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Old 03-24-2011, 09:30 PM   #1
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Default 3 wire 240v GFCI question

So my control panel has 2 separate inputs each with its own inline gfci, one 120v running to the power in and neutral busses, then a 240v that runs through a breaker, then a contactor (coil powered by the power bus and controlled with a 2 POS on/off switch) to the element. When I tried to auto tune the PID today, as soon as I switched the element to the on position, both GFCIs tripped, killing the power. When I did a little more research, I saw that a ground/neutral fault can trip the GFCI and since the 3-wire 240v outlet has the ground and neutral tied together, and that is in turn grounded with everything else (including the 120v inlet and receptacles), could thi have caused both GFCIs to trip, and if so, how would I wire that ground to avoid this issue? If this does not seem like the genesis of the problem, plese let me know and I can try to put up a more comprehensive diagram to get this bug figured out. Thanks for the help!

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Old 03-24-2011, 11:25 PM   #2
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I sort of have a mental picture of your setup. I do believe you problem is as you describe. You cannot tie ground and neutral together. Also you cannot tie neutral fron 2 sources together. However, I think it would help a lot if you posted a diagram to really get to the root of the issue and for anyone to offer suggestions..

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Old 03-24-2011, 11:37 PM   #3
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Some (most?) GFCI's have a circuit to detect neutral to ground faults, so if you have a circuit loop where one side of the loop goes through the GFCI and the other side doesn't, the GFCI will detect a fault. Here's a description of how that works:

http://www.codecheck.com/cc/gfci_principal.htm

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Old 03-25-2011, 07:31 AM   #4
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Here is a crude sketch of the power in, with all the grounds mounted to the same post in the control box.



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DeafSmith
That's what I am thinking since both of the GFCIs tripped as soon as the the 240v input drew current. So if that is the case, how would I go about grounding the neutral/ground from the 240v input safely yet separately from the other grounds in the panel

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Old 03-25-2011, 04:04 PM   #5
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I can't see an obvious problem from your diagram - nothing that would cause the GFCI to trip when turn on the element. One other possibility to look for is a connection between the 240 volt hot line and the 120 volt hot line - this would have the same effect as a grounded neutral. I didn't see one in your diagram, but double check your wiring.

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Old 03-25-2011, 04:08 PM   #6
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Also chech the neutral between the 120V source and ground or the ground from the 240V supply.

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Old 03-25-2011, 04:41 PM   #7
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I don't like to mix power sources. I am not an electrician but I can't see how it is safe to have the neutral from 120v source connected to the 240.

Personally I would have rewired the 240 to include neutral and not installed the 120v line. Seems safer and less chance for error.

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Old 03-26-2011, 03:16 AM   #8
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I really can't reconcile the description and the diagram to understand what is connected to what here.

You are using the two hot lines from your incoming 240V cable for the element. That I can see and understand.

You are using the hot and neutral from your incoming 120V cable for pretty much everything else.

What are you doing with the third wire from that 240V cable and the ground wire from 120V cable?

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Old 03-26-2011, 03:55 AM   #9
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Perhaps I am miss reading this but have you installed a GFCI on the 240V circuit?

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Old 03-26-2011, 04:14 AM   #10
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Quote:
Originally Posted by Wberry View Post
Perhaps I am miss reading this but have you installed a GFCI on the 240V circuit?
I believe he's got one of those 240v cords with an inline GFCI on it.

First post mentions that both the 240V feed and the 120V feed have GFCI.
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