Stray Voltage in crawl space

[roy g]

roy g

roy g

had a simular problem . Customer kids getting shocked on chain link fence when dew or grass damp. finally located neighbors dryer feed conductor laying across metal vent pipe. insulation was skinned only enough for slight leakage and not solid ground so as to trip breaker. It was probally about 60 feet from the fence.

 

[ptonsparky]

I know this has been posted as Stray Voltage, but it may not be stray voltage.

This issue is to find out if it is stray voltage or groundfault voltage. That is not always easy, and may take some time.

If I remember correctly, stray voltage is only when the primary neutral is invloved, otherwise you aren't getting a tingle from stray voltage. Technical difference, even if it feels the same.

 

[hurk27]

If I remember correctly, stray voltage is only when the primary neutral is invloved, otherwise you aren't getting a tingle from stray voltage. Technical difference, even if it feels the same.

I too have a problem with what we call stray voltage or not, I have read quite a few papers written on the subject, and it can get confusing when they are not all on the same page, there are those who believe that stray voltage comes from the grid from many miles from the location, I just cant fathom this, I do believe that most all stray voltage is a problem of a very close source to the offended area, whether it is the secondary neutral or primary neutral it's all the same to me, it's a load seeking a return path through the offended customers property grounding system which rises the voltage of this grounding system to a point above Earth potential.

 

[gar]

090830-2000 EST

If all water piping in the house is metallic, bonding exists around the water meter, all outside piping to the home is a continuous conductive path, and this house has a sound neutral connection to the transformer center tap, then the earth around said home will have a voltage relative to the water line of maybe not over 200 MV. This assumes no stray current in the earth.

Suppose there is stray current in the earth, then there should be a voltage gradient in the earth and thus from the earth to the water pipe.

A screw driver is an adequate probe when connected to a high impedance meter to check ground voltage relative to another similar probe or the water pipe.

When running these tests open the main breaker, if one exists, or all breakers otherwise. Now voltage measurements should be unrelated to anything in said home.

Use a long extension with the EGC directly connected to the water line before the water meter. The EGC of this cord connects to one input terminal of your meter.

On a rough map of the yard record ground voltage measurements at various locations, and the crawl space.

Depending upon the results you may be able to predict the source of the ground current.

If you see more than a few hundred millivolts there is a problem.

Report back on your results.

If the map of voltages is unclear on a source direction, then I can describe a technique that may identify the direction of stray current flow.

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[don_resqcapt19]

090830-2000 EST
If all water piping in the house is metallic, bonding exists around the water meter, all outside piping to the home is a continuous conductive path, and this house has a sound neutral connection to the transformer center tap, then the earth around said home will have a voltage relative to the water line of maybe not over 200 MV. This assumes no stray current in the earth.
...

Would not the voltage to remote earth (earth outside the sphere of influence of the underground water pipe or other electrodes) have a voltage that is equal to the voltage drop on the grounded conductor between the utility transformer and the main bonding jumper at the service disconnect? If you turn the building main disconnect off, then this voltage drop will go away, but you will still read the voltage drop of the utility primary neutral if the supply transformer is connected line to neutral.

 

[gar]

090831-0607 EST

don:

Assume the home load on the transformer is zero, then there is no current flow on the neutral from the transformer center tap to the home's main panel due to the home's load. But there may be current on this neutral from some other load. This could be adjacent homes or the transformer primary neutral.

For any reasonable current on the neutral from other sources than the home of concern, say under 20 A, there will be very little voltage drop along the underground water pipe, even say 400 ft of pipe.

If the service neutral is in good condition and its current is moderate, then at most the voltage drop from the transformer's ground rod to the home's grounding electrode (primarily the supply water pipe) should be in the few volt region.

This would not result in a very large voltage drop between two points in the earth in the general region between the transformer and the home due to this source of current.

Other currents may exist in the ground from some other source. These would produce their own gradient.

Obviously all the currents in the earth are vector summed to produce a composite current vector at any chosen point.

If there is a point source of current a long distance away and the sink for this is a long water pipe, then the current is moderately uniformly distributed along the long water pipe. Thus, current density along the pipe is small compared to the current from the point source. The current density and the near surface resistance of the soil will determine the voltage gradient in the earth.

This does not quite respond to your question. I was trying to find the data on my experiments from a test rod in my backyard to my water pipe, but I have not found it yet. Have to do some other work right now.

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[tryinghard]

Get the utility company involved first.

 

[don_resqcapt19]

gar,
It is my understanding that the voltage reading from any point on the building's grounding system to "remote earth" (sometimes defined as a point on the earth at least 50' away from any grounding electrode) at any location will be equal to the voltage drop on the service grounding conductor. This voltage should only be a few volts, as you said. The voltage drop on the utility primary neutral can be much greater and it will also show up when you read voltage from the grounding system to remote earth.

 

[gar]

090901-0736 EST

don:

I do not have large ground currents in my yard and therefore it may not be typical.

If I use my pole transformer ground rod as my reference point and go to various points in the yard I do not have a uniform voltage difference as I move to varying points. Many of these points are more than 50 ft from the pole ground rod and/or the house. In the following quote I had not yet refined some of my measurement methods and as an expedient I used an outside receptacle EGC as the reference rather than directly from the water pipe or the pole ground rod.
From my post # 12 at http://forums.mikeholt.com/showthread.php?t=99809

In my yard I have an adjacent neighbor that shares a transformer with me. Fairly certain our pole has a floating delta on the primary, and any other neighbors are serviced from the same delta, but different transformers. There is a ground rod at the pole. In the house the service neutral is connected to a 1" copper water supply line that runs about 150 ft to the street.

I used an outdoor outlet to connect to the house system ground. Using an extension and a Fluke 27 I made measurements relative to the house ground. The voltage at the pole ground rod was about 35 millivolts, and at the conduit going into the house about 35 millivolts. This is at the side of the house and about 30 ft from pole to meter, and about 75 ft of 0000 copper. My 35 millivolt levels are probably from equipment line filters.

Going toward the rear of the yard about 60 ft the voltage increases to about 80 millivolts. Swing toward the opposite side of the yard it increases further to about 180 millivolts. Thus, there are currents from the neighbors

The voltages I read are small, generally less than 150 millivolts. However, as I discovered and reported in a recent post I saw a repetitive variation of several hundred millivolts. Also I reported that the vector angle of current flow varied as I moved around the yard.
See posts #68 and #71 at http://forums.mikeholt.com/showthread.php?t=114269&page=2

Thus, I can not assume any particular point as a useful remote earth point.

More later.

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[gar]

090902-0850 EST

fourteen/two

From your first post:

If I have no power in the house how could I still be getting the voltage on the pipes?

What tests have you run since your earlier posts?

If the pipes are conductive all the way thru the house, the water meter, the outside water supply line, and the supply line is moderately long, then the pipes should be at about the potential (voltage) of the earth around the water supply line.

If there is a source of current into the ground that flows to the water supply line. then there can be a voltage difference from the pipes in the crawl space to the earth in that space. This does not have anything to do with the house being powered if the source of the current is from some other circuit source.

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