small voltage on circuit with breaker off
I replaced a dimmer switch with a regular switch this weekend. After turning the breaker off, I verified it wasn't hot with my multimeter, but noticed that I was still getting 5V AC between hot/neutral and hot/ground. I removed the cover to my service panel and was reading the same thing at the breaker. I turned off every other breaker individually and none of them seem to have this problem.
I then turned off every breaker in the panel, and the problem breaker read 0V. As I turned on individual breakers, the problem breaker voltage increased about .1-.2 volts per breaker. There were a few breakers that reduced the voltage instead of increasing it. These are duplex 20A breakers. The breaker on the same duplex increases it by about 2 volts. The two breakers on the duplex unit below the problem breaker increase it by about 1 volt each.
While this was occurring, every thing that I am aware of was completely disconnected from the circuit. I spent a while crawling around in the attic and could not find anything that tied into this line between the service panel and the switch I was replacing.
Is it possible that some sort of induction effect is causing this voltage? Why wouldn't it occur on any other circuit? Is it unusual? It is unsafe? Do I need to contact an electrician to check it out?
Probably some induction effect, sometimes referred to as phantom voltage. Any AC current carrying wire can induce currents, usually very minute, in nearby wires. Current won't actually flow unless there is some load, and your meter is such a load although only a minute amount of current will flow through a voltmeter even if the wire is actually live.
The more sensitive your meter is (higher impedance) the more likely you will measure this. In general, digital voltmeters are more sensitive than analog (needle) meters.
If you plug in and turn on an incandescent lamp using this circuit with the breaker off, and it is indeed induced current/voltage, that will drop to zero as seen on any voltmeter.
I agree with AllanJ. In general when measuring on power circuits using high-impedance meters the readings often will be misleading. For one example, I was reading 120v out of the panel yet the circuit was dead, i.e., the lamps and outlets on that feed were dead. I then realized that I should have used my simple GE Voltage Tester. I found that the connection out of the breaker was poor, but good enough to pass a small current readable on a DVM.
Problem solved and lesson learned. Voltage went to 0 when I plugged in a small table lamp.
Thanks AllanJ and oilseal.
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