I think I see the problem.
You have an open somewhere on
one of the hots.
From the top: Your house has two phases of hot; 120VAC/_0 degrees, and 120VAC/_180 degrees.
Second: If you take a multimeter and stick one probe on the ground frame and put the other probe on either the Big Red Wire or the Big Black Wire, you'd expect to get 120 VAC to either of those wires.
But you said that you put the probes of the multimeter
across the Big Red Wire and the Big Black Wire and got Zero volts. Whoops!
Now, as a thought experiment, suppose that the breaker connected to the Big Black Wire happens to be busted and is open. And you've got the Mobile Connector plugged into the socket.
Under this scenario, there's 120/_0 degrees on the Big Red Wire since its breaker is in good working order. That voltage goes all the way down to the socket, into the wire in the mobile connector, through A Lot Of Resistance (but not infinity resistance) in the Mobile Connector, back into the
other hot wire in the Mobile Connector, back to the wall socket, onto the Big Black Wire, and There It Stops. 120VAC/_0 degrees on
both the Big Red Wire and the Big Black Wire.
Remember that there's not a lot of current flowing around in here; the multimeter has an internal resistance of MegaOhms and the internal resistance of the Mobile Connector is probably a couple of hundred kOhms.
Result: 120 VAC/_0 degrees on both Big Wires, so the voltage difference between them is Zero. Check. The voltage difference between the Big Red or the Big Black and Ground/Neutral is 120 VAC, check.
Like I said, you got an open somewhere. Could be:
- The duplex breakers clip onto the two hot bus bars. One or the other of the clips might not be working right.
- Breakers are mechanical devices. The get broken. If you're confident in your abilities (and, seriously, don't do this if you're not), turn off the power to the breaker panel, pop the breaker out of the panel (it clips, like I said), and check for continuity from a clip to the red wire and the other clip to the black wire. Then turn off the breaker and verify that both sides go to open. Use the Ohms function on your multimeter.
- Check continuity of the red wire to one of the hots on that NEMA14-50 socket contacts and verify that there's no continuity from that red wire to any of the other blade connections.
- Check continuity of the black wire to the other hot on that NEMA14-50 socket and verify that there's no continuity from that black wire to any of the other blade connections.
You're likely going to find something busted in steps 1-4 above. Fix whatever it is, put the breaker back where it came from, turn on the power, and check things.
By the by: If you're a-gonna do this, you darn well should have a helper with a rope/bunch of clothesline, standing physically clear of
you. If you get stuck on something and can't unclench, they're to throw the rope over you and pull you off. And if they're not happy with the idea of that job, then get an electrician in to do the work.