earth has nothing to do with it.

Same for your generator. If you bond the generator frame to the frame of the truck, the truck cannot become energized because the hot wire that touches the truck will instantly operate the circuit breaker because it has a fault path back to the generator neutral.

*Answer this question: Drive a rod into the Earth. Take a generator and insulate it from the ground, by say setting it on a dry wooden pallet. With the generator running, what would happen if you touch a hot wire from it to the rod driven in Earth?*

InPhase277


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## goose134 (Nov 12, 2007)

> *Answer this question: Drive a rod into the Earth. Take a generator and insulate it from the ground, by say setting it on a dry wooden pallet. With the generator running, what would happen if you touch a hot wire from it to the rod driven in Earth?*


Ok, You first.


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## jrclen (Oct 23, 2007)

goose134 said:


> Ok, You first.


Classic post. :laughing:


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## RePhase277 (Feb 5, 2008)

goose134 said:


> Ok, You first.


:thumbup: I have done it! I demonstrated this to my crew, several of which had the same misconception about the Earth and grounding.

But I want the OP to answer this with what he thinks will happen. We know how poorly grounding is misunderstood by even veteran electricians. There is no shame in it. It is apparently a complicated subject. I want to help this fellow out.

InPhase277


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## Greg (Aug 1, 2007)

> *Answer this question: Drive a rod into the Earth. Take a generator and insulate it from the ground, by say setting it on a dry wooden pallet. With the generator running, what would happen if you touch a hot wire from it to the rod driven in Earth?*


I'd say nothing would happen because the generator is isolated just as if it was mounted on the truck with rubber tires.


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## gilbequick (Oct 6, 2007)

Nothing would happen. At a house, if you didn't have a neutral from the POCO and only the driven ground rods, you can touch the GEC directly to the lug on the breaker and it won't trip. The only ground in town is the one the POCO brings to your service via their neutral (grounded) conductor.


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## jrclen (Oct 23, 2007)

I agree, nothing would happen.

There is no restaurant in the ground (earth) that the electrons wish to go to. They want to return to the source and will use the ground to get there.
(one of my early instructors explaining this)


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## goose134 (Nov 12, 2007)

I agree that nothing would happen. I just saw a good opportunity for a funny.

Electron restaurant in the ground?


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## jrclen (Oct 23, 2007)

goose134 said:


> I agree that nothing would happen. I just saw a good opportunity for a funny.
> 
> Electron restaurant in the ground?


It was his way of explaining to us that electricity doesn't want to go to earth just for the heck of it, like most untrained people seem to think. :thumbsup:


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## Mackie (Mar 16, 2008)

If you didn't see the post in the other forum...

At work, we install water cooled gas powered generators into dedicated fiber optic splicing trucks. The gens are internally bonded but we DO bond the ground to the neu at the load demand center.


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## cdnelectrician (Mar 14, 2008)

SparkyinCanada...What are you feeding with the genny? From my experience you are still required to ground that whole thing to earth each and every time you move it or use it. We have set up a few temp trailer mount diesel generators to feed malls and buildings while we service the HV switchgear and ESA required a ground to the building/station grounding system. However I would check with your inspector about the bond in the panel you are supplying...my thoughts are if it was bonded in the genny as well as the panel then you will get current flowing through the ground as well as neutral. Same as a service...ground at the main switch...everything after is a bond. And the neutral only gets grounded twice....at the utility side, and at the main switch. The whole reason behind grounding the hell out of everything is to avoid any kind of difference in potential.


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## abellm (Oct 4, 2009)

Sparkyincanada/Mackie,

The truck with the mounted Generator is INTERNALLY bonded 'neutral to ground', good.
Now, if the truck / generator frame became energized, this condition should facilitate the opening of the OCP therefore protecting any persons in contact with the truck.
If the load center is the only point of switching (having OCP) then it would make good sense to bond the N/G in the load center even though this has been done internally on the generator. (I did not catch if the gen. had any OCP near the point of N/G bonding)


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