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Mobile connector flashes 2x - “ground loss”

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A more directly applicable test would have been to insert, say a 5 ohm resistor into the circuit, and see how the DMM is affected while measuring that resistance. But I didn't have one handy.
OK, I realized that I do have one handy, namely my little compact fan + toaster element (resistive heater) space heater, with a 2 prong plug and simple mechanical switches. When I turn the space heater to the medium setting, then I can measure across the 2 prongs of the plug to get the cold resistance of one of the two heater elements, in parallel with the windings on the motor.

So motivated by an interest in science and goaded by qdeathstar's intransigence, I was able to take the following resistance measurements with my DMM.

14.8 ohms = Space heater configured as above
0.1 to 0.3 ohms = Unloaded N-G resistance at receptacle
14.9 to 15.2 ohms = Unloaded N-G in series with space heater
17 ohms = N-G in series with space heater, with hair dryer on fan only (low load)
41 ohms = N-G in series with space heater, with hair dryer on low (medium load)
0 ohms = N-G in series with space heater, with hair dryer on high (high load)

As you can see, the DMM result depends on the current that is flowing on the neutral conductor that is being measured. So if you try to take a measurement on a powered circuit, your result will be off.

So for hopefully the last time, as the manual tells you, only take resistance measurements on unpowered circuits.

Cheers, Wayne
 
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OK, I realized that I do have one handy, namely my little compact fan + toaster element (resistive heater) space heater, with a 2 prong plug and simple mechanical switches. When I turn the space heater to the medium setting, then I can measure across the 2 prongs of the plug to get the cold resistance of one of the two heater elements, in parallel with the windings on the motor.

So motivated by an interest in science and goaded by qdeathstar's intransigence, I was able to take the following resistance measurements with my DMM.

14.8 ohms = Space heater configured as above
0.1 to 0.3 ohms = Unloaded N-G resistance at receptacle
14.9 to 15.2 ohms = Unloaded N-G in series with space heater
17 ohms = N-G in series with space heater, with hair dryer on fan only (low load)
41 ohms = N-G in series with space heater, with hair dryer on low (medium load)
0 ohms = N-G in series with space heater, with hair dryer on high (high load)

As you can see, the DMM result depends on the current that is flowing on the neutral conductor that is being measured. So if you try to take a measurement on a powered circuit, your result will be off.

So for hopefully the last time, as the manual tells you, only take resistance measurements on unpowered circuits.

Cheers, Wayne
Thanks for the real world test!
If you get really bored, what ACV and DCV readings do you get under those conditions?
 
Mistaken about what? I am correct. There us no change in the result of the test. Even your test showed this.
Did you completely miss post #121?


Cheers, Wayne
 
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Did you completely miss post #121?


Cheers, Wayne

That, again, is completely irrelevant. The result, that the ground is good, won’t change.
 
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That, again, is completely irrelevant.
You're apparently missing the point of the test I did, so let me explain:

We have a dummy resistance with two terminals R1 and R2. We have a branch circuit receptacle with terminals N and G. I connected R1 to G, and then measured resistance R2 to N. The dummy resistance represents a fault in the EGC, and we want to know if we can consistently measure that resistance when the branch circuit is powered.

So I imposed varying loads on the branch circuit (via the other receptacle in the duplex) while measuring the resistance R2 to N. The measured value changed with the imposed load. Including, at high load, reading 0 ohms, even though the resistance was actually about 15 ohms.

The conclusion is that you can't take a reliable resistance reading when part of the circuit is energized.

Cheers, Wayne
 
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The conclusion is that you can't take a reliable resistance reading when part of the circuit is energized.
I should say "with a DMM".

What can be done is a different sort of impedance test, and I think there are more expensive purpose built meters that do it. Namely, for two conductors that have power between them (H-N, or H-G if there's no ground fault protection upstream), impose a varying controlled load while recording both voltage and current.

The resulting graph of V vs I should be linear, and the slope is the impedance of everything between the source (e.g. utility transformer) and the point of measurement, including the source's internal impedance. Other concurrent loads shouldn't interfere much if they are fairly constant.

This is similar to what the DMM is doing, with several differences: the imposed load is AC instead of DC; the DMM assumes the zero load voltage is zero (unenergized circuit); and the imposed load would be much greater than the DMM injection current, on the order of 1 to 10A, I think.

Comparing the H-N impedance to the H-G impedance would then tell you if you have a bad ground (assuming low H-N impedance, which implies a good H and N).

Cheers, Wayne

P.S. For energy forum followers, If you've noticed that @Vines has referred to a "loop impedance test" done on a residential service to determine the maximum number of allowable PowerWalls, I believe that's the test described above. For this use it's actually determining the maximum allowable export current on the service. Because when exporting power, the inverters raise their voltage to push power to the grid through that loop impedance, which is the flip side of the usual voltage drop considerations. And if the loop impedance is too high, and the inverters try to push too much power, the required voltage would rise too high and cause to the inverters to trip off (rather than risk damaging other equipment).