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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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