I just replaced a 3-way switch, and when I was testing, I noticed that there
was 30 volts (Radio Shack analog meter) at the light socket when the
switches were off. I tried using one of the little idiot lights that glow
with voltage, and indeed, it glowed weakly. The light and the 2 3-way
switches work perfectly though. This circuit is fed by old knob and tube
wiring. Should I worry that something nasty is going on?
An analog meter even though not as sensitive as digital/electronic
one, might pick up such an induced voltage.
Such a meter with a fairly typical one milliamp full scale deflection,
on its 30 volt scale will have a resistance of 30,000 ohms.
On a 100 volt scale perhaps 100,000 ohms.
No experience with K and T wiring but those wires tend to be
seaparated; not twisted or close together as with modern wiring. Would
that not make induction from an adajcent live wire more likely?
You could put a bulb in series with the feed wire from the fuse,
leaving the bulb out of the fixture, just to make sure there is no
current leaking through?
You could have a shared neutral. I have one in my house and I have to
turn off the breakers for both circuits to avoid a situation like you
describe. The breakers should in fact be ganged. (It's on my list!)
See this site: http://www.thecircuitdetective.com/twocircuit.htm
C'mon, 30 VOLTS due to a shared neutral?
That would take more than just a "shared" neutral, it'd take a very
poorly connected or high resistance neutral, in which case it's unlikely
that as the OP said, the light and the switches work perfectly.
More likely slight resistive or capacitive leakage currents tweaking the
test meter, as several others have said already.
Since it takes at least about 50 volts AC to light a neon lamp, and even
more for the NE-2H, A1C and similar ones in testers but the meter reads
only 30 volts, what you have is a situation with low, highly limited
I suspect you have a switch at the end of a run of wire, and capacitance
between the two wires is allowing a small amount of AC current to flow.
If the switch in question is at the end of a run of wire, this is
- Don Klipstein ( email@example.com)
I have the exact same situation in the house addition I'm building, two new
3-way circuits wired as in the photo at the end of a wire run. With the
switches in the off position, one circuit read 29 volts across the load wires,
and the other read 31 volts. So I hooked up a temporary single light bulb
thinking I'd see a very faint glow in the filament. Nothing doing. No glow,
and then with the light off, when I read across the terminals of the bulb
socket, I read zero volts.
You probably need a low impedence meter to measure the voltage,
instead of high impedance that puts no load on the circuit. Usually
that means analog, with a needle, rather than digital.
And stop posting urls with [img] in front of or behind them. Each
real user of usenet has to edit them out. It's better that you edit
them once, than everyone of us having to do so.
On Tuesday, November 17, 2015 at 3:11:19 PM UTC-5, Micky wrote:
That would account for why he had 30 volt with no bulb and 0
volts with it. A high impedance meter can read what amounts
to stray voltage. With even a small load, it disappears.
As to whether he has some real problem, who knows, because all
we know are the two voltage readings. Does the light work?
It's a high source impedance (what causes voltage drop). It doesn't take
much load to drop the voltage to 0. A LED with appropriate resistor my
I have the same situation with solid-state relays (used for holiday
light control), "off" isn't quite off and a LED load will light (dimly).
Any non-LED load drops the voltage to 0.
I see no purpose to the [IMG] (perhaps some web forum thing?), but don't
have to edit them to click on the URL (which my newsreader, Thunderbird,
37 days until the winter celebration (Friday December 25, 2015 12:00:00
AM for 1 day).
Capacitive coupling from a hot wire into an ungrounded wire will give exact
ly the results the OP mentioned, If there is a third wire associated with
the first two wires, and that wire is ungrounded at either end, it will als
o pick up a signal triggering a sensitive voltage tester. But, if that thi
rd wire is grounded at either end, there should be no voltage induced/capac
itively in the ungrounded second wire. If the third wire is gounded at the
near end, nothing should be measured. If the third wire is grounded at th
e far end and at the near end, and carries some return current, there can b
e magnetic coupling to the third wire and some voltage may be measured.
On Saturday, November 21, 2015 at 12:53:17 PM UTC-5, Tony Hwang wrote:
xactly the results the OP mentioned, If there is a third wire associated w
ith the first two wires, and that wire is ungrounded at either end, it will
also pick up a signal triggering a sensitive voltage tester. But, if that
third wire is grounded at either end, there should be no voltage induced/c
apacitively in the ungrounded second wire. If the third wire is gounded at
the near end, nothing should be measured. If the third wire is grounded a
t the far end and at the near end, and carries some return current, there c
an be magnetic coupling to the third wire and some voltage may be measure
I measure voltage all the time using a digital high impedance VOM.
You just have to understand electricity 101 to know how to use it
and understand what you are reading. And if you want to turn it
into a low impedance meter, they have shunt resistors that you can plug in.
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