False negative, non-contact voltage tester, tick tracer

[mikeadams2]

We had a situation in which a tick tracer/voltage tester indicated that there was voltage at the switch for a furnace when the switch was turned off. When the switch was turned on, the voltage tester indicated that there was no voltage. I just can't remember how it was wired. It was an old switch. I'm racking my brain as to why the voltage cancelled out. Could have been 240v or 120v. Anyone ever run into this?

 

[kwired]

Neutral being switched?

What exactly were you testing, conductors attached to the switch itself? If so switched neutral could very well do that.

 

[K8MHZ]

Kwired has probably the best answer.

Tick tracers are great, but you need a good brand (My favorite is Greenlee) and you need to know the nuances of the tester. It makes a difference if you touch the wires being tested, and where you touch them. It takes a while to learn exactly what your tick tracer is telling you.

I suggest that until a person learns the quirks of both tick tracers and DVOM's to do troubleshooting, they use a solenoid tester. My Ideal tells me if there is current, what the voltage is, if it's AC or DC and, if there is no power, it's a continuity tester. All with no buttons or controls. It just works.

 

[mjc763]

I have found you get a more accurate reading if you hold your finger on the top as your touching the wire. This will prevent most of the false positives.

 

[PetrosA]

I'm going with Kwired's answer too.

I've had a few tick tracers over the years, and I don't think even the worst ones ever gave me a false negative. I use Fluke tracers now exclusively for the last 5 years or so and have never, ever had a false negative.

That being said, there are situations were a tick tracer will show no voltage where it does actually exist, but it's not a false reading - Romex with wet paper inside will not light a tick tracer, nor will any kind of MC, AC or BX cable unless the jacket is hot. You need to know enough to be aware of these traps.

 

[kwired]

I'm going with Kwired's answer too.

I've had a few tick tracers over the years, and I don't think even the worst ones ever gave me a false negative. I use Fluke tracers now exclusively for the last 5 years or so and have never, ever had a false negative.

That being said, there are situations were a tick tracer will show no voltage where it does actually exist, but it's not a false reading - Romex with wet paper inside will not light a tick tracer, nor will any kind of MC, AC or BX cable unless the jacket is hot. You need to know enough to be aware of these traps.

You need to be a little aware of how these testers work, they detect capacitive coupling. Wet paper inside NM jacket makes for a poor capacitor and you don't get the same thing you do if that paper is dry. Grounded objects in close proximity also can alter expected results. OTOH a lack of grounded objects can make everything appear to be "hot". This makes such testers work great for testing for open equipment grounding conductors, inspectors around here use them to test for grounding of switch yokes and at luminaires without even removing any covers. If it doesn't light up they move on, if it does further investigation is warranted. They sometimes do give false reading for reasons not worth spending too much time figuring out why it did give a false reading.

 

[mikeadams2]

Thank you so much for the responses. My best guess is that it was a switched neutral. The tick tracer may have been picking up stray voltage at first and then stopped for whatever reason. It is a Klein that beeps slowly for anything under 50v and rapidly for anything over. It was beeping rapidly but again, it was an old house and I'm sure there was stray voltage kicking around. I was holding it up to the conductors on the switch, that I'm pretty sure were in an old BX jacket. Thank you again. I must have been over thinking it

 

[PetrosA]

A switched neutral isn't "stray" voltage. A "neutral" wire returning from a load to a switch is carrying whatever system voltage you have. It will knock you on your butt if you touch it and you're grounded. Consider it an extension of the hot wire through whatever resistance the load offers which will limit the amperage it can deliver, but not the voltage.

 

[wyreman]

tic = widow maker
best brand is hioki by far

 

[wyreman]

stray voltage is when you throw a roll of cable in the attic and pull down one end and measure across a pair and you get some kind of digital numbers
and your klein tic turns red

then you could get out a magnet wiggy
or, this if you're feeling brave:thumbsup:

 

[kwired]

A switched neutral isn't "stray" voltage. A "neutral" wire returning from a load to a switch is carrying whatever system voltage you have. It will knock you on your butt if you touch it and you're grounded. Consider it an extension of the hot wire through whatever resistance the load offers which will limit the amperage it can deliver, but not the voltage.

Has full voltage to ground when the switch is open on the side connected to the load. Close the switch and it is at ground potential, plus any voltage drop present, short conductor run and this VD is nearly zero.