Earth Ground Safety

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Old 03-09-03, 06:41 AM
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jacdan
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Question Earth Ground Safety

I am sometimes the demo man on older residential and smaller commercial remodel jobs and I need some help in regards to earth ground safety. As I understand it many of these older buildings used the water pipes as a means of grounding their system instead of the grounding rods. Much of my work entails adding on to the foundation, and ripping out the old incoming water/sewer lines and thats it until the other trades come in. My plumber people have warned me about pipes being hot, can I check the pipes with a voltage sensor prior to working on them? If their is a fault somewhere in the wiring system and I lop off the incoming water pipes will this put users at risk? Would power have to be shut off at the box or does the utility have to shut it off at the pole? I need someway to check this since sometimes we don't get an electrician on the job until later, and I don't want to put myself or anybody else in danger.
 
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Old 03-09-03, 08:46 AM
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I don't know of an easy answer to your dilemma. There are proximity voltage sensors available for less than fifteen dollars US but I don't know that would be enough. If the buildings service grounded conductor (neutral) is open or has a high impedance connection the voltage might not become detectable with a proximity detector until you disconnected the old Grounding Electrode Conductor (GEC) to the old water pipes.

Because metal water pipes that interconnect several homes provide a parallel pathway for the neutral current to flow back to the transformer Xo via the service conductors of other buildings the voltage might go up on the GEC only after you disconnect it. The presence of ground rods will not prevent this as the impedance of the return path through the earth is much higher than that through the metallic piping system serving several buildings supplied from the same transformer.

One thing that might work would be to buy one of the less expensive current meters. By placing its sensing element around the existing GEC or the pipe itself you could measure the current flowing in the piping. Anything more than five amps would be a red flag. The same kind of current sensing meter could be used to measure the current flow in the service entry wiring grounded conductor. An absence of current or a very low current in that conductor relative to the current flowing in the metallic piping would be a strong indicator of danger.
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Tom
 
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Old 03-09-03, 09:12 AM
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Another option might just be a good pair of rubber gloves when you disconnect it, then look for arcs and you should be able to tell if there is current on the line.
 
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Old 03-09-03, 09:32 AM
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Originally posted by sberry27
Another option might just be a good pair of rubber gloves when you disconnect it, then look for arcs and you should be able to tell if there is current on the line.
Uhmmm NO! Don't do that!
Grounded tools in the hands of other workers might go high relative to the earth. If any of those persons are in intimate contact with the earth they could be injured or killed. Also any connected 120 volt loads could be damaged or destroyed.
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Old 03-09-03, 10:59 AM
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I recently was helping my girl friend move out of her apartment which was in a large 30's house and I "just happened" to look at the utility attachment and I noticed that the 2 Neutral wires were not connected. I notified the owner and told him the underground water-service pipe was conducting current between the house and the utility pole. This obviously was a shock-hazard If a plumber were to cut thru the pipe while it was conducting current.

The safest procedure is to work on the pipe with the power Off.
Please know that when you remove the water-service pipe to a building supplied with electricity you may be removing what is Grounding Electrode for the building's electrical system and the removal of the GE would create a hazard.

After you have exposed the water-service pipe you could drive 2 Ground rods in the bottom of the trench-the deeper into the soil the better- and run a Grounding cable from the rods to the interior Grounding clamps. This will act as a temporary Ground and will be useful later because the Code now requires "Supplementary" Grounding Electrodes, usualy Ground-rods, to augment the "Primary" GE, usualy the water-service pipe.

A worker protected by rubber gloves and operating a Sawzall which is supplied from a GFI receptacle is a Must.----Good Luck!!!
 
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Old 03-09-03, 12:40 PM
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Thanks Tom, I had forgot thet we had discussed this somewhere else and that you had mentioned that the voltage could shoot up upon disconnection. Seems to me now upon reflection that part of the solution was to use an amprobe first. I live in a rural area where there isnt much city water but I do run across it sometimes for pipe thawing where we want to disconnect the ground from the water service before heating piping. Thanks for the reminder, especially this time of year.
 
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Old 03-09-03, 01:20 PM
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texsparky
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A clamp on ammeter will let you know whether or not there current is flowing on the ground wire.
 
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Old 03-09-03, 06:16 PM
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jacdan
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Thank you everybody, I appreciate the information. I do have a greenlee clampon which I can check for juice to give me a heads up prior to any work. Since most of times there is no way of knowing whether the electrical contractors have done any inspections prior to any site work, I wanted to have some basics before beginning.
 
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Old 03-09-03, 06:53 PM
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Bridge around your cut.

If you are ever unsure of how much current the pipe may be carrying then take a piece of emery cloth tape and clean up the pipe a distance on each side of the point of your cut. Clamp around the cut prior to cutting with a battery jumper cable. That will keep you from becoming part of the circuit. When we were constructing the building that now houses the WTOP news studio in Washington DC the plumbers went to cut out a section of pipe and the resulting arc welded there saw to the pipe.
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Tom
 
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