Residual voltage when light switch off

18 years ago

I just installed a ceiling fan in a bedroom of a home I just bought, built in 1951. To make a long story short, I discovered that there were 16.5 volts across the two wires in the ceiling box, even when the wall switch is off. With the switch on, the voltage is 120, which matches the voltage throughout the house.

I decided that the wall switch, made in 1951, must be bad, and this seemed to be supported by the fact that, with the switch open, I read a resistance across it of a range of 1 MegaOhm to 55 Megaohm, depending on the trial. My plan was to just replace the switch, as soon as I picked up a new once, so I went ahead and installed the fan, but now i'm wondering if I should take it down again, and verify that the residual voltage is indeed gone after replacing the switch. At the same time, I couldn't imagine any other scenario (ie. "custom" wiring by the previous owner) that would explain 16 volts with the switch off, but normal line voltage with the switch on.

Does anyone have an opinion about this?

Thanks.

Sort by:Oldest

Comments (6)

hendricus
18 years ago
There was a thread here a week ago that covered the same thing, phantom voltage.
journeyman1
18 years ago
If you were touching the meter leads, you were measuring your own resistance. If the switch was made in 1951, it would be a pushbutton switch. I would change the switch and forget trying to understand the 16 volts, if it is indeed real voltage.
Related Professionals
Dalton General Contractors · Nashua General Contractors · Binghamton General Contractors · Huron General Contractors · North Platte General Contractors · Longview General Contractors · Hilo General Contractors · Terre Haute Home Remodeling · San Diego Home Remodeling · Birmingham Bathroom Remodelers · Gainesville Bathroom Remodelers · Carbondale Bathroom Remodelers · Southaven Bathroom Remodelers · San Diego Bathroom Remodelers · Victoria Bathroom Remodelers
pjb999
18 years ago
Digital Multimeter? I never knew about the phantom voltage issue til I'd already returned a couple as faulty, if you want to be accurate when looking for o volts across something, you should use an analogue meter. Digital's great for everything else ;)
itsunclebill
18 years ago
If the switch was made in 1951, it would be a pushbutton switch.

Toggle switches have been around since at least the early 1920s and weren't uncommon at all in the early 50's
brickeyee
18 years ago
The digital meter has an input impedance into the 10s to 100s of Mega-ohms.

You are measuring voltage capacitively coupled from the hot to the neutral (and ground).
The capacitance is between the wires, with the insulation acting as the dielectric.
At 60 Hz the impedance is very high (on the same order as the meters input impedance) and so you see a fraction of the 120 V.

If you want to see if the voltage is phantom, place a load on the circuit and measure across the turned off load.

You should see zero volts.

A small 7 watt light bulb in a socket with some clip on leads is all you need for a load.
Geoff Ris
5 years ago
This thread is super old, but just had an experience similar to this. I was reading 31V at a ceiling light I took down with the switch off. After reading about phantom voltage, I took the voltage reading at the switch to ground when it was off and still got 31V. I replaced it with a new switch and both locations now read 0V with the switch off. Digital multimeter, from an electrician. Seems the only explanation is that the switch was leaking voltage, but I don't see that coming up as an explanation in my googles.