Double pole voltage reading

[102 Inspector]

So last night my brother in law calls about a circuit breaker. He was getting 120 volts off each terminal, however no reading when crossing both terminals. This went to the water heater and he said it felt warm a couple days ago. He checked the busbar to the breaker slots and got 240 volts, but still did not get and reading when going through the breaker assembly. Why would the breaker read 120 volt off each terminal and no reading when crossing over both. Simply looking for explanation for my interest. Thank you. Also, I was not there and only going by phone conversation.

 

[jimdavis]

One pole of the circuit breaker is open / failed, creating an open circuit when you try to read the line-to-line voltage. The 120 volts that you read on that terminal is feeding through the water heater element from the other pole of the breaker and back to that terminal.

 

[iwire]

I agree with Jim

 

[102 Inspector]

That makes good sense when you actually stop and think about it. Since I was not there doing the readings, it just seemed very strange the way he was describing it. I assumed he had disconnected the load from the breaker before reading, but probably did not since he was frustrated. thanks for the explanation.

 

[Jraef]

The other possibility, if he just installed it himself and it has never worked, is that he used a "twin" breaker. Those are not the same as a 2 pole breaker in that they only have one stab, so there is only 2 circuits off of the same leg.

But if it was working correctly and no longer is, then the breaker has failed as mentioned, or someone tried to "roll their own" two pole breaker by using 2 one pole breakers and a handle tie bar. When one breaker trips, it often does not flip the other breaker off, so he just has to reset the tripped one (after determining why it tripped).