Explain this one…
Hello all,
I moved into an old house, built in 1850 actually. In 1970 a do-it-all type contractor renovated a few rooms into an apartment with electric.
I’m re-installing a ceiling fan. One was already there before, but it broke and was taken down. It is controlled by 2 3-way switches. The power comes in at the fan with a black, neutral and ground. Two 14-3 wires go to each switch from there. There is also a 14-2 wire which I assume goes to some electric outlets, but I haven’t tested that. There is nothing plugged in anywhere in the apartment while I am testing. The wiring is hot to black of one 14-3, to common leg of 3 way, to red white jumpers which are connected in the fan box, then black from the other 3 way is the switched hot for the fan.
When I test the voltage for the fan, I get 120V, but if I turn either 3 way off I get…43 volts (or so). I think once or twice I got 50, but mostly 43 volts. I am testing between the hot after the 3 ways and ground/neutral, same result. I opened up both switches and replaced them, still 43 volts when off. I check the continuity of the wires between the fan box and the switches and I get a good tone on my meter. I did notice about 5-6 ohms resistance, but I was check continuity through a series of alligator clips and an extension cord.
So here’s the kicker. I took the 3 ways out and left the bare wires sticking out of
the switch boxes (red, white, black). So now power comes into the fan box, connects to the black of one of the 3 way boxes and terminates at a dead end (no switch). The jumpers (red/white) are not wired to anything either at the fan or the wall switch. With the breaker on I get 120 volts black, and, you guessed it, 43 volts on the jumpers. Even better, this is on both sets of jumpers, running to separate boxes. If I turn the breaker off I get 0 volts everywhere.
I tried something bold and touched one of my 43 volt jumpers to ground. No spark. When grounded the voltage is, of course, 0. When I disconnect it from ground it instantly goes back to 43 volts. When touching the ground I check with an amp meter and got no recordable amperage. So basically I decided this is bullsh*# power, on the level of static electricity, maybe induced or something, but not enough to be a danger.
But where the heck is it coming from??
You’re using a digital multimeter, aren’t you? You’re seeing a so-called “ghost” or “phantom” voltage caused by capacitive coupling between the wire you’re measuring and an adjacent live wire. The high input impedance of a typical DMM places almost no load on the circuit and therefore allows this voltage to remain high. If you use a low-impedance measuring device, such as a “wiggy” solenoid tester, an old analog voltmeter, or an electrician’s multimeter with a low-Z input, the circuit will be loaded enough to make the ghost voltage disappear. Hooking a load, such as a light bulb, across the circuit will also eliminate the ghost voltage.
