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Thread: Service call

  1. #1
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    Service call

    Went out today to a friends who lost a phase from the poco, poco came put in a bridge and gave them temporary power.

    Soon after I got a call that one outlet didn't work so I went to check and got 78v changed the breaker and all was good .

    I never got a voltage reading like that from a bad breaker usually it works or does not ag all, anyone ever come across that

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    Could the burnt up SEC have anything to do with the breaker going bad or just a coincidence

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    Quote Originally Posted by nickelec View Post
    Could the burnt up SEC have anything to do with the breaker going bad or just a coincidence

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    I can't answer (with confidence) about what caused the breaker to go bad. But I would guess you weren't getting a true reading with the 78V. It was probably "ghost voltage". If you measure it with a meter with low "Z" or a solenoid type tester I bet you wouldn't have read that voltage. You could even put a "pigtail" socket with a 75W or 100W bulb across it and then measure it. If the voltage was "true" you would see a dimly lit bulb and the voltage would read correctly on the meter. If it was "ghost voltage" the bulb wouldn't be lit and the meter would read 0V or close.
    If you aim at nothing, you will hit it every time!

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    So u think it was a back feed of some sort

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    Quote Originally Posted by nickelec View Post
    So u think it was a back feed of some sort

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    I was thinking more of capacitive coupling from adjacent conductors.
    If you aim at nothing, you will hit it every time!

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    Got ya

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    Quote Originally Posted by nickelec View Post
    Went out today to a friends who lost a phase from the poco, poco came put in a bridge and gave them temporary power.

    Soon after I got a call that one outlet didn't work so I went to check and got 78v changed the breaker and all was good .

    I never got a voltage reading like that from a bad breaker usually it works or does not ag all, anyone ever come across that

    Sent from my SM-G950U using Tapatalk
    Exact same thing happened last summer. It was actually a Square-D outdoor panel from 1977, and the 200A main breaker failed, giving 86V on one of the legs. This was on a Saturday, so I couldn't have the POCO come out to disconnect power until Monday, even though Home Depot sells replacement breakers. I knew the breaker was bad because the line side lugs were each reading 120V to ground.

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    Quote Originally Posted by jeff48356 View Post
    Exact same thing happened last summer. It was actually a Square-D outdoor panel from 1977, and the 200A main breaker failed, giving 86V on one of the legs. This was on a Saturday, so I couldn't have the POCO come out to disconnect power until Monday, even though Home Depot sells replacement breakers. I knew the breaker was bad because the line side lugs were each reading 120V to ground.
    That 86 volts possibly goes to zero if you turn off all the two pole breakers. Back feed through a 240 volt load (water heater is most common) puts that load in series with the panel bus that has open supply. Actual voltage measured will vary as load connected to the bus with open normal supply conductor changes.

    With a typical 4500 watt water heater and a loss of the supply on "L2", you end up with 120 volt input on "L1", all 120 volts connected to L1 work normally. Everything on "L2" gets 120 volts from L1, has the ~12.8 ohm water heater element in series and returns back to L2, and all the loads connected to L2 have that 12.8 ohms in series with them, the more load you are trying to supply the more voltage will be dropped across the water heater element.
    I live for today, I'm just a day behind.

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    A good double-check is to measure the voltage across line-to-load of each pole of the breaker, as you would with fuses.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  10. #10
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    Quote Originally Posted by LarryFine View Post
    A good double-check is to measure the voltage across line-to-load of each pole of the breaker, as you would with fuses.
    I live for today, I'm just a day behind.

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