Voltage Sniffer - False Positive

[Michael_Leavitt]

The following question was forwarded to me by a fellow inspector. Please read and offer any facts, educated guesses or speculations. The real question is not where the voltage came from, since there was none, but what could have been throwing the voltage sniffer technology into disaray?...

I had something weird happen today. There were three wires hanging down from a beam. I put my voltage detector (the C898 sold in Professional Equipment) and it indicated they were hot. A few minutes later, the owner comes down and points out that the other end of the wires were hanging down from the other end of the beam. So now I look pretty silly. I went and got my other voltage pencil (Circuit Alert GVD-504A) and checked them again, and again it beeped and lit up.

So what's going on? The wires were not connected to anything electrical. Any insight would be appreciated. I wasn't impressing my client...

[ June 16, 2004, 06:37 PM: Message edited by: Michael_Leavitt ]

 

[ryan_618]

Re: Voltage Sniffer - False Positive

Did you try it with a meter?

 

[Michael_Leavitt]

Re: Voltage Sniffer - False Positive

The person who sent me the question is a home inspector and did not carry with him any kind of digital meter. He had a two light neon tester, a three prong tester, and his voltage sniffers. Knowing this, and knowing that this is all that a majority of the independent home inspectors carry with them I am wondering what was lighting his sniffers. Of course the residential inspector?s role is not to technically evaluate the residential electrical system. But we need to know the limitations of the tools that we are using every day. I use these sniffers daily while in partially finished basements and attics to help me identify where the wiring is live and where it truly has been aborted..... So what type of things make a voltage sniffer act like there is actually voltage present?

 

[bphgravity]

Re: Voltage Sniffer - False Positive

I would personally check the batteries and contact the manufacturer for assistance. They may have some data on false-positive reports on their product.

Did this "fellow inspector" perform any tests on the tester itself to see if it is operating properly otherwise?

I also wouldn't trust the owner's word, or anyone elses for that matter on where the other end of the wires are and if they are connected to anything. Is there a possibility there is a junction box in between the two open ends that has an energized source init? There are just too many possibilities.

 

[jimwalker]

Re: Voltage Sniffer - False Positive

I have a suggestion but you won't like it.Let real electricians do electrical testing.What he read was stray voltage.This just goes to prove his qualifications.Why are people paying good money for unskilled people ? Qualified inspectors would have handled this.
Lets look at it this way,most counties and cities require an inspector to at least passed a masters test,why should we exspect less from him ?

[ June 16, 2004, 07:37 PM: Message edited by: jimwalker ]

 

[roger]

Re: Voltage Sniffer - False Positive

Michael, Bryan is right and to add another possibility, are these conductors in close proximity to other electrical apparatus i.e. flourescent lights, fans, A/C equipment, another cable wraped around its length, etc...?

If so there is a slim possibility of some induction on these conductors.

Roger

 

[jimwalker]

Re: Voltage Sniffer - False Positive

exactly my point.induction.Hot sticks should not be mis used.They indicate possible live voltage not actual voltage

 

[roger]

Re: Voltage Sniffer - False Positive

Well, if thats what you meant, why didn't you just say that and save some key strokes? :roll:

Roger

 

[jimwalker]

Re: Voltage Sniffer - False Positive

sorry thaught i was clear on first post.I use hot stick before i use klines but never assume voltage just because it says it is there.$10 checker saves $25 klines and shock other than that dont trust them

 

[Michael_Leavitt]

Re: Voltage Sniffer - False Positive

Thanks for the replies. I guess that we will have to ignore Jim?s response due to a superiority complex .. Please keep in mind that home inspectors are a front line form of marketing for licensed electricians. When we find miswirings we immediately get our clients convinced that they need to give the master electricians the big bucks for troubleshooting and repair.

Here is another great response from Douglas Hansen that helps me understand the weakness of the tool in question.

Michael,
It helps to remember that these "voltage" detectors don't necessarily go off from the presence of a source of line voltage - they go off when they sense something acting as a capacitor. They tell you if the metal you are holding it to is ungrounded.

In general, I like the red detector that PE sells (don't know if it is the model number you cite below) because it is not supposed to go off at low voltages. The Fluke detector is rated 90V to 600V. The little yellow one that chirps, the Extech, and the Greenlee are all too sensitive for my tastes. Sometimes you can hold them in an attic with knob and tube wiring and the magnetic fields off the wire will set them off even when you are several feet from the wiring.

My guess is you had some kind of uncancelled magnetic field, either from knob and tube or from a subpanel with a bonded neutral, and the loose wires were just a good capacitor to help detect it. If you had grabbed onto one of the wires, your voltage sensor would have shut off. Of course, that isn't going to be your intuitive response when the darn instrument is telling you the thing is live!

Take your tester and try it on the metal frame of your toaster. With the toaster off, the tester should register nothing. With the toaster on, the metal frame should light up your tester. It's not telling you the metal frame is hot - it's telling you it's ungrounded.

DH

 

[roger]

Re: Voltage Sniffer - False Positive

Welcome to the forum Michael. LOL

Roger

 

[don_resqcapt19]

Re: Voltage Sniffer - False Positive

Everyone also needs to know that these non-contact voltage testers also produce false negatives in some cases. They should never be used for safety purposes.
Don

 

[cs409]

Re: Voltage Sniffer - False Positive

well what did we learn in this post! 1st, some testers are correct and some are not. some people know how to use them and some dont. and most of the time we have no idea why the tester didnt work or why it worked when it shouldnt work. so tomorrow someone will use a tester and darn, they will get a weird reading.......... some will understand why some want, but one thing for sure, as with the above post, no one will really know the answer why! but as always, some good ideas, thoughts, will show up as usual.....

 

[gvmv]

Re: Voltage Sniffer - False Positive

I came in contact with this last week when we were pulling out old circuits in some Walker Duct. My greenlee voltage tick would show there was voltage but my colleagues Klein one would not. From moving things around it seemed mine would pick up the slightest field coming from other wires. After some testing out of curiosity I found that the greenlee will say there is current in a wire if you are testing it within about 6 inches of a wire that has voltage in close proximity.

 

[big john]

Re: Voltage Sniffer - False Positive

"Hot sticks" (non-contact voltage sensors) are relatively good for determining when the power is off, but:

</font>

• <font size="2" face="Verdana, Helvetica, sans-serif">They won't work on direct current.</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">They won't work on shielded conductors. So testing armored-cable, metal-clad cable, or conductors in metallic conduit is out of the question.</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">They might not work on twisted conductors. For example, the hot stick might have a difficult time detecting voltage in three-conductor NM, because the conductors are twisted together inside the jacket.</font>

<font size="2" face="Verdana, Helvetica, sans-serif">While false negatives aren't very common, I still tap the conductors with the back of my hand before I actually grab them with my fingers, just to make sure they're dead.
EDIT: The above statement was made because it suggests the possibility that a hot-stick will fail detect voltage. Always use the common sense and a proper tester when trying to determine if a conductor is energized.

Though, because of capacitive and inductive coupling, false positives are rampant. If you ever hook an oscilloscope to any de-energized conductor in a building, you'll probably get a 60 Hz signal, because at some point, somewhere, it's coupled to a live line. Hot sticks are pretty sensitive to this, and are useless for trouble-shooting because of it.

-John

[ June 17, 2004, 05:00 PM: Message edited by: big john ]

 

[bphgravity]

Re: Voltage Sniffer - False Positive

Originally posted by big john:
...While false negatives aren't very common, I still tap the conductors with the back of my hand before I actually grab them with my fingers, just to make sure they're dead...

I think this is a very poor statement to make. This is certainly not an "OSHA APPROVED" method for determining de-energized circuits. If this is a practice you perform, I suggest you keep it yourself and not share with persons of unknown experience or training that are frequently visiting this forum. I feel the responsible thing to is edit your post and delete this statment. It serves no purpose and suggests and unsafe and quite frankly, a deadly practice.

 

[cs409]

Re: Voltage Sniffer - False Positive

Don't edit your post,,,

"Not saying dont" but make sure you do proper test to make sure all is off.....

BUTT

Any one grabbing/touching any exposed wire with his/her bare hand or any conduit, or J box and or panels etc verses tapping with back of hand first ,,,is asking for trouble. No matter how its been tested! PERIOD.

 

[charlie b]

Re: Voltage Sniffer - False Positive

I disagree with you on this one Bryan. Please look at it again, carefully and in context. Big John is not saying that touching a circuit is his method of detecting the presence, or absence, of voltage. He is saying that he has already done the usual and appropriate steps to remove voltage, and he is ready to start work (grab them with my fingers). By that time, all should already have been made safe.

What he is doing is taking one final, extra, not-necessary-but-why-not-do-it-anyway-just-for-added-safety step. If you touch a live circuit with the back of your fingers, there is a good chance (though even this is not certain) that the current will cause your fingers to form a fist, thus pulling them away from the source. I would call it an acceptable practice, but here again, only on the condition that you take the usual and appropriate steps to remove voltage first.

 

[don_resqcapt19]

Re: Voltage Sniffer - False Positive

Charlie,
I agree with Bryan. If the only test equipment that has been used is a noncontact tester, then the wires in question must be considered hot and not be touched. No one should ever trust their safety to this type of tester.
Don

 

[Michael_Leavitt]

Re: Voltage Sniffer - False Positive

Thanks to everyone who responded. It was a real pleasure tapping into your knowledge base. The more I learn, the more I realize I don?t know. Thanks again for helping me in my quest.