10 VAC on open circuit

[jcoffel]

I have been a lurker for some time, but this is my first post. I have been very impressed with the knowledge base here, so naturally, this is the first place I thought to ask this question.

A friend was finishing his basement and asked me about 10 volts that is showing up at the lighting outlet of his three way circuit when it is turned off. I went to his house and verified it, but after checking his wiring and methods, can find no explanation for it. I went over the entire branch circuit and checked for open or poor neutrals, mis-wired outlets, etc. but found nothing out of the ordinary. I even pulled his switches and verified their proper operation. There is currently nothing energized on the circuit.

I've done commercial/industrial work for years and never seen (noticed) this. I don't think the issue is that it's a three way circuit, but am at a loss for ideas.

Any help would be appreciated. Thanks.

 

[iwire]

Re: 10 VAC on open circuit

This is most likely just the result of live conductors running side by side with your dead conductor.

A digital volt meter is very sensitive to this, try this suggestion from Scott T.

Put the leads of the Digital Volt meter on the conductors as before, now with some coordination put the leads from a set of wiggys on these same terminals.

The load of the wiggys will draw down this induced voltage, the Digital should now read zero.

The voltage reading of 10 volts is 'real' but it has very little current behind it.

If with both meters connected you still see 10 volts something is then strange.

 

[jcoffel]

Re: 10 VAC on open circuit

Thanks. I had a suspicion that it could have been induced, but I've had long "discussions" about such theories as applied to the systems I'm used to dealing with. I've used the argument that if you can generate power with a garbage can coiled with wire under a power line, then induced voltages are "real" even if low in a case such as this.

Am I correct to assume this is less common when dealing with my usual metal box / MC cable / metal framing member wiring methods?

 

[dereckbc]

Re: 10 VAC on open circuit

jcoffel, I agree with Bob, you are probable reading a phantom voltage. DVM's have extremely high input impedances (< 2M-Ohm), and will read any induced voltage from nearby conductors, radio stations, etc.

The method suggested will work, it just simply puts a load on the device under test. Another method is to put 100K-ohm resistor across the input of the meter, or just use an old-fashion analog meter.

Shielded circuits may prevent the phantom voltage if it is the only circuit in the raceway, and again, it may not.

To prove this to yourself, turn on your DVM, set it to the lowest AC setting, connect the leads to the meter, leave the leads open, and observe the readout as you move around.

[ March 22, 2004, 09:51 AM: Message edited by: dereckbc ]

 

[tom baker]

Re: 10 VAC on open circuit

We've been spoiled by DVM's. One advantage of the older analog simpson 260 type is they had a very high input impedance, and loaded the circuit more.
The simpson 260 was such a popular meter they came out with an DVM version in the same case!

 

[jcoffel]

Re: 10 VAC on open circuit

I used to have an analog 260. I loved that thing. The only problems I ever had were blown fuses (my own fault).

Anyway, induced voltage mentioned above was enough to set of non-contact voltage tester on my Ideal 61-702 clamp-on.

Thanks for being here for questions such as mine. Any community needs knowledge resources and leadership, and you guys are a valuable asset to our electrical community.

Jamie