Portable Generators and Ground Rods

[iwire]

I am trying to understand what a ground rod does to enhance safety for a portable generator.

From what I have learned from many of you at this forum, a ground rod installed at a service will help open a utility OCPD if a high voltage conductor lands on the bare neutral conductor.

Also it seems most agree that the ground rod may aid in the dissipation of a lightning strike from the utility system.

Now I install a generator to power an outdoor concert on temporary structures, stage, light towers, control risers (front of house) etc.

Can anyone tell me what the ground rod connected to the generator frame does for us in this situation?

Is it solely for lightning protection and if so how would it help and what is it protecting?

At the voltages the system would be operating (120/208) the ground rod has no chance of operating an OCPD

I am not trying to change the code I would just like to know why we do this instead of doing it because the code says so.

Thanks, Bob

[ August 21, 2003, 09:10 AM: Message edited by: iwire ]

 

[don_resqcapt19]

Re: Portable Generators and Ground Rods

Bob,
What section of the code requires the use of a grounding electrode with a portable generator?
Don

 

[iwire]

Re: Portable Generators and Ground Rods

Don, I am happy you asked that as I have questions on that too.

Perhaps I should have used the term "Vehicle-Mounted Generator" as I am thinking of one on a trailer.

I will post the section even though I know you are aware of it.

250.34 Portable and Vehicle-Mounted Generators.

(B) Vehicle-Mounted Generators. The frame of a vehicle shall be permitted to serve as the grounding electrode for a system supplied by a generator located on the vehicle under the following conditions:

(1)The frame of the generator is bonded to the vehicle frame, and

(2)The generator supplies only equipment located on the vehicle or cord-and-plug-connected equipment through receptacles mounted on the vehicle, or both equipment located on the vehicle and cord-and-plug-connected equipment through receptacles mounted on the vehicle or on the generator, and

(3)The non?current-carrying metal parts of equipment and the equipment grounding conductor terminals of the receptacles are bonded to the generator frame, and

(4)The system complies with all other provisions of this article.

Now in the situation I describe in my first post the load on the generator is not cord-and-plug-connected.

The Load, 400 amps, 800 amps is hardwired to the load terminals of the generator terminals and feeds portable power distribution equipment, cord and plug connected from that point on.

In this case I believe the code requires a ground rod as we do not meet 250.34(B)(2).

Thanks for any help you can give me here.

Bob

 

[don_resqcapt19]

Re: Portable Generators and Ground Rods

Bob,
If you hard wire it, you are correct that the code would require a grounding electrode system. Put the portable power distribution equipment on the trailer and you don't need a grounding electrode. I, like you, see no electrical advantage to provide a grounding electrode system for this application. In my opinion, the grounding electrode system may actually slightly increase the danger of electrical shock. I've done some experiments with a 15 kW unit mounted on our rescue truck. With the hot of a 120 volt circuit supplied via a truck mounted GFCI, I can put the hot on anything away from the truck and not be able to flow the 4 to 6mA required to trip the GFCI. This tells me that under these conditions, you can touch the hot conductor and not receive a serious shock as long as you are not touching anything connected to the grounded or grounding conductor of the generator system. If you connect the generator system to a grounding electrode, you would have a return path back and the GFCI would operate under these same conditions. I even did the same thing once outside in a light rain and still could not trip the GFCI. The trailer mounted unit will normally have a metallic foot in contact with the ground and would likely trip the GFCI if you did the same experiment.
Warning do not try these experiments at home.
Don

 

[charlie b]

Re: Portable Generators and Ground Rods

Bob: You are correct in saying that a ground rod will not impact a 120/208V system?s ability to trip an OCPD ? the resistance of dirt is too high. It could have an impact on power systems that operate at medium voltage or at a utility?s transmission voltage. My understanding of the ground rod is that it serves two purposes. One is to limit the voltage between any phase and planet Earth to the nominal level (e.g., 120 volts or 277 volts) to which it is entitled. The other is to establish a low-resistance path for lightning to find its way to planet Earth.

Lightning is a ?sort-of-exception? to two of our most treasured rules: (1) Electricity requires a complete path, and (2) Current will take all available paths back to its source. The truth behind both rules is that Nature seeks a balance. The excess of negative charges built up in a cloud is thrown (by Zeus?) at the dirt (or any anything between the cloud and the dirt), where there is an excess of positive charges. Balance restored. Without a ground rod, a lightning strike on (for example) the frame of a vehicle that carries a generator would deposit its excess of charge onto that frame. The next person who touches the frame would serve the role that the ground rod (and the GEC) should have served: the person becomes the path through which the excess negative charges finally make their way into contact with the dirt?s excess of positive charges.

 

[karl riley]

Re: Portable Generators and Ground Rods

Charlie b, I like your use of "planet earth" when that is exactly what is meant. It helps us to get away from the ambiguous meanings of "ground". I like the British use of "earthing".

And by the way, I often wonder whether we just assume lightning will follow the paths we provide for it or whether there is a large body of evidence showing that it dutifully flows through a ground rod to earth and harmlessly dissipates out into planet earth. I have never researched this (waiting for retirement).

Karl

 

[charlie b]

Re: Portable Generators and Ground Rods

Most of the NEC?s rules are readily understood in the context of the potential danger that could arise from violating the rule. I agree with you, Karl, that this one isn?t so evident. I equate this particular NEC rule with the rule of my youth that says I shouldn?t stick my arm out the window of a moving car. I have no idea of the probable level of danger, but am nevertheless comfortable with the notion of obeying the rule ? just in case.

 

[pierre]

Re: Portable Generators and Ground Rods

Correct me if I am wrong, but I believe the ground rod for this application will direct the lightning hit to 'Mother Earth' and keep it from traveling along the conductors to the load being served.

Pierre

 

[charlie b]

Re: Portable Generators and Ground Rods

There should be no connection between the vehicle frame and any of the hot conductors. Therefore, for the lightning strike to propagate down the hot conductors, it would first have to jump through over one megohm?s worth of insulation. The driving force behind lightning might be strong enough to make that jump, and the ground rod would certainly be a more attractive (bad pun ? sorry :roll: ) path for the lightning to follow. So I conclude that you are not wrong.

 

[iwire]

Re: Portable Generators and Ground Rods

Originally posted by don_resqcapt19:
Bob,
If you hard wire it, you are correct that the code would require a grounding electrode system. Put the portable power distribution equipment on the trailer and you don't need a grounding electrode.

Don again you hit the nail on the head, I read that section the same way, if it is all cord and plug connected no grounding electrode is required.

Many people read that section as all the outlets have to be mounted on the generator.

Originally posted by don_resqcapt19:
I, like you, see no electrical advantage to provide a grounding electrode system for this application. In my opinion, the grounding electrode system may actually slightly increase the danger of electrical shock.

I was thinking the same thing, all we have done is create a return path on what was an isolated system.

Originally posted by don_resqcapt19:
Warning do not try these experiments at home.
Don

No I won't.

The lightning protection angle IMO sounds like a cop out as we have limited knowledge of what lightning will and will not due.

I have no doubt that if lightning hits this electrical system some of the current will head for the rod but IMO it will head out in every direction and do as much damage to the system as if there was no rods.

JMO, from an installer, Bob.

 

[iwire]

Re: Portable Generators and Ground Rods

Originally posted by charlie b:
Without a ground rod, a lightning strike on (for example) the frame of a vehicle that carries a generator would deposit its excess of charge onto that frame. The next person who touches the frame would serve the role that the ground rod (and the GEC) should have served: the person becomes the path through which the excess negative charges finally make their way into contact with the dirt?s excess of positive charges.

Charlie I appreciate your response to my questions, but I feel like you are pulling my leg here.

Are you telling me that the generator is going to hold a charge beyond the duration of a lightning strike?

I equate this particular NEC rule with the rule of my youth that says I shouldn?t stick my arm out the window of a moving car. I have no idea of the probable level of danger, but am nevertheless comfortable with the notion of obeying the rule ? just in case.

To me the difference here is we know that is our arm makes contact with an object it will do damage and we know that keeping our arm inside will prevent this.

It seems we do not know if a ground rod will do a thing in the case of a lightning strike on a isolated electrical system.

I find it odd that there are rules in the code that seem not to have science to back them up.

That said I will continue to put the ground rods in as that is the code and I agreed to follow it when I got my license.

Bob

 

[charlie b]

Re: Portable Generators and Ground Rods

Originally posted by iwire: Charlie I appreciate your response to my questions, but I feel like you are pulling my leg here. Are you telling me that the generator is going to hold a charge beyond the duration of a lightning strike?

Not for the world would I try to pull your leg. On a side issue, is this your shoe that I have in my hand?

Seriously, like all current, lightning comprises ?charge in motion.? Unlike all other types of current, this charge is not circulating in a closed path. Rather, it is moving towards the opposite charges that reside in planet Earth, hoping to find a mate and to settle down (i.e., restore balance of charge). But if lightning hits the frame of the vehicle that holds the generator, and if there is no path from that vehicle to planet Earth (discounting the highly insulated path that includes the rubber tires), then where would the charge go? Now we have one conductive body (the vehicle frame) with one type of net charge (negative), another conductive body (planet Earth) with the opposite type of net charge (positive), and insulated space between them. Sound like a capacitor? A capacitor is exactly what you have! And what happens with a person grabs both sides of a charged capacitor (for example, by standing on Earth and touching the frame)? The lucky person lives to say ?ouch.?

 

[iwire]

Re: Portable Generators and Ground Rods

Originally posted by charlie b:
Not for the world would I try to pull your leg. On a side issue, is this your shoe that I have in my hand?

A friendly word, grab the pant leg instead, you have know idea what I have stepped in before.

I hesitated to use the word capacitor as that is specifically made to hold a charge.

I have a hard time thinking of just a block of steel holding a charge, is this anything like a static electricity charge?

Bob

 

[charlie b]

Re: Portable Generators and Ground Rods

It's exactly the same as a static electricity charge. And the word "capacitor" does apply, in the sense that it would function the same way, even though it was not by intentional design.

 

[don_resqcapt19]

Re: Portable Generators and Ground Rods

Charlie,
If the lightning jumped across miles of sky to get to the generator, why wouldn't it jump a few more inches to earth? I can't imagine the generator, acting as a capacitor, holding all, or even a small portion, of the energy of a lightning strike.
Don

 

[iwire]

Re: Portable Generators and Ground Rods

Thank you for the response to this so far.

Now my question is why is a ground rod required if the generator is hard wired but not if it is plugged in?

What has changed that a ground rod will be needed if the unit is hardwired?

What prompted the CMP to make this distinction?

Bob

 

[websparky]

Re: Portable Generators and Ground Rods

Hi Charlie,

Sorry to disagree with ya! But when this event we call "lightning" occurs, all of the electrical charge is dissipated and spent. There is no residual charge left in the object that served as a conductor.

One of the reasons objects are "grounded" to the earth is to prevent a side flash or a spark generated by the lightning strike because the strike is trying to connect with earth, not the object in-between.
A ground rod in this instance serves a two fold purpose. One, it makes a low resistance path to earth for static discharge and lightning events and two, it is there equalize potentials.

Dave

 

[gwz2]

Re: Portable Generators and Ground Rods

445.3 refers to 702, which this generator may be.

702.10 Portable Generator Grounding.
702.10(A) (SDS) refers to 250.30.
702.10(B) ( non SDS ) EGC to be bonded to the system grounding electrode.

 

[iwire]

Re: Portable Generators and Ground Rods

it is there equalize potentials.

Equalize potentials between what?

The generator frame and the earth?

If we agree the generator frame will not hold a charge how is this difference in potential created?

Why would this difference of potential exist in a hard wired generator but not in a cord and plugged connected generator?

The only way I can see for a difference of potential between the earth and the generator frame would be if one of the "hot" (unbonded) conductors from the generator was in contact with the earth, now add as many ground rods as you want and there will still be a difference of potential between the earth and the generator frame.

As for keeping a low resistance path to the earth for lightning wouldn't what Don said be true?

If the lightning jumped across miles of sky to get to the generator, why wouldn't it jump a few more inches to earth?

What I am getting at is pretty much what Karl said

I often wonder whether we just assume lightning will follow the paths we provide for it or whether there is a large body of evidence showing that it dutifully flows through a ground rod to earth and harmlessly dissipates out into planet earth.

Don't get me wrong as long as the code asks for a rod I will put one, they are cheap and install easy with a demo hammer, I just want to understand why the code requires it.

gwz2

I am asking about a generator with no connection to a utility system and where no grounding electrode system exists.


Thanks all, Bob

 

[russ]

Re: Portable Generators and Ground Rods

I have always treated generators that are the only power source as seperatly derived systems.
The only time I run into these situations is the local church carnivals
The carnivals, and the generators in trailers that power them, are bonded by way of the ground conductor to the rides and concession trailers/booths. It's always seemed to me that the ground rods would help dissipate a lightning strike on anything struck (rides and booths) that are bonded to the system.

Russ

[ August 23, 2003, 07:50 PM: Message edited by: russ ]