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I lost one phase of power yesterday, so only half of my circuits are working. We've had heavy windstorm where I'm at and I'm trying to diagnose the issue. I'm assuming there is a loose connection at my overhead service line. Power company is supposed to be coming out eventually but they are quite busy with all the outages.

When I measure line-to-line it should read 240V, but it reads 2V. When I measure each phase to ground they are 120V for the good phase and 118V for the bad phase. Why am I getting a reading of 118V?

I shut off all the circuits to the bad phase. If I turn on some of the circuits, the line-to-ground voltage of the bad phase drops. If I measure voltage on some of the circuits that are turned OFF from the bad phase, I am getting up to 20V.

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  • Did you measure line to neutral?
    – crip659
    Nov 6, 2022 at 13:24
  • Yes, Line to neutral is what I have been measuring. I measured from the main breaker where the service wires land, and the lug where the neutral wire lands. Also both sides of the bonded neutral bar which gives the same reading.
    – APP
    Nov 6, 2022 at 13:28
  • What you are seeing is exactly what one would expect with one phase dead.
    – Hot Licks
    Nov 6, 2022 at 13:53

1 Answer 1

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Turn off your main breaker and re-measure. With everything still connected, you are going to get confusing readings.

Let's say to have two phases, A and B, 120V to ground each, 240 phase-to-phase. You've lost phase B.

Compared to the high impedance of a digital multimeter, household loads are practically dead shorts. The schematic of the power company and your house now looks like this if you have loads still connected. Phase B is connected to Phase A through a tiny resistance, so it's not dropping any voltage. It's basically equal to Phase B, so you see ~120V B-N but ~0V A-B.

Power  |                           |                           |
Co     |                           |                           |
       |                           |                           |
-----  |                           |                           
       |        120V loads         |         120V loads        |
House [A]----[tiny resistance]----[N]----[tiny resistance]----[B]
       |                                                       |
       +-----[tiny resistance]---------------------------------+
                240V loads

Shut off your main breaker, and you'll see 120V A-N but ~0V A-B and B-N.

You should turn off all of your 240V circuit breakers until the power company fixes this outage. Until it is repaired, all of your phase-B circuits are being powered through whatever 240V loads happen to be connected at any given time. That's not good for anything involved, as it can cause wildly varying voltages.

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  • Thanks for the explanation! I did turn off all circuits connected to B-phase.
    – APP
    Nov 6, 2022 at 14:22
  • Great answer! @Buzz 240V circuits are connected across A and B phase, so need to be shut off also. One leg going dead does not take them out of the game. In fact often water heaters will regain temperature simply by being in series with the 120V loads on the dead phase. Then the thermostat cuts it out, the 120V loads go stone dead and the homeowner is "WTH just happened?"
    – Harper - Reinstate Monica
    Nov 6, 2022 at 21:57

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