Electricity is out in a bathroom

Discussion in 'Electrical Forum discussion & Blog' started by milik, Dec 25, 2007.

  1. milik

    milik New Member

    Messages:
    2
    One of the lights is out in a bathroom. When switch is in off position I can measure 67volts across switch when it is in on I measure 0 volts. I can measure 120v across circuit breaker. and is not tripping.
  2. 480sparky

    480sparky In the Trades

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    Measuring across the two terminals of a switch is meaningless. What is the voltage between each terminal and ground when the switch is on? If they're both 120, the switch is OK, and you need to look elsewhere.
  3. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

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    It's not totally meaningless, but darn near. In this case, he's dropping zero volts across the switch when the switch is ON, which is normal, but the light should be lit. However, 67 volts across a (presumably) open circuit (when the switch is OFF), is definitely NOT normal (should be 120), so something ain't right. (I'm assuming a high-impedance voltmeter here.) You can quickly test the switch by a) turning off the breaker, b) removing the switch, and c) jumping the 2 wires together, to simulate an ON switch. When you turn on the breaker, the light should light. How old is the house, wiring, light, and switch?
    Last edited: Dec 25, 2007
  4. Bob NH

    Bob NH In the Trades

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    If you are measuring 120 Volts across the circuit breaker it is tripped or failed open. A breaker that is closed will have ZERO volts across it.

    Perhaps you mean 120 Volts between the breaker output and neutral. If that is what you mean then the breaker is closed as it should be if not tripped.

    When you measure something like 67 volts at a circuit with a digital meter it is ambiguous and often caused by an open circuit (switch off in your case) combined with the characteristics of the meter. Digital meters draw very little current (microamps of less) and the induced voltage on the wire is often enough to cause a voltage indication. If you connect an incandescent bulb to the circuit (turn on the switch in your example) and measure the voltage you should measure zero volts if the bulb doesn't light when the switch is on. That means you really have no power on the circuit.

    If the output of the breaker is 120 Volts relative to the neutral bar then you have an open circuit (disconnect) somewhere after the breaker.

    Try moving the breaker to the OFF position and then to the ON position and see if it works.
  5. milik

    milik New Member

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    2
    Update

    Here is an update.
    1) Switch is fine. replaced it with new same problem.
    2) Circuit breaker is OK.
    3) Light fixture is disconnected So it's not it.

    I'm thinking if perhaps wire is damaged somewhere and sort circuiting. But not sure how to check for that.
  6. 480sparky

    480sparky In the Trades

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    149
    If the light fixture is disconnected, then there's the problem.

    Or are you saying there's a short somewhere after the switch?
  7. BrianJohn

    BrianJohn DIY Senior Member

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    VA
    Actually. A breaker that is closed WITH NO load connected will have "0" volts across it. Utilizing a multimeter a circuit breaker that is closed with a load connected will have some voltage drop across it. Though this voltage is very low (hopefully), in the millivolt range. The millivolt/FOP*/VD* test is a common test to trace, repair and/or verify repairs of thermal discrepancies.


    *FOP Fall of Potential
    *VD Voltage Drop
  8. hj

    hj Moderator & Master Plumber Staff Member

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    power

    That is a good indication that the light is being fed from a GFCI outlet, which has tripped. The low voltage is the feedthrough from the GFCI's circuitry.
  9. kd

    kd New Member

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    207
    Milik, you need to buy a rubber standard light bulb socket with the two wires sticking out, or you could use a porcelain socket and attach a black and a white wire. Screw a bulb of about 25 watts or so into the socket. Turn off CB. Connect bulb to hot and neutral at the switch. Turn on CB. Does it light? If so, go to light outlet. Do same test after hot wiring the switch (connect two black wires to switch together). If no light. Check inside both boxes for broken wires. Then test continuity of wires running from switch to light outlet. The neutral may be fed from a different box. Do not get into replacing wire inside the wall until you are sure that that is the problem. Do not get frustrated and try to use the bare grounding wire as a neutral=Code violation=hazard.
  10. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

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    HJ, can you elaborate? I've never seen a diagram of the internals of a GFCI breaker or outlet, but I thought tripped meant tripped -- where does the "feedthrough from the GFCI's circuitry" come from?
  11. Mikey

    Mikey Aspiring Old Fart, EE, computer & networking geek

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    That's why the pros get the big bucks...
  12. hj

    hj Moderator & Master Plumber Staff Member

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    Location:
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    Gfci

    The internal circuitry of the GFCI is using the light's neutral, which is continuous with the "hot" wire at that time and he is measuring that power in a series setup. When he opens the switch the neutral "power" is interupted and then the potential difference occurs at the switch. It could also indicate a broken neutral to the GFCI, with the same effect..
  13. Alectrician

    Alectrician DIY Senior Member

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    689
    Need more detail to trouble shoot this over the net.
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