Phantom Wire - Very Strange

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DanM
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Joined: Mon Aug 07, 2017 6:39 am

Phantom Wire - Very Strange

Post by DanM » Tue Dec 05, 2017 8:28 pm

So I was going to start putting in some light fixtures in the ensuite and master bedroom just now, and I tested the wires with my voltage pen to double check that they weren't live. The weird thing is one wire tested positive for AC, but it's not connected to anything! I know the pen doesn't test for how much AC voltage, or watts, or whatever, it just looks for any AC whatsoever but this seems really strange. I couldn't test if there was actual 120VAC in the wire since my wife wouldn't agree to lick the end of the wire while I recorded scientific data from a safe distance... she's not very helpful sometimes.

Anyway, what on earth might cause this 'phantom wire' to have AC in it? It's literally not connected to anything, I went in the attic and double checked and it's not pierced anywhere and rubbing against another live wire. It's just a straight piece of 14-2 from a switch to a light box. It's not just the intermittent beep the pen makes when my hand or whatever I'm touching is staticky, it's the full on constant beep of 'oh dear god no there's AC here. Danger, Danger Will Robinson.' Also, the line that would power it isn't even connected to the panel yet (that wire, and all the other stuff tests negative for AC). I'm gonna run out and get a multimeter so I can test and see how much power is actually there, but hopefully someone can explain this phenomenon to me. I'm quite puzzled right now.

I've attached pictures of the two ends, I didn't take a pic of it in the attic but it just goes up the wall from the switch, over a few trusses, and down into the light box.
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Aaron
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Location: St. Paul, MN

Re: Phantom Wire - Very Strange

Post by Aaron » Tue Dec 05, 2017 8:55 pm

Those buzz sticks can sometimes give you false positives, they pick up energy in the wire/cable inductively to notify you of the likelihood of live current. But what it may pick up instead is residual inductive energy that is running on a live cable stapled alongside it that bleeds into the cold cable you're probing. It's really a tool that I have found to be sort of an ambiguous nuisance in many cases.

The only certain way to know whether the cable is live or not is to use either a volt meter or a simple two-lead tester tool with a lamp that lights up when you actually tap power from the cable. But because your cable is sheathed, it's not possible to test it that way.

Unless you know beyond any shadow of doubt the cable *must* be dead, please turn off the power at the panel just to be sure. Because perhaps that cable *is* live because you simply just forgot you connected it at the other end. (This has happened to me LOTS. I simply just forget.)

It's also very good practice, at rough in, to physically terminate cable in junction boxes right away. Push through 6" of length, leave a loop of cable slack outside the box and secure it nearby with a staple. Then secure the clamp inside box. Then strip the sheath off from within the box while you already know the cable is already dead.

DanM
Posts: 129
Joined: Mon Aug 07, 2017 6:39 am

Re: Phantom Wire - Very Strange

Post by DanM » Tue Dec 05, 2017 9:37 pm

It must be the pen giving a false positive. I ran out and bought a multimeter and it's reading no voltage. And I know with 100% certainty that the cable isn't powered. I call an electrician in to do the panel stuff since I'm not that confident, so it's definitely not connected to anything. I checked for any physical connections, in case I was dumb and connected it to something it shouldn't be, but it's basically a loose length of wire not attached to anything at either end.

And yes normally I would put it in the switch box at rough in, but there's an existing live wire in the box currently so I left both ends out for now just so it didn't accidentally become live while the other end was hanging out of the fixture box. I was going to setup the fixtures, then switch the breaker off for the old wire that currently powers the light and tear the old wire out, connect my new wires to the switch in the existing box, then get the new circuit connected to the breaker.

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Aaron
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Location: St. Paul, MN

Re: Phantom Wire - Very Strange

Post by Aaron » Tue Dec 05, 2017 11:15 pm

Ok, sounds like you know what you're doing then. Carry on and stay safe!

MindBender
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Location: Vancouver Island, Canada

Re: Phantom Wire - Very Strange

Post by MindBender » Thu Dec 14, 2017 7:54 pm

Here is what I learned about those testers... if the wire is physically disconnected but still runs next to live wires you will get false positive. It become like an antenna.

However, if the wire is still connected buy power has been turned off, the tester will work great.

This had me scratching my head until I figured out this behaviour... it maybe the ground and neutral that prevent false results...

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Aaron
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Location: St. Paul, MN

Re: Phantom Wire - Very Strange

Post by Aaron » Fri Dec 15, 2017 2:41 am

Yeah basically. It picks up inductance which electrically is similar to RF.

At least it errs on the side of caution with a false positive rather than a false negative. If it’s more likely to alert than not, then hopefully its user will be that much safer and double check.

On the tone injector and inductive pickup tools that low-voltage installers and telecom guys use, there’s a little dial to adjust the sensitivity of the pickup tool so you can adjust for false positives.

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