Current flowing in ground wire

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  #1  
Old 05-06-05, 08:54 AM
ElDerfo
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Current flowing in ground wire

Hi. After upgrading my residential electrical service to 200A, I was doing some routine checks when I found that there is anywhere from 1.5A to 5A flowing through the main grounding electrode (presumably, from the utility's pole). Even if I open the service disconnect, this current is still flowing. I contacted my utility, who came out yesterday, measured and told me that it is acceptable to have up to 6A stray current flowing from the utility's center tap to my grounding system. That doesn't seem right to me. Is this situation really acceptable? Thanks.
 
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  #2  
Old 05-06-05, 12:41 PM
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I would really hope that the utility company would be qualified to say what is acceptable (I don't know but it seems wrong). Anyway, do you have a metal water pipe for an electrode? I am wondering if a neighbor that shares transformers has a loose neutral connection.
 
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Old 05-06-05, 12:53 PM
ElDerfo
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Thanks, Phil. I have one ground wire going from the service disconnect to the incoming copper water pipe (bonded on both sides of the meter) as well as to two ground rods. Your theory about a grounding problem at a neighbor is certainly a possibility. Since the current I'm measuring varies so much (between 1.2A and 5A) throughout the day, I am inclined to think that the variation is due to other activity on my side of the transformer. My initial theory is that the utility's center tap is not properly grounded (and hence presents a difference in potential between it and my grounding system). I would appreciate any other thoughts that anyone has on this. Thanks.
 
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Old 05-06-05, 02:40 PM
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When two houses share both the same POCO transformer and a common underground metal water piping system there will always be some current flowing on the grounding electrode system since it is a parallel connection to the grounded service conductors(Neutrals).

If your neighbor has a loose/bad neutral connection, the current you are measuring would more than likely be much higher than 5 amps.
 
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Old 05-08-05, 03:50 PM
ElDerfo
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Thank you, "Mr. Fault." What I don't fully understand is: If my water pipe is at ground potential and the neutral coming from the utility co is at ground potential, why is current is flowing? 5 Amps seems like a lot. Thanks.
 
  #6  
Old 05-08-05, 05:13 PM
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The potential of ground varies a bit from place to place. And it takes very little voltage difference to drive a lot of current though pipes with little resistance.
 
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Old 05-08-05, 05:59 PM
ElDerfo
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Thank you, Mr. Nelson. Your explanation made everything click. By Ohm's law, it only takes a couple of millivolts between two grounding locations to generate that kind of current. I am now enlightened.
 
  #8  
Old 05-08-05, 08:25 PM
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Also consider that return current is not necessarily seeking ground, but to return to it's source's neutral. It will take any path available, even if it means through a neighbors grounding electrode system.
 
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