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Thread: Voltage Sniffer - False Positive

  1. #11
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    N.C.
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    Re: Voltage Sniffer - False Positive

    Welcome to the forum Michael. LOL

    Roger
    Moderator

  2. #12
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    Feb 2003
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    Illinois
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    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
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  3. #13
    Join Date
    Jun 2003
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    114

    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.....

  4. #14
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    Sep 2003
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    Travelling Electrician
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    32

    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.

  5. #15
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    Jun 2003
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    Portland, ME
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    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 ]
    Sometimes I even fix something that I wasn't responsible for breaking.

  6. #16
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    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. :mad:
    Bryan P. Holland, MCP

  7. #17
    Join Date
    Jun 2003
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    114

    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.

  8. #18
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    Feb 2003
    Location
    Seattle, WA
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    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.
    Charles E. Beck, P.E., Seattle
    Comments based on 2014 NEC unless otherwise noted.

  9. #19
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    Feb 2003
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    Illinois
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    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
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  10. #20

    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.
    Michael Leavitt
    Orem, Utah
    Monday Morning Messenger

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