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Re: 34 volts, induced? [Re: gfretwell] #176109 03/20/08 02:50 PM
Joined: Aug 2001
Posts: 7,520
pauluk Offline
I think that the proliferation of digital meters with their umpteen-digit presentation has resulted in lot of people losing track of the difference between precision and accuracy. I've seen some real cheap DMMs (made you-know-where) in which the accuracy on A.C. volts is something like 4% plus 2 counts -- Worse than a good quality analog meter with a conventional d'Arsonval movement -- at least at the upper end of the scale.

I still have a Simpson 260 which I like to use, along with an old Avo 8, not to mention several lesser meters. I also have an old BPL (British Physical Labs) SuperRanger, which with a basic sensitivity of 1000 ohms-per-volt is much less susceptible to "phantom" readings on power work.

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Re: 34 volts, induced? [Re: pauluk] #176111 03/20/08 05:10 PM
Joined: Dec 2002
Posts: 32
wiking Offline OP
Junior Member
You guys are awesome. It's great to have this kind of resource of wisdom and experience. Thanks.

Re: 34 volts, induced? [Re: wiking] #176125 03/21/08 12:27 AM
Joined: Oct 2007
Posts: 56
SP4RX Offline
I learned about this the hard way a few years ago. I was also troubleshooting a 3-way situation and was using a voltage sensor to figure out what was live and what wasn't. The sensor, however, had a sensitivity adjusting dial on it which must have been set a little too high. It was indicating live wires which had no way of being live. I know this because I had disconnected the wire at both ends of the pipe run and verified that it was the same wire at both ends.
Mystified at this point, I got out my DMM and took a "real" measurement which was around...30-some volts. It seemed to me that the voltage pretty much had to be a result of induction from the other conductors in the pipe.
This phenomenon can really leave a guy scratching his head for a while until he figures it out.

I agree with wiking, it really is great to have access to the kind of wisdom available here.


Re: 34 volts, induced? [Re: SP4RX] #176130 03/21/08 09:04 AM
Joined: Oct 2002
Posts: 482
Zapped Offline
I find this effect especially prevalant in old, cloth wrapped wire (circa '50s?). Used to drive me nuts when I was starting out. An inductive "sniffer" even lights up on an unconnected conductor at times.

Re: 34 volts, induced? [Re: Zapped] #176210 03/25/08 05:20 PM
Joined: Mar 2004
Posts: 391
BigJohn Offline
The reason I gave up my Vol-Con Elite was because of false-positives on de-energized wires. Same situation as a lot of the ones here: A 277 volt three-way I was trouble-shooting; wires run in pipe. Kept getting strong 120V readings on some of the wires. All the wires are the same color so I'm cussing the original installer thinking I have two different voltages in the same box with the same wire colors. Can't even remember how I finally figured out I was reading induced voltage on de-energized travelers, I just know it took me a long while to do it.
I never really liked the term "phantom voltage" because thinking it isn't really there could turn you into a ghost!
I agree. Went to T/S a light a helper was working on. We're getting approximately 120V on a neutral wire. Being the studious guy, he suggests "phantom voltage", and I was inclined to agree. The neutral was in a conduit about 150' long with about fifteen other conductors. Normally ghost voltage is nothing, but in that case it would've been more than enough to knock someone on their butt. Even with two wiggys on the wire, there wasn't enough of a load to drop the reading below 100V!


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