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Can This Be Done?

Posted By: iwire

Can This Be Done? - 01/01/06 03:07 PM

Last Month there was a storm on Cape Cod MA, this caused wide spread power outages. As a result we where called in to tie in three 800 KW temporary generators into three separate supermarkets.

Each unit was a very new trailer mounted Caterpillar genset.

While making up the connections we could not help but notice this sticker.

[Linked Image]

Should we have packed up and gone home?

[This message has been edited by iwire (edited 01-01-2006).]
Posted By: Celtic

Re: Can This Be Done? - 01/01/06 04:40 PM

It's early...shouldn't the sticker state: At least/minimum 25 ohms to ground?
Posted By: sierra electrician

Re: Can This Be Done? - 01/01/06 06:09 PM

You could have used the building gnd system.
Posted By: iwire

Re: Can This Be Done? - 01/01/06 06:13 PM

Quote
You could have used the building gnd system.


We did. [Linked Image]

However I still have no reason to believe that the buildings grounding electrode system provided less than 25 ohms of resistance.
Posted By: trekkie76

Re: Can This Be Done? - 01/01/06 06:18 PM

does this have anything to do with it being on a tralier? I would suspect they did mean 25 ohms or less, not at least 25 ohms.
Posted By: sierra electrician

Re: Can This Be Done? - 01/01/06 06:22 PM

Normally when Bldg Steel, UFER, Water & Gas are all bonded you will be well below 25 ohms.

[This message has been edited by sierra electrician (edited 01-01-2006).]
Posted By: iwire

Re: Can This Be Done? - 01/01/06 06:34 PM

I would not say you normally assume it will be less than 25 ohms.

That depends entirely on the 'dirt' in the area.
Posted By: iwire

Re: Can This Be Done? - 01/01/06 06:41 PM

To expand a bit.

What difference does 1, 10, 25 or 100 ohms to ground make anyway?

A fault on the load side of the generators breaker will not use the earth for fault clearing.
Posted By: sierra electrician

Re: Can This Be Done? - 01/01/06 06:46 PM

I would also never assume it. In my experience in my area this is what I have found.
When it is "criticle" to acheive the requested ohms you should always measure it.
Posted By: BigJohn

Re: Can This Be Done? - 01/02/06 01:35 AM

This isn't exactly on topic, but it's related to something I've always wondered:

It seems like everyone talks about earth impedence being less-than or equal-to 25 ohms. That doesn't really make sense to me. How can earth have a definite resistance? Your resistance has to be between two points on a circuit, and assuming the ground rod is A, where is B? And if you increase the distance between the two points aren't you increasing the resistance?

What's going on when people talk about "earth impedence?"

-John
Posted By: BobH

Re: Can This Be Done? - 01/02/06 04:04 AM

The warning label was probably written by a lawyer, not an electrician, based on misunderstood information. Thank God electricians fully understand bonding and grounding, huh! [Linked Image]
Posted By: XtheEdgeX

Re: Can This Be Done? - 01/02/06 05:28 AM

Here's a definition of impedance.
http://www.encyclopedia.com/html/i1/impedanc.asp
Posted By: macmikeman

Re: Can This Be Done? - 01/02/06 09:00 AM

In order to properly measure the impedance of Earth", one must first measure from a ground rod driven to an 8' depth into the Earth and one driven to an 8' depth driven into the Moon, and one driven to an 8' depth driven into Mars. Now we have a good 3 point system from which to measure the impedence of the Earth. Its the impedance of space which will get you every time. It is over 25 ohms.
Posted By: George Corron

Re: Can This Be Done? - 01/02/06 03:28 PM

Guys,
Good answers, and lots of good thoughts.

The sticker is there to indicate that you should insure there is less than 25 ohms (though an arbitrary figure, CAT and others, use it because every electrician understands it) between the generator and what you are standing on.

The last thing anyone (and certainly CAT) needs is a difference of potential between a trailer sitting there in the open where anyone can touch it, and the trailer which is mounted on rubber tires.

A fault or inductive build up is one thing, the gen has plenty of safeties to deal with that, but if someone is touching it during that period, we want to make certain they don't get hurt.

They also don't want the electrician, or tech, to get zapped trying to crawl up on the trailer when trying to service.

There's a good half dozen proper ways to hook up a ground to a gen depending on your service type, etc. They're just trying to make sure you know it needs to be grounded at the frame to nearby ground, if you don't, it's on you, cause they put the sticker there.
Posted By: iwire

Re: Can This Be Done? - 01/02/06 03:41 PM

Quote
A fault or inductive build up is one thing, the gen has plenty of safeties to deal with that, but if someone is touching it during that period, we want to make certain they don't get hurt.


George that can not be prevented no mate how low the resistance to ground is.

It will be imposable to 'connect' the generator frame to earth in a way that would eliminate the danger during a fault.

This is exactly why I posted this thread, there are no real answers.

In some ways ways it would be better if the generator and the building it was suppling where isolated from the earth but that is not possible either.
Posted By: gfretwell

Re: Can This Be Done? - 01/02/06 03:59 PM

I was just over at my wife's construction trailer, powered by one of those 36kva generators. They have a 4 wire feeder into the trailer. the gen set is bonded and they have 2 ground rods. The trailer frame is also somewhat grounded by the 16 tie down anchors screwed into the dirt.

For the purposes of insuring there is no difference in potential between the trailer and the generator I think the EGC in the 1/0 feeder is doing a great job. Who cares if it is not "ground" as compared to the sales trailer a quarter mile away?

Now if they strung a LAN cable between them we would have a problem. They are letting Sprint DSL deal with that.
Posted By: BobH

Re: Can This Be Done? - 01/02/06 04:05 PM

Iwire, personally I think this label leads to misunderstanding and possibly a false sense of safety for those who do not understand grounding and bonding. They would be better off just stating that the generator should be hooked up by a 'qualified electrician' and leave it at that.
Posted By: skipr

Re: Can This Be Done? - 01/05/06 08:04 AM

The human body has roughly 25 ohm's of resistance and heart beat pulses at 60hz. So the path to ground needs to be less, so current choose's the least resistant path to ground in the event of contacting live parts. try checking resistance on yourself, you will see a many different reading
s as you place probes at different locations on yourself. OH and DSA (department of state architect's) does a earth ground test every time I install rods on high school campus electrical systems.If 25 or less is not achieved, I must correct, either by soil treament or driving another rod no more than 6 feet from first.
Posted By: e57

Re: Can This Be Done? - 01/05/06 09:39 AM

"heart beat pulses at 60hz" I would not rely on that... On average humans, 72 beats a minute, higher if you smoke and drink coffee non-stop like myself. [Linked Image] Far cry from 60 cycles, a second.

Also be wary of placing meter probes on the body, many have voltage that could be not cool going across the heart. Especially those used to check ground resistance.

Anyway, I think this sticker is intended for situations where the the gen set is not part of a building ground, and not perminatly bonded to the neutral, making it a seperately drived system. It would then require its own rod(s) at 25 ohms or less. Or, if it were a stand alone power source.

Tied to the buildings ground system (Bonded neutrals and all), and with a EGC sized for it, you would not need a seperate rod at all. Making said sticker a superphilus decoration.
Posted By: resqcapt19

Re: Can This Be Done? - 01/05/06 12:14 PM

skipr,
Quote
The human body has roughly 25 ohm's of resistance

Most studies on this show that the average person has a resistance of 500 to 1000 ohms, not 25.
Quote
so current choose's the least resistant path to ground in the event of contacting live parts.

It just doesn't work that way. Current takes all paths and as long a the fault exists, the grounding electrode provides no increase in safety for people. Ground rods and grounding electrodes do not get rid of the voltage, it is there until the fault clears and the grounding electrode system does not play much of a part in the fault clearing process. The big players in that process are the main bonding jumper and the equipment grounding conductors.
Quote
I must correct, either by soil treament or driving another rod no more than 6 feet from first.

The code requires that the second rod be at least 6' from the first, and it is most effective when it is spaced twice the length of the rod from the first rod.
Don
Posted By: BobH

Re: Can This Be Done? - 01/05/06 06:15 PM

The human body has roughly 25 ohm's of resistance and heart beat pulses at 60hz. So the path to ground needs to be less, so current choose's the least resistant path to ground: quote

I hope you're not being serious here, this is a gross misunderstanding of the concept of bonding and grounding. There is no way the human body has 25 ohms of resistance as this varies greatly between individuals and I would seriously doubt anyone would have a resistance that low. Sorry to be so blunt but what did you expect posting info. like that on a electrical forum?
Posted By: Ryan_J

Re: Can This Be Done? - 01/06/06 05:39 PM

The generator frame is bonded to the equipment grounding conductor, which will definatley have a resistance of less than 25 ohms. [Linked Image]
Posted By: energy7

Re: Can This Be Done? - 01/06/06 05:43 PM

From the comments, I think I'm missing something. When I read the post and saw the image, I thought: SO, this is pretty standard, and is a standard requirement whenever I inspect generators for the circus, movie shoots, car dealer tent sales, etc. I'll at least see a ground rod, maybe two if we're in sand (within a mile of the ocean). Or, generator is connected to an existing grounding electrode system.
NEC art. 250-30 and 250-34 cover the grounding system pretty well: If you don't meet the exceptions in 250-34(a) or (b), then you're a separately-derived system under 250-30, your grounding system is just about like a building.
What have I missed?
Posted By: iwire

Re: Can This Be Done? - 01/07/06 10:58 AM

skipr Has decided his wisdom is not needed here.

I removed his 'resignation' post.

skipr, you are welcome back here any time.

Bob
Posted By: iwire

Re: Can This Be Done? - 01/07/06 11:08 AM

energy7

Quote
From the comments, I think I'm missing something.


Perhaps.

I do not think any of us disputes that the NEC sometimes requires grounding electrodes for generators.

The reason the sticker caught my eye was that it specified 25 ohms or less.

If that label is part of the generators listing and labeling than 110.3(B) becomes a factor.

It may be imposable to achieve less than 25 ohms to ground.

Picture your generator at the circus, what do they do, install one or two ground rods?

Have you ever tested them to see if they had 25 ohms or less?

And then we get into the need for 25 ohms at all.

Why 25, what will that do for us.

The generators we installed where portable generators entirely cord and plug connected.

The generators themselves where not bonded as they where tied into bonded switch gear.

In this case the generators frame was grounded by the EGC we ran from the switchgear with the feeders.
Posted By: iwire

Re: Can This Be Done? - 01/07/06 12:08 PM

To encourage further discussion where is the requirement that states a circus generator needs an electrode?
Posted By: gfretwell

Re: Can This Be Done? - 01/07/06 05:46 PM

Careful Bob. You are going to start the "is a floating neutral safer than a bonded one" argument.
Some portable generators do not bond the neutral (like Honda) while most others do.
I know our lab supplies and convenience outlets in IBM machines used an isolation transformer that effectively lifted the neutral/ground bond.
It prevented ground loops when you used a scope but it was also supposed to be a safety thing.
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