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My 3 prong 240v outlet shows 240v when measuring across red and black wires. However, when testing either hot to the third wire I get 0 volts. My understanding is it should be 120v.
What does this signify?
At the breaker box each wire going into the double breaker show 120v when testing from it to the neutral bar.
The third wire is not connected, either broken somewhere along its path or there's a missing connection in one of the junction box leading to the outlet.
If there's multiple connection boxes from the panel to the outlet, you can test at each to figure where the problem is. Either a nut came loose somewhere or a wire was cut/torn.
The question doesn't make it clear whether you are in a location with 240 and ground, or 240 with neutral and ground, but the answer remains the same, the third wire is disconnected somewhere.
Yes, this is a lost neutral or ground.
Keep in mind that if this is a NEMA 10 type receptacle, hot-hot-neutral (no ground), then a lost neutral is a rather dangerous situation. Such connections typically bond neutral to the chassis on the assumption that the neutral wire rarely fails, and bootlegging ground in this way is less unsafe than having no ground. I wouldn't know; I'm not an actuary.
