Feedback Or Stray Voltage

toastermaker

I searched "All open Forums" for "feedback" and "stray voltage" Unfortunately I got tons of hits about users wanting feedback on a subject
and nothing on stray voltage.

In residential work I often read low volts on a circuit even when the breaker is off. My boss called it feedback and said its normal.
Usually this is in amounts of 7 volts or less. On occaision it is higher, up to around 12 volts.


My question:

What is the cutoff point where I should start to consider the source as somthing other than normal feedback?

Thanks for any replies,

mdshunk

This is commonly called "phantom voltage", and you'll only measure it with a digital multimeter. More properly, it's capacitive coupling in a 3 wire cable or inductive coupling in a 2 wire cable. When a cable runs near an energized cable, that "dead" cable picks up some energy, in much the same way that a transformer works. If you put a load on that cable where you're measuring a few volts (like turning on a light on that circuit) or us a loaded type voltage meter (like a Wiggy), the voltage will be absolutely zero. The few volts you measure has absolutely no current potential. Add the smallest load (resistance or inductive), and the voltage is zip. It's just being induced on that dead cable by other energized cables.

toastermaker

Thanks mdshunk,

Is there something of an upper limit to this phantom voltage?



Thanks again,

mdshunk

That might depend, in part, on how good your DMM is. I've measured into the 30's on the dead traveler of a 3 way switch circuit. That 3 wire cable has the conductors wound around each other inside the cable jacket. There is much inductance in that case. You might have noticed this phenomenon also when checking for a hot conductor with a non-contact voltage probe (the glow stick sort of thing). You sometimes get a false "hot" with those on a dead conductor because of this so-called phantom voltage.

toastermaker

Thank you very much mdshunk,