240V 4-prong outlet


Old 03-17-06, 08:38 AM
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Unhappy 240V 4-prong outlet

My appliance (a GE speedcook oven) suddenly stopped working. It is completely dark, and my suspicion that it is not getting power. The same happened several years ago, and a repairman then traced it to not getting 110V from outlet and fixed it. This time I anm trying to tackle it myself.

Naturally, the first thing I decided to check the outlet. It has four wires, red/black/white and a bare grounding wire. What I measure is:

250V between red and black
120v between red and white
120V between black and white
0 between white and bare

It seems right to me, but I do not think I have a good understanding of how this setup should work.
So is outlet working right and I should be looking at the appliance itself, or is it the same problem as before?

An explanation of how it works would also be welcome.

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Old 03-17-06, 08:49 AM
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Your voltage reading are correct.

In Noth America a 240 volt appliance like a stove or dryer has both 240 volt and 120 volt compnents. For this reason new installations are wired with four wire cable, containing the two hot wires (from opposite legs of the incoming power), a neutral, and a ground.

In your case the red and black are the two hot wires, the white is the neutral, and the bare wire is the ground. Your meaurements confirm this.

Since you have the proper voltage, start looking at the appliance for a problem.
Old 03-17-06, 10:29 AM
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Thank you - now I understand it much better

The next step would be to check the internal breaker (if it has one) and/or power supply of the appliance itself. I hope it is possible to do it without removing the appliance from the wall. Oh well..
Old 03-23-06, 12:54 PM
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The Fix

I did look inside the appliance (took the front cover off without removing it from the wall) and saw a small ceramic fuse in 110V circiut. Tested the fuse with ohmmeter - was blown. Managed to get another fuse (a feat in itself), popped it in - bingo!

The oven has been working fine for the last 5 days, so apparently that was a one-time event rather than some short that keeps blowing fuses.

Anyway, thanks for help, racraft.

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