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Thread: low voltage or amp??

  1. #1

    Default low voltage or amp??

    I was trying to hook up an extra receptacle in my garage. Above it I found a line that I thought was live. I have a little meter that you hold up to the wire (non contact) and it makes a fast beeping fast flashing noise if it is live. It seemed to register live, but not as fast as other lines that I tested. To make a long story short, there wasnt much on the line, I could actually touch it with my bare hands and not feel a thing.

    Could it have been a 12 volt line?? the wire seemed like regular 12-2 wire, but older (the house is nearly 50 years old) It was a white wire.

    Also, the wire was actually joined in a junction box in the attic to a similar wire. BOTH wires registered as live, or almost live, even when disconnected from each other.

    There had been water damage. Could water damage the breaker box and produce this problem? or a bad ground?

    Help!

  2. #2
    Join Date
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    Noncontact voltage testers are known to false positive.
    If that wire runs close to another live wire it can pick up enough signal from it to be detected. Non contacts are good for quick tests, but I never use them to verify something is dead enough to touch. They wont pick up 12V. Most say they need about 50v to work.

    Also a good hot but bad neutral will make it alert live but it's not a complete circuit, so something such as a light will not work.
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  3. #3

    Default clarification and meter recommendation?

    Quote Originally Posted by Mr T View Post
    Noncontact voltage testers are known to false positive.
    If that wire runs close to another live wire it can pick up enough signal from it to be detected. Non contacts are good for quick tests, but I never use them to verify something is dead enough to touch. They wont pick up 12V. Most say they need about 50v to work.

    Also a good hot but bad neutral will make it alert live but it's not a complete circuit, so something such as a light will not work.
    I dont understand the last sentence. For example, if this was a load for a switch controled light, then it might get a false positive?

    Are you saying the neutral may have been broken and it is a bad circuit - bad wire??

    Is there a meter you would recommend for me? I really sort of hate disconnecting a wire nut and putting it back on, for fear of not getting a good contact and making more problems.

  4. #4
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    A non contact tester does not rely on a complete circuit to indicate. If the circuit is strong and there's no interference in the way, they can detect electricity a foot away from the circuit. They are good for a quick test but when it comes to troubleshooting or verifying a circuit is safe to touch,I put it away and get out my in-contact test equipment. I can rub a non-contact tester on my cat and it will go off the same as if I held it to a live outlet. (When was the last time you changed your cat's batteries?)

    Since you mentioned outlet, a 3 light outlet tester works great. They can detect reverse wiring and open neutrals (if the circuit has a functioning ground). They use neon bulbs which will complete the circuit being tested in various forms. Just plug it in and read the light pattern.

    You can also get a neon test light. This has 2 probes (can be used on outlets, switches, lights and loose wire) which can test any type of 120V AC or higher circuit. It relies on completing a circuit to work and is reliable for any AC circuit over 50V (up to about 600V if you want to get technical). Both of these testers cost less then $10 and can be found anywhere. Some electrical theory knowledge is needed to use this.

    A voltmeter is the most versatile test tool. It can test AC and DC circuits as well as batteries. It will tell you the voltage, not just that something is there. They can test other things like resistance and current. Some can test for continuity ( a complete circuit). You do have to know how to use it and understand electrical theory to get accurate results, and they can glow brightly inside or shoot several feet into the air if used incorrectly.. dont ask how I know that..... They cost from about $10- several hundred. You will be fine with a basic model. Harbor Freight often has a coupon for a free meter that is pretty decent for basic use.
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  5. #5
    Join Date
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    One more thing with non-contact testers. They can detect voltage in a wire that really has no voltage but is close and run parallel to a live cable. This happens in abandoned wires that are not connected. So if it is going off on a white wire, that wire could just be run close to a separate live wire and the electric field is coupling to that unterminated wire. Ground one end of the wire, and it won't show anything. I was trying to trace a bad switch box, and all the wires in it were showing as hot because they were run together and going through the same holes in studs (and there was a disconnected neutral). Had to get the meter out to figure it out.

    The other way to get 120V on a white wire is to have it be disconnected or broken at one end of a normal circuit. If you plug a load into the circuit, voltage flows from black, through the load, to the white, seeking the grounded conductor. If the connection to the grounded/neutral/return terminal is broken or disconnected, you have a no kidding 120V on that white wire but the load will not function. Touching the free end of that white wire will bite you if the load is turned on, as you are completing the path to ground. This is a common junior electrician mistake.
    Mark
    Kent, WA

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