Phantom voltage reading on switch loops

jamiedolan

Is it normal to have a phantom voltage reading on the non-energized conductor of a switch loop?

My rms ideal multimeter reads 15v on a switch loop, on a conductor to ground even with that conductor disconnected at both ends if there is current on the other wire. i.e. 12-2 romex (brand new install). power on 1 conductor, other conductor disconnected (no fixture connected, switch off) reads about 15vac. Same reading with the wire physically disconnected from the switch.

I suspect this is a normal for a digital meter and that it is just picking up that reading due to the other cable that is enerize in the loop. I just went to check a set of wires were dead at a fixture box and saw the 15v reading. The wire that reads 15v to ground does NOT light a bulb tester.

When the wires are connected and an pigtail socked hung, my RMS clamp meter showed identical draw and return on the conductors with nothing on the ground.

I don't really thing were taking about a problem here, it just threw me off to see that reading. Can anyone confirm this is a normal finding?

Jamie

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Jamie Dolan - Neenah, WI

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micromind

This is completely normal. The impedance (resistance) of the voltage input of your average digital multimeter is around 11 megohms. It doesn't 'load' the circuit enough to eliminate phantom voltages. A test light, or a solenoid-type wiggy loads it enough.

The phantom voltage you're seeing is actually capacitive coupling. A capacitor can be defined as two conductors separated by an insulator. Any cable with two or more wires in it fits this definition. In an AC circuit, any flow of current will induce a voltage into the other conductors. If they're not connected to anything, there will be a voltage induced relative to the voltage of the current-carrying wires.

Linemen call this 'induction'. It can be lethal. Quite a few years ago, we were building a 12.5 KV line on poles below a 69 KV line that was energized. This line was about 3 or 4 miles long, and didn't have any transformers on it yet. The neutral was grounded. The phase wires were also grounded by grounding jumpers installed at one end.

As a demonstration to the apprentices, I disconnected the grounding jumpers, waited a few minutes, and moved one of the jumpers toward the isolated line with a hot-stick. About 5 or 6" away....SNAP! A real nice blue/white arc. Certainly enough to send a person to the next life!!

This served as a very real reminder of the old lineman statement 'If it ain't grounded, it ain't dead'.

The current induced by capacitive coupling is one of the very few things that will cause a GFI to trip when there is no actual ground fault. It takes about 300-400 feet of 2 wire cable to generate 5 milliamps though.

Rob

jamiedolan

Thanks for the confirmation on what it is I was seeing. On 120v lines will you ever see a higher induced voltage than about 15vac that I saw tonight?
Jamie

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Jamie Dolan - Neenah, WI

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Yoyizit

I'm surprised it is so low. I don't think your meter has a very high input impedance.
50' of Romex should give you about 500kΩ of cap. reactance at 60 Hz, with 120v giving you ~300uA of current to ground. The reactance makes a voltage divider with the resistance of your meter.
Without getting into vector calculations your meter's impedance might be roughly 50kΩ on whatever scale you used.
The phantom voltage tends toward 60vac or 120vac, depending on whether a ground conductor is in the same cable.

jamiedolan

Yes, it had a ground in the cable (12-2 romex with ground) The ground was connected / bonded.

I can look at the manual for the meter, will it tell me the input impedance? Does that mean I have a low quality meter? It is a $100 RMS meter.

It was only about 15 feet of romex also. So maybe that explains the lower voltage.

Jamie

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Jamie Dolan - Neenah, WI

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Yoyizit

For an analog meter it might say 100kΩ/volt or a digital meter may have a constant high impedance like 10 or 20 MΩ, independent of what scale you select.
The Wiggy may come in at 200Ω/volt and my $15 analog meter is 1000Ω/v and these are both powered by the voltage being measured, not like a battery powered DVM containing amplifiers.
Post a link to the specs for your meter. There is a lot of info in there, and a lot that is unsaid.