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If I measure the voltage across the ground prong of a receptacle and a water pipe, I get around 42 mV of voltage. The ground prong is connected to a grounding conductor, which is connected to a ground bus bar at the main panel, which is connected to a #8 AWG conductor, and finally connected to an 8ft. ground rod.

The power system is: 240 V, 60 Hz split-phase (residential)

These are my questions:

  1. What could be causing this voltage? Is this an induced voltage?
  2. Or is this just a measurement error of my multimeter?
  3. Is this voltage a safety issue? Should I be concerned?
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  • \$\begingroup\$ Is it AC or DC? \$\endgroup\$
    – user16324
    Commented Mar 1, 2017 at 13:36
  • \$\begingroup\$ Sorry forgot to include. It's AC. \$\endgroup\$
    – Darwin Bautista
    Commented Mar 1, 2017 at 13:40
  • \$\begingroup\$ By the way the correct term is either equipment grounding conductor or just ground, or my preference, equipment safety ground since it's not a conductor in the usual sense. In a bizarre idiomatic (some say idiotic) use, the US National Electrical Code and many derived standards use the term "grounded conductor" for neutral. What they mean to say is "the conductor (wire intended to normally carry current) which is near ground voltage because it's bonded to ground back at the main panel". I prefer "neutral". \$\endgroup\$
    – Harper - Reinstate Monica
    Commented Mar 3, 2017 at 3:04

3 Answers 3

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The house ground is bonded to the supply ground at some point. There might also be a ground spike and a connection to copper piping somewhere.

Although the ground circuit is not suppose to carry any current, it does due to capacitive or varistor leakage. This current will cause a voltage drop and is what you measure.

Ask an electrician to ensure that the piping is bonded to the earth spike to be safe, if the regulations allow it.

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  • \$\begingroup\$ If the ground circuit does carry a tiny bit of current (just enough to have a few mV difference with respect to true ground), is this cause for concern? Or is this normal with any AC circuit (considering that current-carrying conductors can induce current in nearby conductors)? \$\endgroup\$
    – Darwin Bautista
    Commented Mar 1, 2017 at 16:56
  • \$\begingroup\$ No concern, small differences are normal, but if the wiring is suspect ground continuity tests are required. \$\endgroup\$
    – skvery
    Commented Mar 1, 2017 at 17:26
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Proper electrical work should be done by trained people. That said, all conductors have resistance. Using the equation:

V = I X R

...we see that only a little resistance (R) is necessary to develop a small amount of voltage (V) given a load is plugged into the outlet and is drawing current (I).

If there is no load plugged into the outlet, then you are likely measuring noise. Most multimeters have a very high impedance (R). So, again using the above equation, we see that only a very small amount of current (I) that we can likely attribute to noise will case a small amount of measurable voltage (V).

It should be noted that this observation may be attributed to ground loops. A ground loop situation can range from annoying (for example the buzz heard from an audio amplifier) to deadly (for example performers electrocuted on stage).

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  • \$\begingroup\$ Yes, I'm aware of Ohm's law. Also, I'm not the one who did the wiring; I'm just verifying if all the receptacles are properly wired. If you read my question carefully, I mentioned that I'm measuring the voltage across a supposedly ground reference (grounding conductor) and another ground reference (metal pipe). I'm not measuring the voltage across line and ground. To confirm if this is a measurement error, I measured the voltage across a metallic water outlet and another disconnected water outlet. I got exactly 0 V. So it means that my ground conductor has a non-zero potential. \$\endgroup\$
    – Darwin Bautista
    Commented Mar 1, 2017 at 15:10
  • \$\begingroup\$ Please realize may people will read your question. Some are likely the ones who did the wiring. Others may actually have discovered a real hazard. I re-read both the question and answer several times. Ground loops are differences in grounds. Grounds might include pipes. Much of this depends on how electricity has been installed for you. The answer briefly touches on how ground loops can create small voltage differences and points you to a wikipedia page. In the end, it is very difficult to tell you if you have discovered a hazard. Different locations use different rules. \$\endgroup\$
    – st2000
    Commented Mar 1, 2017 at 15:46
  • \$\begingroup\$ You say: "...ground prong is connected to a grounding conductor, which is connected to a ground bus bar at the main panel". So, could any of these items ("grounding conductor" or "ground bus") be also used for the neutral side of your power lines (assuming this is the way electricity is properly installed in your location)? If so, consider a load at your location is drawing current. Consider a small amount of voltage developing because of this current between the point where the the rod is connected and the point where the ground to the receptacle is connected. (please continue reading) \$\endgroup\$
    – st2000
    Commented Mar 1, 2017 at 16:01
  • \$\begingroup\$ (please continue reading) There may still be a problem. That is, this is not to say there is no problem. This is only offering one possible explanation. \$\endgroup\$
    – st2000
    Commented Mar 1, 2017 at 16:03
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That small voltage, 42 milliVolts, is why audio people suggest the entire set of equipment: amplifier, CDplayer, vinyl-record player+RIAA preamp, etc etc, all be powered from the same outlet.

Imagine this problem, but 10X or 100X bigger, in a rolling mill with 1,000 horsepower motors moving the rollers. How to instrument such a beast of noise and voltage drops and electric fields and magnetic fields? Optical isolators and/or differential sensing.

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  • \$\begingroup\$ This does not answer the cause (1) and whether or not it's a safety issue (2) or if it's a measurement error (3). Temporarily downvoted. \$\endgroup\$
    – try-catch-finally
    Commented May 9, 2017 at 4:28

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