240 V circuit now delivers 120 V

[Roger Spendlove]

My water heater stopped working, and I've diagnosed that the wires coming out of the wall are supplying only 120 V, instead of the 240 V that the heater needs. The breaker is a double-pole 30Amp breaker.
My question: is it possible that half of this breaker has failed, and that simply replacing the entire breaker will allow the circuit to resume supplying the 240 V that it was providing before?

 

[Reach4]

First confirm that you are measuring the voltage between the two hot wires rather than one hot wire and a ground. Each hot would show 120 V to ground.

My question: is it possible that half of this breaker has failed, and that simply replacing the entire breaker will allow the circuit to resume supplying the 240 V that it was providing before?

I think you want to take voltage measurements at the breaker.

 

[Sponsor]

 

[LLigetfa]

I've diagnosed that the wires coming out of the wall are supplying only 120 V

If the two wires are lifted from the load and the voltage measured, it should not be possible to read 120V regardless of the state of the breaker. The only way to read 120V is to a ground reference unless a mains fault into the panel is affecting more than just the water heater.

 

[Jadnashua]

When measuring voltage, it's all about what you use for a reference. A somewhat off the wall example...I used to have to repair high voltage power supplies for a magnetron tube...the filaments used 5vdc, but the plate ran at 5,000vdc. If one side of the filaments was actually ground, there would be a 4,995v difference to the other end of the filaments. This would almost certainly cause arcs and mess with the operation. So, what really was going on was one side of the filaments had 5,000vdc on it, and the other had 4,995v, or a 5vdc difference. You have to know where to measure, and what it means, when measuring any voltage on anything.

Your home has 240vac coming in. You get 120vac by referencing the middle of the transformer to each outer end separately. If you try to measure to ground on a 240vac circuit, it would be easy to measure 120vac if you didn't reference the proper place (ground would only be a safety point, not what you'd use to measure for 240vac). As said above, you have to measure between the two hot input leads...leave ground out of the equation entirely.

 

[hj]

Unless one of the wires has "gone to ground" you would NOT read 120v between the wires. If one of them failed, you would read ZERO and that might need a new breaker to fix it. WE have no idea HOW you are testing so cannot tell you WHAT conditions would give you the readings you indicated, or if they are actually showing defects.