Voltage Appearing Between White Neutral And Gnd Wire ?

[Robert11]

Hello:

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 ?

Thanks,
B.

 

[sofie]

Robert11:
It's the current (not the voltage) that you should be most worried about....
your meter is probably sensitive enough that you are seeing voltage from
leakage or capacitive coupling Put a 1.0 K restore in parallel with your
meter probes.... if you still measure much more than 0.2 V RMS then you
might have a problem.

 

[Zorro]

Several possibilities:
The distribution is TT type (private houses).
The neutral is grounded into the transformer cabinet far away your house.
Quite normal to have some volts difference towards the earth ground which is
grounded in your home.

It could be possible also thet your own ground connection is impedant (bad
ground). In case an apparatus is leaking, current will go through this
impedance increasing the local ground vs the deep earth ground the
transformer is connected to. Dangerous. You'd better to verify every
equipment connected to the mains in order to see if it's failing. Otherwise,
measure the ground resistance with the proper method.

Should the distribution be TN type (usually private transformers):
The Neutral is connected to the ground via a resistor (1kOhm as far as I
remember). A threshold detector is connected in parallel with this resistor,
monitoring the leakage. This kind of distribution is normally used in
factories or public buildings. You may have 50 to 60 volts between neutral
and ground.

By the way, where are you located ? In Europe the Neutral is always light
blue. The white is reserved to a live return (two-way switch or permutator,
separation of several bulb return lines within the same hose,...).

 

[CJT]

Hello:

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 ?

Thanks,
B.

Assuming that circuit feeds other appliances, they could be drawing
enough current for a resistive drop along the neutral.

The safety ground, with (hopefully) no current flowing, will be closer
to "true" ground potential.

Whether it's a problem depends on the heat it represents that is being
generated in the neutral. Consider E^2/R, with E=2V and R small.

IMHO it wouldn't hurt to get it looked at.

 

[Jim Adney]

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.

I'm assuming that you're in the US, that the white is Neutral, and
that the smoke detectors are the only items in this circuit. If this
is true, then this must mean that there is a high resistance
connection in the neutral line somewhere between where you did your
measurement and the service entrance in your home. In US code, the
Neutral and Ground are connected together at the service entrance.

If you turn the power to these detectors completely off (turn the
breaker off,) you should then measure no voltage between the Neutral
and Ground, and you should then be able to measure the resistance
between those 2 points. It should be only a couple of Ohms at most.
Since the current draw of the detectors should be quite small, the
amount of resistance it would take to develop 2V of voltage drop on
the Neutral would be rather large.

OTOH, if there are other power consumers on this same circuit, and
they draw a few hundred watts, then 2V of voltage differential between
the Neutral and the Ground might be normal.

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