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#96858 - 01/01/06 07:07 AM Can This Be Done?
iwire Offline
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Registered: 01/05/03
Posts: 4391
Loc: North Attleboro, MA USA
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.



Should we have packed up and gone home?

[This message has been edited by iwire (edited 01-01-2006).]
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Bob Badger
Construction & Maintenance Electrician
Massachusetts
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#96859 - 01/01/06 08:40 AM Re: Can This Be Done?
Celtic Offline
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Registered: 02/26/05
Posts: 361
Loc: NJ
It's early...shouldn't the sticker state: At least/minimum 25 ohms to ground?
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#96860 - 01/01/06 10:09 AM Re: Can This Be Done?
sierra electrician Offline
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Registered: 02/12/05
Posts: 219
Loc: North Fork, CA USA
You could have used the building gnd system.
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#96861 - 01/01/06 10:13 AM Re: Can This Be Done?
iwire Offline
Moderator
Registered: 01/05/03
Posts: 4391
Loc: North Attleboro, MA USA
Quote:
You could have used the building gnd system.


We did.

However I still have no reason to believe that the buildings grounding electrode system provided less than 25 ohms of resistance.
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Bob Badger
Construction & Maintenance Electrician
Massachusetts
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#96862 - 01/01/06 10:18 AM Re: Can This Be Done?
trekkie76 Offline
Member
Registered: 06/19/04
Posts: 220
Loc: baileyville, maine, usa
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.
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#96863 - 01/01/06 10:22 AM Re: Can This Be Done?
sierra electrician Offline
Member
Registered: 02/12/05
Posts: 219
Loc: North Fork, CA USA
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).]
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#96864 - 01/01/06 10:34 AM Re: Can This Be Done?
iwire Offline
Moderator
Registered: 01/05/03
Posts: 4391
Loc: North Attleboro, MA USA
I would not say you normally assume it will be less than 25 ohms.

That depends entirely on the 'dirt' in the area.
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Bob Badger
Construction & Maintenance Electrician
Massachusetts
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#96865 - 01/01/06 10:41 AM Re: Can This Be Done?
iwire Offline
Moderator
Registered: 01/05/03
Posts: 4391
Loc: North Attleboro, MA USA
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.
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Bob Badger
Construction & Maintenance Electrician
Massachusetts
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#96866 - 01/01/06 10:46 AM Re: Can This Be Done?
sierra electrician Offline
Member
Registered: 02/12/05
Posts: 219
Loc: North Fork, CA USA
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.
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#96867 - 01/01/06 05:35 PM Re: Can This Be Done?
BigJohn Offline
Member
Registered: 03/06/04
Posts: 391
Loc: Boston, MA
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
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