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Re: Can This Be Done? #96878 01/05/06 02:15 PM
Joined: Oct 2004
Posts: 135
BobH Offline
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?

2017 / 2014 NEC & Related Books and Study Guides
Re: Can This Be Done? #96879 01/06/06 01:39 PM
Joined: Aug 2003
Posts: 1,374
Ryan_J Offline
The generator frame is bonded to the equipment grounding conductor, which will definatley have a resistance of less than 25 ohms. [Linked Image]

Ryan Jackson,
Salt Lake City
Re: Can This Be Done? #96880 01/06/06 01:43 PM
Joined: Jun 2002
Posts: 58
energy7 Offline
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?

Re: Can This Be Done? #96881 01/07/06 06:58 AM
Joined: Jan 2003
Posts: 4,391
iwire Offline OP
skipr Has decided his wisdom is not needed here.

I removed his 'resignation' post.

skipr, you are welcome back here any time.


Bob Badger
Construction & Maintenance Electrician
Re: Can This Be Done? #96882 01/07/06 07:08 AM
Joined: Jan 2003
Posts: 4,391
iwire Offline OP

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


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.

Bob Badger
Construction & Maintenance Electrician
Re: Can This Be Done? #96883 01/07/06 08:08 AM
Joined: Jan 2003
Posts: 4,391
iwire Offline OP
To encourage further discussion where is the requirement that states a circus generator needs an electrode?

Bob Badger
Construction & Maintenance Electrician
Re: Can This Be Done? #96884 01/07/06 01:46 PM
Joined: Jul 2004
Posts: 9,394
gfretwell Offline
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.

Greg Fretwell
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