I have a circuit in my house with a funny characteristic, and I'd like
to track it down. In that circuit, I measure a slight voltage
difference between power and neutral at a light fixture when the switch
to that fixture is off. Yet, my meter shows no continuity when I try to
measure resistance. Other circuits do not show any voltage when they
are switched off.
I assume this means that the neutral on the peculiar circuit is not at
a common ground with the 'real' ground. It does not strike me as
inherently dangerous, yet it seems like I might as well sort out the
problem. Anybody seen this and have a recommendation on
It is probably nothing to worry about. If you are using a digital meter it
is most like picking up some induced voltage from other conductors near it
or the switch could be leaking a very small ammount. How many volts are you
calling 'slight' ?
I'm using a ~20 year old analog multimeter. I measure ~5V on the 30V
scale, but about 0.5V on the 3V scale; so it's just enough to boost the
meter off zero. I was wondering about induction, the circuit in
question is only several meters from the service box, but next to a GFI
Ralph Mowery wrote:
The other folks keep calling it "induced" voltage, but you only really
couple inductively if there's a current in one wire running next to
another one, not just voltage on the wire.
From your description it sounds like you have capacitive coupling
across the switch. The capacitive reactance behaves like a very high
impedance at 60 HZ. For the purpose of understanding what your meter is
showing, you could consider it like a very high value resistor across
the switch, conducting current from the 120 volt source through your
meter's internal load resistance to ground.
Now, your conventional analog meter will have a resistance rating,
something like "20,000 ohms per volt". That means, when set to it's 30
volt scale it will present a load of 600,000 ohms, but when set to its 3
volt scale it will present a load of 60,000 ohms.
That's why you see two different voltage readings, the current flow is
pretty much constant, as the voltage across the capacitive reactance is
nearly 120 volts and varies less than 5 volts between the two
measurements, but the meter's load resistance is changing by 10:1, so
that relatively constant current creates two significantly different
voltages across the meter's input terminals, and that's why your two
Either way, it's nothing to worry about, if your meter is a 20,000 ohm
per volt unit the current flow is approximately 5/600,000 amperes,
about 8 microamps. You prolly couldn't feel that low a current even if
you stuck your tongue on the wire while grabbing metal water pipes with
Induction occurs when the current changes, something happening all the
time with AC current. A constant DC current will not induce anything.
That seems to be one reason for using AC so much, it allows you to use
On 6/11/06 8:20 AM, in article firstname.lastname@example.org,
You may note that as you change the voltage setting by a factor of 10, the
measured voltage is also changed by a factor of 10. That is, the needle
position is the same for the two settings.
What this suggests to me is that you are measuring capacitive coupling from
the hot side of the line across to the disconnect hot side past the switch.
The small capacitance is high impedance compared to the resistance of the
meter. In the end, you are getting constant capacitive current flowing
through your meter.
If you know what your meter resistance is, you can place a resistor of that
value in parallel with the meter. The "measured" voltage will be cut in half
because equal currents are flowing in the meter and in the shunting
Meter resistance is easily determined from the meter's rating in ohms per
volt. Just multiply the ohms per volt by the full scale voltage setting.
-- Ferme le Bush
Basically, your meter is far better than meters of yesteryear. It has
extremely high input impedence, so it doesn't 'labor down' the circuit
it's measuring, even slightly.
What you're measuring (almost certainly) is a small AC voltage that's
induced by the lengths of wiring being in close proximity to each other,
much like a transformer. This voltage has no current delivering
capability, as can be seen by shorting the wire to ground.
This perplexing phenomenon gives people fits until they understand it.
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