Have been trying to get some smoke detectors interconnected, and during the trouble shooting of the problem, I measured (with an analog voltmeter) the voltage between the white neutral, and the bare copper ground wire in the box.
Was very surprised to see that it was about 2 V AC.
Other than the fairly obvious reasons, such as bad ground connections in the service box for the neutral or gnd, or within the wiring chain itself, was wondering if anyone might have any other thoughts or opinions on this.
Might as well add this: The smoke detectors were on line, and functioning, when I measured. The interconnect for the smoke detectors (the third, red, wire ) uses the white neutral (also) as it's return. And, measuring a few outlets around the house showed 0 voltage between the neutral and gnd as one would expect.
But, even if the smokes were dumping something on the white neutral, it being at gnd potential, would "sink" these voltages immediately, I would think, if the neutral was grounded well. So, what might be happening ?
BTW: How "common" is it to see voltages of this magnitude between the white neutral and ground ?
If you find an old analog meter and test it again you will likely see no voltage. Those new digital meters read voltages that get into a wire just because it is close to another wire and does not have anything to draw the current voltage down; much like a static charge.
If you have a large load upstream from the smokes on the circuit, it's possible to get a couple of volts on the neutral relative to ground. In that case, no big.
However, if the smoke detectors are on their own circuit, or you don't have a large load on the circuit, then you've got a loose splice on the neutral somewhere between the measurement point and the grounding bar in the breaker box. Not quite a floating neutral, but thinking about it. Real fun to track down, especially since you usually don't know the wire routing.
I did not note in in my other response, but this is certainly possible and it could be a real problem. I did not really think about it with my first response, but should have. It can be a dangerous situation. So it is worth checking out to make sure what it is.
This may or may not be a problem, and this is why the neutral should never be used to ground an item, and also why the neutral is only bonded to ground at the service entrance (not at sub-panels within the same building).
The neutral is known as the groundED conductor as it is grounded at the service entrance; the ground is known as the groundING conductor as it provides grounding throughout the building. The neutral is meant to carry current and the ground is meant to not normally carry current.
As such, with no current through the grounding conductor, it should be at ground potential throughout the building. Since the neutral carries current, and since our conductors have some, albeit very small, resistance they will develop a voltage drop. The voltage drop is a product of the current passing through it and its resistance (which is proportional to its length). So, it is not unheard of to find a point where there could be
2 volts difference between the neutral and ground.
Now, having said that, I would say that it is a little surprizing to find this difference on a circuit like smoke detectors (the exception would be where this circuit were fed from a sub panel that had some fairly heavy circuits run off of it, mainly on one hot).
The other issue is related to the meter used to measure the voltage. Fairly high impedance meters could be measuring noise or capacitively-coupled signals from the hot in the same cable. A lower impedance (under 50k) would elimnate this possibility. If you don't have a lower impedance meter, you could try using a resistor of about that value (47k is the closest) -- all you do is connect the resistor between the two points (neutral and ground here) and measure the voltage across the resistor.
Don't even bet a penny on it unless the neutral is OPEN on its way back to the panel. The capacitive currents are in the microamp range and it takes LOTS of ohms to create a two volt drop ant those current levels.
[The other postings touched upon this, but sufficiently buried to perhaps be worth repeating.]
You measured with an analog voltmeter, that will _normally_ have an internal resistance in the 5K-50K "ohms per volt" range. Your voltmeter _should_ have that info prominently displayed on the face of the meter. If your meter has that information (and it's not something extreme like 1M ohms per volt), then the voltage difference between neutral and ground is real.
You have to be really careful interpreting results with a DVM (or other devices in the 1M+ ohm/volt range) due to inductive pickup. While a DVM reading would be real if the neutral and ground were properly connected back to the panel, if there was a break in the wires, the DVM could read virtually anything (in fact > 120V in some cases).
[If you're using a DVM, you can try to "load" the voltage you're seeing. A 50K ohm resistor would work. So would a low wattage 120V lightbulb.[+] If you still read 2V, the voltage is real.]
A 2V difference between ground and neutral would be "normal" if there's a highish load in operation downstream of the smoke detectors due to voltage drop in the neutral (IxR), and there was a fair bit of wire between the smoke detectors and the ground/neutral interconnect in the panel.
But it does seem worth checking a little further. Try killing that breaker. Do you still see 2V between neutral and ground? If you do, you probably have excessive resistance in the neutral-ground interconnect in the panel [*]. Check for voltage between neutral and ground on other circuits.
[+] Values chosen so you don't fry something. It'd be best to pick a resistor in the 10K range, say, but you may not have one handy, and it really should be at least 1W - just in case something goes wrong and you get 120V.... Best not to have "test equipment" that will melt - especially if you tried this trick again to test whether an 80V reading was real or not.
[*] A reference was made to a major load imbalance in the two hot legs of your service possibly causing this. No, it wouldn't, because no matter how hard they "pull", resulting in different neutral-hot voltages, they shouldn't pull a solid neutral/ground connection in the panel apart voltage-wise on an unloaded circuit at _all_.
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