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The other day one of the phases of our power faulted and the fuse tripped on the power company side. I measured the voltage at 60-65V. Lights were flickering, and some devices kept running (like a dell computer, it was perfectly happy at 60V).

How does the power back feed from one phase to a 'tripped' line that should read 0V?

Both lines are regular 120V AC mains lines in northern America.

Glorfindel
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Voltage Spike
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    If this is a split-phase installation, I'm wondering if it was leaking through any 240V appliances, like a clothes dryer or oven. – Hearth Sep 16 '19 at 21:37
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    I agree with @Hearth. I suggest unplugging all the 240V appliances and check whether the problem goes away. – joribama Sep 16 '19 at 23:49
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    60 V is half voltage makes me think a distribution transformer is getting half voltage on a primary. Line-to-ground fault? What does the transformer feeding your home look like? One hot or two hots? –  Sep 17 '19 at 02:07
  • If it's coming through a 240V appliance, as @Hearth suggests, it's a common-enough problem to have a name: it's called a "backfeed". – Pete Becker Sep 17 '19 at 12:00
  • @Hearth, how does 240 V leak if the utility fuse has blown? You shouldn't get *any* voltage at the home. Isn't the utility fuse on the primary with the secondaries protected by the home's main circuit breaker? –  Sep 17 '19 at 13:56

2 Answers2

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Most all residential power feeds in the USA are 120/240 Vac split-phase. You have two hot wires (L1, L2) and a neutral coming into your house or apartment or trailer. After passing through your power meter it goes to your breaker panel. It is constructed so 240 Vac appliances connect to L1 and L2 which are 180 deg out of phase. 120 Vac appliances connect to either L1 or L2 in a hop-scotch pattern that helps distribute and even out the loading placed on L1 and L2.

This also helps keep return currents in neutral small enough to be safe.

Because 240 Vac appliances literally pass current to/from L1 and L2, if L1 or L2 fails due to outside lines being cut or a burned out main breaker, the disrupted lines can have some voltage on it, known as backfeed.

To test for this condition just use the breakers to turn off all 240 Vac appliances. If the problem goes away then you have a burned-out main breaker (replace) or a phase wire from the utility is open (call utility service).

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Your dead phase is backfeeding via your 240V appliances.

If your 240V appliances had 0 resistance, your dead-phase side would light up at full voltage.

As things are, the 240V loads are in series with the 120V loads on the dead phase. That is causing the voltage to settle out where it is. You can manipulate that by turning on 240V resistive loads or turning off 120V loads. (I wouldn't run a motor load under these circumstances.)

Keep in mind that your water heater is going to work anyway, so it will be cycling on and off. That is to say, when it cycles on, it will parallel itself with the other 240V loads if any. It will use its share of the power to warm the water. Most water heaters are well enough insulated that the water heater will make progress even at 1/16 normal heat output (60V instead of 240V), and eventualy it will reach target temperature and shut off. You can't do anything about this, except waste hot water.

There's an earlier edit where I talk about phantom voltage. It's wrong here.

Harper - Reinstate Monica
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    @DanNeely in the penultimate paragraph (currently) Harper says "You can't do anything about this [the hot water working anyway], except waste hot water." I say you can - you can turn it off at the breaker. – Chris H Sep 17 '19 at 14:08
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    @ChrisH they are in the US too. If I'm understanding the original question correctly the fault wasn't between a major appliance and the panel, but between the panel and its source (either the main panel if it was a sub-panel or grid if it was the main panel). – Dan Is Fiddling By Firelight Sep 17 '19 at 13:50
  • Don't you have separate breakers serving your water heaters? Here, high load appliances (ovens, electric showers, water heaters) are on dedicated circuits – Chris H Sep 17 '19 at 08:31