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Tester shows volts but circuit won't light

  1. Sep 26, 2007 #1
    I have a two wire outside light connection that the tester shows has 122 volts. It lights the neon circuit tester also. When I connect a regular 15 or 40 volt light it will not light up. Obviously I tested to see that these work fine on a different circuit. I added a two pin plug to the circuit and this shows a light on the tester that says has an open ground. Nothing will power from this either. Can you show 120 volts but no amps? What do I do next to get the light to work?
     
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  3. Sep 26, 2007 #2

    berkeman

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    Sounds like the 15V and 40V light bulbs were blown by the 120Vrms AC mains. How could you connect up low voltage lighting to an AC mains socket? Test the bulbs for resistance with your DVM to see if they are still intact.

    Or maybe you meant to say that the light bulbs were 120Vrms, 15W and 40W? In that case, still test the bulbs for their cold resistance.
     
  4. Sep 26, 2007 #3

    dlgoff

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    If your tester is indicatng an open ground, then you should check where these outside wires terminate (maybe inside somewhere?). Then check that the ground is really grounded. If so, then maybe the wire has been broken (maybe underground leading to your outside light?).
     
  5. Sep 26, 2007 #4
    This is all 120 volt no 12 volt involved. 10 years ago I connected these two wires to a three pin plug that is not grounded. I was sure to connect the line in black to the small side of the plug. The light worked for 10 years. There is another light on the circuit that works. From this light the which turns off and on with a motion sensor the two wires continue to the light that does now not work. The light at the end did not turn of and on with this motion sensor. To these two wires which i labelled correctly the black and the white I connected a three pin plug but did not connect a ground wire. So I expected the tester to say that it had an open ground. I guess my question is how can it show 122 volts at the end and not allow a light or other 120 volt tool to turn? It seems to mean that although the testers show a 120 volts, there is not enough amps to turn on the lights etc? Appreciate your help.
    PS this is my first attempt ever at using a question forum like this so I can't seem to get to all the answers again yet.
     
  6. Sep 26, 2007 #5
    Sorry also what does it mean to test the bulbs for there cold resistance. They all work on the same light connection when pluged into the black wire and white wire holes of a three pin plug. All the bulbs work in another light socket also.
     
  7. Sep 26, 2007 #6

    NoTime

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    What your test results indicate is that there is high resistance in your cable. The most likely culprit is a bad wirenut junction where this cable joins to something else.

    The high resistance will pass enough current to show voltage on your meter or light a neon bulb, but not enough to run an incadescent bulb.

    It is also possible that the cable itself has a break in one of the wires.
     
  8. Sep 27, 2007 #7
    Thank you very much. I found that the resistance in the hot line was .7 or .8 but was up to 63 in the neutral line. By substituting the neutral the light comes on. It is not a wirenut so must be a break in the neutral line. I am still digging. I am still confused as to why it showed the 120 volts. Did that mean I would not have felt it if I touched the circuit? Testers show the volts but not necessarily the potential for current flow? Did it use the ground to transmit the indicated power? Why didn't it blow the fuse if it is grounded?
    Appreciate any help to understand this. what section of an electrical manual should I seek more answers?
    Again thank you for all the help.
     
  9. Sep 27, 2007 #8

    berkeman

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    The way to think about it is the way that real voltage sources (AC or DC) are modelled -- as an ideal voltage source with zero output resistance, in series with a resistor equal to the output resistance of the voltage source.

    To pass power, the output resistance must be low (like you measured for the good wire in your cable). If the output resistance is high, then you will still be able to read the source voltage when using a high-input-impedance measuring device like your voltmeter, since there is very little input current required, so very little of the source voltage drops across the source resistance. But when you connect a power load, that load tries to draw significant current, which causes a large voltage drop across the source resistance, and you don't get enough voltage or current at the load to deliver the required power.
     
  10. Sep 27, 2007 #9

    NoTime

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    Hopefully, Berkeman answered your question.

    Since, you have to dig up the cable I would recommend that you replace it with grounded cabling rated for underground service.
    Regular house wire will get water in it corrode and fail.
    All exterior wiring should have a ground fault circuit breaker as well.
     
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