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#47671 - 01/24/05 07:07 AM Bad Ground Reading
mustangelectric Offline
Member

Registered: 02/08/04
Posts: 496
Loc: Bentonville, AR
Hi,
I wonder what would cause an outlet tester to read "bad ground" when all the grounds are installed?
I have a new out-building that has a 240 V-3 Wire, 60A service to it from a 200A main breaker panel. The building has 3-8 foot two tube fluorescents, a motion detector flood outside, 3-6 outlet surge suppressors designed for shop use, 4 reg recepts and all is fed from 2 gfi on two separate 15A ckts. The 70A panel has an isolated neutral. There is an 8' X 1/2" copper clad ground rod installed with a #8 gec. The neutrals and grounds are separated at the sub panel but not at the main panel. The sub panel is not bonded.
I get this reading from the first outlet on each circuit and then from the slave outlets.
The tester trips the gfi when the button is operated. Does this have anything to do with the ballast in the fluorescents?
Thanks for any replies.
-Regards
Greg


[This message has been edited by mustangelectric (edited 01-24-2005).]
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#47672 - 01/24/05 07:35 AM Re: Bad Ground Reading
dereckbc Offline
Member

Registered: 10/08/03
Posts: 158
Loc: Tulsa, OK
Gregg, something I do not understand. You said the sub-panel is fed from MP via 3-wire, and the SP has isolated neutral bus. I assume by 3-wire you mean L1, L2, & N with no EGC. If that assumption is true, then your meter is correct, you do not have a ground.

If you run 3-wire, meet the conditions of 250-32 ( I think that is the code reference)of no metal paths, then you need a N-G bond in the SP. Pesrsonally I would always require 4-wire to be on the safe side.

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#47673 - 01/24/05 08:14 AM Re: Bad Ground Reading
mustangelectric Offline
Member

Registered: 02/08/04
Posts: 496
Loc: Bentonville, AR
Hi,
Thanks for the reply. Where is the NEC requirement to run a 4 wire service to an out-building? I established the ground at the separate service. Isn't this considered a separate service? Or it is treated as a separate service?

Are you saying that since it is a separate service then I need to BOND the panel and not treat it as a sub? I thought any panel after the first ocpd had to have an insulated neutral except for an out-building?

You must run all of the circuit conductors together for a sub panel in a single building but if you leave that building you can establish a ground at the separate service thus dropping the 4th wire.

So any three wire feeder needs to be bonded at the panel?

A four wire would not get a ground rod so I would not bond at the sub if in the same building.

I almost always run a 3 wire feeder to a outbuilding and install a ground rod there.

If this is treated as the first ocpd then it would have to be bonded.

-regards

Greg




[This message has been edited by mustangelectric (edited 01-24-2005).]
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#47674 - 01/24/05 08:50 AM Re: Bad Ground Reading
nesparky Offline
Member

Registered: 06/21/01
Posts: 650
Loc: omaha,ne
If you run 3 wire to an out building, you must bond at the sub panel. You also need to make sure no other metal path occurs from the main building to the out building. Either a 3 wire method or a 4 wire method can be correct if all the conditions are met. Right now you do not have a ground path back to the main service.
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#47675 - 01/24/05 08:52 AM Re: Bad Ground Reading
Electra Offline
Member

Registered: 07/29/02
Posts: 38
Loc: Portland, Oregon, USA
If you do not take an EGC to the separate building, you must bond the neutral in the panel. Otherwise, the only path back to the source in case of a ground fault, is a very high impedance path through the earth, which is likely not to trip the breaker.

Also, you must always establish a ground reference at a separate building (ground rod, ufer, etc.), even if you do pull an EGC with the circuit. The only exception there is if you are only feeding the building with a single circuit, then you don't have to drive a ground rod.

Laura Jenkins

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#47676 - 01/24/05 09:03 AM Re: Bad Ground Reading
dereckbc Offline
Member

Registered: 10/08/03
Posts: 158
Loc: Tulsa, OK
Do not have a code book in front of me, but the reference is either 250.26, or 250.32 for your application.

It gives you two options for a sepearate structure.

A: is to supply the structure with 4-wire feeder (L1, L2, N, + EGS) just like any other sub-panel. The exception is you have to supply a ground electrode at the new service and bond it to the panel ground. But the neutral bus is not bonded, that bond exist at the main panel:

B: Is more restrictive. Absolutely no metal paths can exist between the two structures. If that condition you can run a 3-wire feeder (L1, L2, & N). With this option you set a ground electrode and GEC, and bond the neutral to the GEC just like setting a service.

As I stated earlier I never specify option B unelss I am 110% certain no metal paths exist like new construction. I use option A 95% of the time. By going Option A route it does not matter if a existing metal path exist or not, it is always safe.

You need to either add the N-G bond if the conditions allow, or add a EGC to your feeder. Either way a ground electrode is required.

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#47677 - 01/24/05 09:07 AM Re: Bad Ground Reading
Roger Offline
Member

Registered: 05/18/02
Posts: 1779
Loc: N.C.
Greg, the article you are looking for is 250.32(B)

Roger

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#47678 - 01/24/05 09:28 AM Re: Bad Ground Reading
dereckbc Offline
Member

Registered: 10/08/03
Posts: 158
Loc: Tulsa, OK
Roger, THX. I can never seen to remember separate structures from common service. Don't ask me how I can remember the requirements and not the numbered reference.

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#47679 - 01/24/05 09:47 AM Re: Bad Ground Reading
mustangelectric Offline
Member

Registered: 02/08/04
Posts: 496
Loc: Bentonville, AR
Hi,
Thanks for the replies. I appreciate the code references and comments.

I am sure that the bonding will give me a good reading. I am glad I tested it and I am glad I asked!

sincerely

Greg



[This message has been edited by mustangelectric (edited 01-24-2005).]
_________________________
Electricity has no respect for ignorance!

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#47680 - 01/24/05 09:49 AM Re: Bad Ground Reading
Roger Offline
Member

Registered: 05/18/02
Posts: 1779
Loc: N.C.
You're welcome Dereck.

Roger

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