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#202963 - 09/02/11 08:14 PM ground voltage  
schenimann  Offline
Member
Joined: Feb 2008
Posts: 193
Western North Carolina
Maybe someone can expain this to me.

I'm working with my plumber on a job. We are installing an underground water tank to use for a small coy pond and an irrigation system. The tank is fed from a small creek and has a submersible pump. I had all the wiring done and the well guy was wiring the pump in the tank and got shocked. They called me and asked me what the problem was. I'm thinking, oh crap, did I leave the disconnect on, did I wire it wrong, etc. The disconnect was off. Apparently I have got some stray voltage on the egc. He got a shock off the ground. When I go back to the panel and turn off the main I get 2v from the egc to earth ground. Is this a utility problem and how do I diagnose it? I wasn't there when he got popped but it was enough to make think about it.


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#202966 - 09/02/11 09:37 PM Re: ground voltage [Re: schenimann]  
sparky  Offline
Member
Joined: Oct 2000
Posts: 5,311
how close to other GEC's wuz the plumber schenimann ?

~S~


#202972 - 09/03/11 12:12 AM Re: ground voltage [Re: schenimann]  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,121
Estero,Fl,usa
The first questions are "is this fed off of a sub panel? Is the neutral bonded there?"

You might be seeing the voltage drop in a 3 wire feeder.

If so that is where I would start.

You should know "ground" is not always "zero volts". That is why we have article 682.


Greg Fretwell

#202974 - 09/03/11 06:29 AM Re: ground voltage [Re: schenimann]  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,223
SI,New Zealand
Originally Posted by schenimann
Maybe someone can explain this to me.

I'm working with my plumber on a job. We are installing an underground water tank to use for a small coy pond and an irrigation system. The tank is fed from a small creek and has a submersible pump. I had all the wiring done and the well guy was wiring the pump in the tank and got shocked.my emphasis

Are you SURE the guy wiring the well pump didn't transpose a couple of wires?
What I'm saying is you can bond and ground everything to the 9th degree, but not enough "let-through" current will happen to blow a protective device.
This is why plumbers should stick to running pipes and leave the complex stuff to us.


#202977 - 09/03/11 02:01 PM Re: ground voltage [Re: schenimann]  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,121
Estero,Fl,usa
Trumpy, you will find "ground" is not really "zero volts" from point A to point B. That is why we have ground electrode systems, article 680 and in this case 682. Once he establishes his equipotential plane and attaches it to the equipment, hence the EGC, everything will be "zero volts" again.

This is simply the consequence of using the earth as a return path for circuit current. I know we say that is not supposed to happen but current takes all paths and that grounded PoCo transformer shares the earth with the grounded service drop conductor. The grounded transformers on L/N transformers also share earth with the grounded conductor going back to the sub station.
Current is flowing through the earth all over the place and there will be some voltage difference between each ground electrode along the way.
To see this, all you have to do is look at the current flowing in that #8 coming down the pole.

[Linked Image]


Greg Fretwell

#202985 - 09/03/11 06:56 PM Re: ground voltage [Re: schenimann]  
schenimann  Offline
Member
Joined: Feb 2008
Posts: 193
Western North Carolina
The circuit is fed from a meter combo, ground and neutral bonded together. From that egc to earth I get the voltage. Is that voltage just floating around in the earth? If so then I am not worried or is it something that needs to be looked at. It actually had nothing to do with the plumber, it goes all the way to the service.


#202986 - 09/03/11 07:22 PM Re: ground voltage [Re: schenimann]  
twh  Offline
Member
Joined: Mar 2004
Posts: 919
Regina, Sask.
It's normal and can get quite high. I've seen over 110 volts between a building with a ground grid and a ground in a teck cable feeding that building from a source few hundred feet away. Where I am, the rural distribution is single line and uses a ground return, so the higher voltages might be unique to my area.


#202990 - 09/04/11 07:30 AM Re: ground voltage [Re: schenimann]  
Alan Belson  Offline
Member
Joined: Mar 2005
Posts: 1,803
Mayenne N. France
Far be it for me to drop a spanner in the works, but this has to be sorted before you put any fish in the pond!


Wood work but can't!

#202992 - 09/04/11 02:32 PM Re: ground voltage [Re: schenimann]  
Meadow  Offline
Member
Joined: Sep 2011
Posts: 66
USA
What your describing sounds like a utility company problem. I would have them check out their system. Ive dealt with this before on diary farms and pools. It comes from the utility company having an insufficently sized neutral or a break in it somewhere and the ground is taking up the slack. Normally the pocos are resistent to the problem and its not uncommon for them to deny it all together. Talk to them though there is a device made that can be put at their transformer that seperates the customer's neutral from the poco's.


#202995 - 09/04/11 03:25 PM Re: ground voltage [Re: schenimann]  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,121
Estero,Fl,usa
I still think this is pretty normal. That is why we have several articles in the code about establishing an equipotential ground array where these objectionable currents will cause problems. (pools, spas, farms and bodies of water).

My bet is that if you could really find a "ground", whatever that is (maybe the ocean), the pond would be grounded and the GEC of the service would be 2 volts above that. Considering all the losses along the way from the power plant, I am surprised it is only 2 volts.
If I was trying to mitogate it I would either increase the size of the ground electrode system at the service or reground the EGC at the pond (the ultimate effect of article 682)


Greg Fretwell

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