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#30652 - 10/24/03 09:53 AM Stray voltages
aldav53 Offline
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Registered: 08/22/01
Posts: 545
Loc: Chandler, AZ USA
In testing some switches in a home I was found a bad 4 way switch. But was picking up some weird voltages from the 3-ways feeding the 4-way, about 50 volts, but not enough current to light anything, just my little neon tester and Fluke meter. Don't know where this was coming from, maybe the wires were coiled up between the runs causing a coil-like (inductance effect) inducing some voltages. Any ideas?
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#30653 - 10/24/03 11:09 AM Re: Stray voltages
pauluk Offline
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Registered: 08/11/01
Posts: 7520
Loc: Norfolk, England
It's most likely due to capacitive coupling.

When the light is switched on, one of the travelers is effectively disconnected by the 3-way switches at each end. Its proximity to the other conductors results in a small amount of capacitance.

Because a digital meter has a very high input impedance (typically 10 megohms), it takes only a small amount of capacitance to produce a substantial reading.

If you shunted the meter with some load, or used a conventional analog meter with a lower input imedance, then you would see a zero or much lower reading.
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#30654 - 10/24/03 11:54 AM Re: Stray voltages
:andy: Offline
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Registered: 10/18/03
Posts: 289
Loc: Germany
once at work i hat opened the breaker of a receptacle line for servicing. with the small neon screwdriver tester, i could see no difference between breaker on and breaker off. measuring with a tester with coil to montior the voltage then showed me the line was off.

Background: The cable was running in a troughing with several others. as both ends of the phase conductor were unconnected, the wire collected the field fromt he others...
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#30655 - 10/24/03 12:20 PM Re: Stray voltages
Scott35 Offline

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Registered: 10/19/00
Posts: 2707
Loc: Anaheim, CA. USA
As Paul has said, you are seeing the results of Capacitive Coupling (or some type of Reactive Coupling).

The Voltage reading on the DVM is from its high input Z (Impedance).
The Neon Lamp of the tester (probably a Proxy, non-contact type tester) is also seeing same thing.

Try reading with a Wiggy. The problem will go away.

p.s. With the Wiggy connected to the same circuit, try testing with the DVM; should have a very much lower Voltage reading now!

Scott35
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#30656 - 10/24/03 01:31 PM Re: Stray voltages
aldav53 Offline
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Registered: 08/22/01
Posts: 545
Loc: Chandler, AZ USA
That makes sense, thanks..
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