electrical oddity

[JamesW]

Hi All,

I ran into an oddity I can't quite figure out.

A tenants central air conditioner would not turn. Put a digital voltmeter on the connections -- got 0 volts. Traced the wires to the fuse box and pulled the cover.

Testing the 220V circuit breaker -- I read between one output lug of the breaker and the ground lugs. It read 120 volts. Tested the other output lug and read 120 volts.

Then I read between the two output lugs expecting to see 220 volts but there was nothing. I changed the breaker, flipped the switch and the A/C fired right up.

Why was I reading 120V on each lug but nothing across the two hot legs?

[trader_4]

I can only think of two things:

Somehow the breaker was defective and inside the output side had the two legs shorted, but only one leg was connected through the breaker. Which seems very unlikely, but then the AC wasn't working either, and then was, so maybe it is possible.

Operator error when testing, eg not a good connection, etc.

[dpb]

Third alternative -- the digital meter picked up phantom across the two legs to ground but was enough leakage internally across them to show true reading. An analog meter or with a load in parallel would probably have shown zero for those, too.

The OP noted the AC ran _after_ replacing the defective breaker, not with it.

[Ralph Mowery]

It was a good possiablilty that the voltage/current was going out on one good leg/lug, then through some 240 volt device and back to the other output lug. That output lug would be open. I bet if you put the voltmeter across the other side (hot side) of the breaker you would have shown 240 vlots. Also if you put the meter across both the line and load side of the breaker you would have shown 120 or 240 volts. Don't recall which off the top of my head.

[micky]

The full 120 volt reading is unusual, afaik, for phantom voltages, but you shoudl still get yourself a meter with a needle, even the cheapest one to check for times like this. Digital meters definitely will show high voltages even where the available amperage is just about zero. Analog meters have a load in them, a lower input impedance, that will use up the avaiable current.

[gfretwell]

You were seeing the same 120v twice. With one open leg on the breaker, the other side was feeding through the load to the open contact. This is not a phantom voltage thing. It is real voltage with plenty of ma to kill you.

[Clare Snyder]

Even a home-made test light would tell the story.

[JamesW]

You folks are the best! Great answers!

I forgot to mention in the original post, the other 220v circuit breakers all read 220v across their breaker output terminals. This one circuit breaker was the only one to give a zero reading across the two output terminals.

[gfretwell]

Exactly what I would expect to see if one pole of the breaker was open. Did you try cycling the original breaker a couple times?

[trader_4]

We have a winner! I was picturing this with just the breaker, no load. With a load, what you describe is what it was.

[gfretwell]

The transformer in the air handler is enough load to get you ~100ma on the open side. Even in series with a 100w light bulb, it would still be glowing yellow. A little C7 would be almost full brightness.

[invalid unparseable]

You can also get a digital meter with the Low Z setting to eliminate ghost voltage readings.

[dpb]

On 5/22/2020 3:45 PM, [e-mail] wrote: ...

In re-reading I see I misread original post -- I thought he had said he had disconnected the load or that the load was isolated...if that isn't so as re-reading makes it seem likely, then feedthrough is the culprit indeed...

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