Non contact voltage tester showing positive?

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Old 01-30-15, 06:34 PM
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Non contact voltage tester showing positive?

I have a situation where a non-contact voltage tester is showing current on a conductor that should not be carrying current.

I have two junction boxes, box A and box B.



There is an EMT conduit between A and B. It had to run under the concrete slab because there is an entrance between them, and the conduit connections are at the bottom of each box. I pulled the conductor in box A and I see movement in that conductor in box B, it's a continuous run.

In box A, there are only three conductors going into this conduit.

One black conductor (HOT)
One white conductor (NEUTRAL)
One orange conductor (SWITCH LEG)

In box B, the same three conductors.
One black conductor - connected to a switch for a ceiling fan
One orange conductor - connected to a switch to power the receptacle at box A
One white conductor (NEUTRAL)

Now the mystery. The disconnected the orange conductor in box A AND box B. That conductor is NOT connected to ANYTHING. It's capped with a wire nut on both ends. I jiggle the orange conductor in box A, and I can see the orange conductor in box B shake a little.

It's not connected to ANYTHING. Yet when I turn power on and the black conductor has current, my non-contact voltage tester is picking up current in the orange conductor...on both ends.

How is this possible? My non-contact tester tests fine on other similar situations.

Is it possible this is a false positive? Or is there another explanation?
 
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Old 01-30-15, 06:53 PM
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The tick tracer will pick up induced current from nearby wires. It is always best to use a multimeter to determine the hot wires in your boxes
 
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Old 01-30-15, 06:54 PM
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How is this possible? My non-contact tester tests fine on other similar situations.

Is it possible this is a false positive? Or is there another explanation?
It's probably a slight induced voltage. Now you know why they are only for a quick test and aren't reliable for troubleshooting. These non-contact testers have been a huge money maker for the manufacturers and box stores, but they also get a lot of inexperienced DIYers in trouble. Follow your test with a multimeter and get an accurate result.
 
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Old 01-30-15, 06:54 PM
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Induction. Non-contact testers give many false positives. They are only good for doing a quick test to see if there might be a hot wire in a box. To be 100% sure you should use a multimeter.

With some experience you might be able to tell a false positive from a real one. I can even guess the voltage (120 or 277) on a hot wire by how far away I hold the non-contact tester from the wire.
 
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Old 01-30-15, 06:55 PM
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The non contact testers job is to warn of high voltage. You put it near the wires and it went off. You were warned of high voltage. That's all it's good for.
 
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Old 01-30-15, 07:07 PM
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I thought the induced voltage happens only when the system is not grounded...now the system is EMT & metal boxes so it should be grounded.

Do I need to start hunting for an ungrounded device somewhere...or this is just the reliability (lack thereof) of a non-contact voltage tester? I'll use the multimeter tomorrow, it's been collecting dust somewhere.
 
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Old 01-30-15, 07:19 PM
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or this is just the reliability (lack thereof) of a non-contact voltage tester?
They are only somewhat reliable to warn that voltage is present somewhere near where you are testing. For anyone inexperienced, they are very close to useless. They DO NOT take the place of a good multimeter. But, they are a money maker.
 
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