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 Thaiman 04-01-2011 11:03 AM

voltage on ground wire - multiple ground problem?

I am helping a friend work on his 1959 home and ran into a curious problem. the house was originally wired with two wires and in the kitchen and garage a bare wire was connected from the outlets to the cold water pipes as a ground. At some point in the 80s a sub panel was added and the wiring to this panel was three wire with grounds connected to the neutral bus at the panel.

As we remodel the house we have been running ground wires to all of the outlets and switches and tying them all together so that we can eventually get everything grounded. As I was connecting two ground wires together i noticed a small spark and checked the voltage across it (55ish volts) I was concerned about a short on one of the circuits and started swicthing off breakers to try to isolate the problem circuit. As breakers were switched on and off the voltage would decrease sometimes and increase others. The amperage across the grounds was low (maybe 5 mA).

The only thing I can think of is perhaps some of the voltage that is going back to neutral at the panel and hitting the ground cicuit there is bleeding through the ground wires and going to the cold water pipe ground. Does this make sense? If it does, is the correct solution to disconnect the ground at the cold water pipe and run a gorund wire back to the panel?

Would it make sense to add a utility box in the attic that is connected to the panel via a heavy guage wire (#6?) and then running the new grounds from the recepticles to the utility box?

 brric 04-01-2011 11:20 AM

Voltage on a ground is the result of improper grounding and/or bonding.

 Thaiman 04-01-2011 11:25 AM

Quote:
 Originally Posted by brric (Post 621297) Voltage on a ground is the result of improper grounding and/or bonding.
that is what I was thinking. Is the best solution to remove the ground from the cold water pipe and tie it back to the panel ground?

 a7ecorsair 04-01-2011 11:44 AM

There is a main panel and a sub panel. How is the main panel grounded? Does the main panel have two bars for connecting neutrals and grounds or just one?
How was the sub panel connected to the main. Is there a wire connecting the sub panel ground to the main panel ground? How many neutral and or ground bars are in the sub panel? There should be two and they should not be connected in any way. All the white wires should go to one bar which would be connected back to the main panel neutral. This bar should not have a bonding screw and it should be mounted on insulated stand-offs.

 brric 04-01-2011 11:45 AM

Quote:
 Originally Posted by Thaiman (Post 621302) that is what I was thinking. Is the best solution to remove the ground from the cold water pipe and tie it back to the panel ground?
Not the best solution but it might help, doubtful though. More likely neutrals are improperly tied to grounds somewhere in the system.

 Thaiman 04-01-2011 11:47 AM

Quote:
 Originally Posted by a7ecorsair (Post 621308) There is a main panel and a sub panel. How is the main panel grounded? Does the main panel have two bars for connecting neutrals and grounds or just one? How was the sub panel connected to the main. Is there a wire connecting the sub panel ground to the main panel ground? How many neutral and or ground bars are in the sub panel? There should be two and they should not be connected in any way. All the white wires should go to one bar which would be connected back to the main panel neutral. This bar should not have a bonding screw and it should be mounted on insulated stand-offs.
I'll check on this and let you know. I appreciate the quick feedback.

 AllanJ 04-01-2011 11:48 AM

Some ground wires were not actually connected to ground (were not bonded to the panel ground or ground rod) and you may have been measuring phantom voltage.

You would do well to remove the ground wires connecting receptacles to drain pipes and little by little install ground wires back to the panel.

In those parts of the house that are not scheduled for tearing open the walls, you can run the ground wire out of an outlet box, down to the baseboard, and up and around doorways to get to the panel. Other outlet boxes on the same circuit and along the way can have short ground wires coming out and attached to this ground wire.

Where plumbing fixtures are isolated electrically because of a section of plastic pipe between them and the rest of the plumbing system, it is recommended but not mandatory to run a wire from the fixture to where metal pipe resumes. This wire needs to be sized as the largest power wire (usually #12 or #14) running nearby.

 SD515 04-01-2011 12:23 PM

Well water or city water? Might be a back-feed on the water pipe if city water. Caused by a loose neutral in someone's panel, meter box, service entrance connection, etc. (may be yours, may be your neighbors)

 AllanJ 04-01-2011 03:36 PM

Add a #6 copper wire between the panel ground bus bar and the main cold water pipe entering the house, close to (withing 5 feet of) where the pipe comes through the foundation and before the meter. (Use #4 wire if the panel is more than 100 amps). With this installed you don't need to worry about back feeding of electricity from someone else's panel through the water main.

You may also need a #6 ground wire fromt he panel ground to a ground rod but that can come later if not already there.

 Pistol Pete 04-01-2011 04:36 PM

The cold water pipes at various places in the house might not be connected electrically to the supply pipe coming in from the street. In my house there was a section of polybute pipe between the supply pipe and the copper line that fed the water heater. The ground from the panel was connected at the water heater and there was about 40 volts when I measured across the section of plastic pipe. The only reason I noticed it was that we had opened the wall to replace some leaking polybute, so now it's all copper and all good.

Another possibility is if a water softener was added. Some of the bypass manifold thingies are plastic and could interrupt your ground continuity.

 dmxtothemax 04-01-2011 06:26 PM

What were you using to measure the voltage ?
digital volt meters and non contact volt sticks
can produce false redings because of there high impedance.
Check it again with a low impedance anologue meter or test lamp.
Some times many circuits can all contribute each a small amount of
leakage currant to the earth line.
Electronic devices with power filters seem to be the main
contributers.
How ever appliances with heating elements also contibute.

Otherwise get an electricain to check the whole earth system
perhaps it is not wired correctlly.

Quote:
 Originally Posted by Thaiman (Post 621286) I am helping a friend work on his 1959 home and ran into a curious problem. the house was originally wired with two wires and in the kitchen and garage a bare wire was connected from the outlets to the cold water pipes as a ground. At some point in the 80s a sub panel was added and the wiring to this panel was three wire with grounds connected to the neutral bus at the panel. As we remodel the house we have been running ground wires to all of the outlets and switches and tying them all together so that we can eventually get everything grounded. As I was connecting two ground wires together i noticed a small spark and checked the voltage across it (55ish volts) I was concerned about a short on one of the circuits and started swicthing off breakers to try to isolate the problem circuit. As breakers were switched on and off the voltage would decrease sometimes and increase others. The amperage across the grounds was low (maybe 5 mA). The only thing I can think of is perhaps some of the voltage that is going back to neutral at the panel and hitting the ground cicuit there is bleeding through the ground wires and going to the cold water pipe ground. Does this make sense? If it does, is the correct solution to disconnect the ground at the cold water pipe and run a gorund wire back to the panel? Would it make sense to add a utility box in the attic that is connected to the panel via a heavy guage wire (#6?) and then running the new grounds from the recepticles to the utility box?

 brric 04-01-2011 07:03 PM

Quote:
 Originally Posted by dmxtothemax (Post 621504) What were you using to measure the voltage ? digital volt meters and non contact volt sticks can produce false redings because of there high impedance. Check it again with a low impedance anologue meter or test lamp. Some times many circuits can all contribute each a small amount of leakage currant to the earth line. Electronic devices with power filters seem to be the main contributers. How ever appliances with heating elements also contibute. Otherwise get an electricain to check the whole earth system perhaps it is not wired correctlly.
This could be news to some of us that work on these circuits every day. Would you please cite some references as to what devices are allowed to "leak" current to the equipment ground of an electrical circuit.

 dmxtothemax 04-01-2011 09:55 PM

I didn't say they where allowed too!
But the fact is some do.
Like I said they usually have an inbuilt power filter,
these power filters usually have capacitors from both mains
lines to ground,
this is where the leakage currents come from.
Also heating elemants with metal jackets can have
some leakages to earth.
In Australia these are allowed to go as low as 100 k ohms.

This situation might not be ideal or even legal in some situ's
but it does happen.

 SD515 04-02-2011 09:58 AM

Quote:
 Originally Posted by AllanJ (Post 621403) Add a #6 copper wire between the panel ground bus bar and the main cold water pipe entering the house, close to (withing 5 feet of) where the pipe comes through the foundation and before the meter. (Use #4 wire if the panel is more than 100 amps). With this installed you don't need to worry about back feeding of electricity from someone else's panel through the water main. You may also need a #6 ground wire fromt he panel ground to a ground rod but that can come later if not already there.
Throwing this link out there again.....

http://ecmweb.com/grounding/electric_shocking_truth_grounding/index.html

With all due respect, I don't agree with saying there isn't anything to worry about back feeding from someone else's system.

In a situation where you or your neighbor has a loose neutral on a municipal water system that is bonded properly, this only masks the problem, it doesn't fix it. Current would still be flowing on the grounding electrode conductor, between your panel and the water pipe. It can be a dangerous situation if someone has to disconnect that GEC, including the pipe outside in the ground.

I do agree the water pipe needs to be bonded...it's going to be your main grounding electrode. The water meter also needs to be jumped and bonded on both sides, and a ground rod driven and bonded to supplement the water pipe (ground electrode).

We haven't heard yet from the OP if this is city or well water.

 AllanJ 04-02-2011 11:49 AM

Minute currents can flow in the ground wire simply because the ground wire is juxtaposed with the hot wire in a Romex cable. Additional leakage can occur from equipment or appliances because the internal wiring or circuit boards are positioned close to the chassis. This is the cause of phantom voltage. If a particular section of ground wire was not bonded to the panel neutral a significant voltage between that section and something that is bonded to the panel neutral can be measured with a sensitive voltmeter.

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