DC voltage on copper water lines

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How many volts are you reading ?

Is this residential, or commerical, or industrial ?

I have measured a few hundred milli-volts between my GEC & earth a few feet away at my residence.

Welcome to the Forum !!
 
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If you have even order harmonic producing loads that use half-wave diode feed like bi-level dimmers, you will see a DC voltage between neutral and ground.

If you see DC voltage with no load in use, its galvanic action as someone suggested. The connection between different metals and water cause them to become a battery with the more reactive metal becoming compromised.
 
110315-2338 EDT

Across my water meter I read 0.0 millivolts but I must push the probes hard into the pipe to get to non-oxidized copper. But across a dielectric coupling on the cold water line to the gas water heater I read 0.4 millivolts. But 0.0 on the hot water coupling. Years ago the person that installed the water heater did a poor job. Sweated the fittings during installation. So who knows whether the dielectric got shorted on the hot water side or not.

As others said define the points where the voltage measurements were made. Make sure you really make contact with base metal in the copper pipes. What might be in the pipe path between the two measurements points? Go to other areas and measure over the same pipe length and see what voltage you see.

If you have volts of difference, then you possibly have big problems. Copper pipe is quite low in resistance. Thus, amps to get volts of drop.

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gar said:
110315-2338 EDT

Across my water meter I read 0.0 millivolts but I must push the probes hard into the pipe to get to non-oxidized copper. But across a dielectric coupling on the cold water line to the gas water heater I read 0.4 millivolts. But 0.0 on the hot water coupling. Years ago the person that installed the water heater did a poor job. Sweated the fittings during installation. So who knows whether the dielectric got shorted on the hot water side or not.

As others said define the points where the voltage measurements were made. Make sure you really make contact with base metal in the copper pipes. What might be in the pipe path between the two measurements points? Go to other areas and measure over the same pipe length and see what voltage you see.

If you have volts of difference, then you possibly have big problems. Copper pipe is quite low in resistance. Thus, amps to get volts of drop.

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So called "dielectric nipples don't really do anything. It's just a piece of galvanized steel pipe with plastic liner. It doesn't provide provide galvanic isolation between the two ends. If you want galvanic isolation the only way is to use a polymer braided flexible connector. I'm not sure if dielectric unions provide true galvanic isolation either.
 
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