Have been trying to get some smoke detectors interconnected, and
during the trouble shooting of the problem, I measured (with an analog
voltage between the white neutral, and the bare copper ground wire in the
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.
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.
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
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.
"Never ascribe to malice what can equally be explained by incompetence."
[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
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_.
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
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