Question about small voltage between neutral and ground

voodoochile · 11-01-2012, 08:26 PM

Hi,

I have a 15A circuit that powers one outlet and 4 basic light bulb fixtures in my unfinished basement. I just added the 4th light fixture today. The outlet is always powered while the lights are controlled by a switch (which is nothing unusual :-)

I usually sanity check my work before putting covers on lights and outlets by measuring the voltage across the hot and neutral wires and also by doing a quick continuity check with my voltmeter between the neutral and ground wires (by usually, I mean the 3 or 4 times I've done minor wiring projects at home before). Since neutral and ground are at the same potential, I expect to get a steady beep with my meter (i.e. essentially no resistance). When I test this specific circuit, I am getting an intermittent beep rather than the typical continuous one. I measured the voltage across the ground and neutral wires and got a couple of hundred mV. I realize that is not much, but it surprised me as it is higher than what I would expect to see.

With the switch off, I can get the steady beep at the outlet, but as soon as I switch the lights on, the continuity test starts beeping intermittently at that outlet.

I doubt this is a cause for huge concern, but I am confused as to why this is different from what I've seen before.

Any thoughts as to why this is happening? Does anyone else do these types of continuity checks to test the ground and neutral connections?

Thoughts?

kbsparky · 11-01-2012, 08:33 PM

You are reading the difference in potential (VOLTAGE) between a current-carrying conductor (NEUTRAL) and a non-current-carrying conductor (GROUND).

All wire has resistance, and while the circuit is in use, you will find that there is a difference between the 2. The more load you have, the greater the voltage difference is.

This is why you must keep them separated beyond the service panel. You don't want any of that return current jumping over to your ground wire, and all the enclosures and other metal parts connected to it.

voodoochile · 11-01-2012, 09:08 PM

Thanks for your quick response and input - what you're saying makes sense to me so thanks for explaining it.

In general, is there a good way to check that a ground wire actually has a proper path to ground? Obviously, I can visually see whether my connections at a specific box are good, but how do I know for sure that everything upstream is also good (e.g. a severed ground wire)? Is it sufficient to do the continuity test I described earlier when there is no load on the circuit, or is there a better way?

Thanks!

kbsparky · 11-01-2012, 09:16 PM

Best way to check is to use a "wiggy" solenoid-type voltage tester. It applies an actual load when testing to ensure a good reading.