220 volt circuit troubleshooting question

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Old 03-18-11, 10:18 AM
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220 volt circuit troubleshooting question

I have a 220V line that runs my air conditioner.
It stopped working.
The breakers were not tripped. I get a reading of 220V at the breakers.
The voltage between hot leads to the air conditioner is Zero. However from each hot lead to ground is 110V.
What can cause this?
 
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Old 03-18-11, 11:18 AM
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After more reading, it seems my posting above cannot be true. So I just verified the voltages. At the panel, reading across the output leads of the ajacent, ganged-together breakers, there is 220V.
At the A/C (Evaporative Cooler) input there is Zero Volts hot to hot. However there is 110V from each hot to ground.
Anybody have any idea what I am doing wrong?
Thanks.
 
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Old 03-18-11, 11:31 AM
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Each leg of a tandem breaker would read 120 to ground. however, between each hot you would read 0 as they are fed from the same leg. Unless you just replaced the breaker I do not think this would apply.

You could temporarily switch with another two pole breaker and check your readings.
 
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Old 03-18-11, 12:34 PM
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The readings you have do not seem possible. Where exactly are you taking the readings at the A/C unit? Is this with the disconnect switch on or off? What kind of meter are you using?
 
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Old 03-18-11, 01:05 PM
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More Info

The breakers are tied together so they must be switched together.
I am measuring the "output side of the breakers". When I turn them off, the voltage is zero between each other and to ground. When they are switched on then I measure 220V between the two hot wires and 120Volts between each hot wire to ground.(Using standard digital voltmeter on the 300 volt range. AC setting.)
At the A/C unit there is a switch. I am measuring voltages at the input side of the switch, while the switch is off. How ever I get the same results on the equipment side of the switch when it is on.
The unit was working for months just prior to its failure.
I can't see how this can happen!
Any ideas?
Thanks
 
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Old 03-18-11, 03:13 PM
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Can you measure the voltage at the input side of the disconnect switch that should be mounted near the ac unit? Even with it off (which it might have to be to open the box) and your breaker on there should be voltage coming in to the disconnect. If you get voltage there then turn the disconnect on and check the voltage when it enters the ac from the disconnect. Also check the connection for the ground wire coming from the disconnect to the ac to be sure it is well connected. Also the ground to the switch you are checking voltage at. Could be the ground is not good therefore the voltage cannot flow. Some are grounded by the bolts and contact to the frame so you may have to remove the switch to check if there is rust, corrosion or dirt keeping the ground from being made.
 
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Old 03-18-11, 03:59 PM
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The grounding conductor is for safety. It plays no part in the normal operation of the circuit.

Unless you have turned off the breaker in the house panel you should always have voltage on the line side of the disconnect. You will only have power out to the unit with the disconnect turned on or the blade inserted. Your readings sound perfectly normal.
 
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Old 03-18-11, 04:12 PM
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Originally Posted by Sam Basset View Post
The voltage between hot leads to the air conditioner is Zero. However from each hot lead to ground is 110V.
How did you get this specific measurement, and can you repeat it?
 
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Old 03-18-11, 04:27 PM
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Your voltage readings suggest an open in one "side" of the 220 volt circuit .. One 120 volt reading is the voltage-to-Ground on the side of the 220 volt circuit that is intact.

The other 120 volt reading-to-Ground is the meter "in series" with the interior wiring of the HVAC equiptment which connects to the two supply terminals. With one supply terminal "Live" , and the other terminal open , the interior wiring acts as an "extension" of the wire connected the "Live" terminal , = a 120 volt reading to Ground at the "Dead" terminal.
 
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Old 03-18-11, 07:48 PM
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PATTBAA hits the nail on the head. You have lost one leg somewhere in your circuit, perhaps at a junction box. The one good leg is feeding 120 volts thru the unit and back to the disconnect switch and all the way back to the fault in the circuit which gives you a 120 volt to ground reading from both lugs and a "0" volt reading across them.
 
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