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Old 08-10-2014, 09:04 PM   #1
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Had a customer needing AC repair which I gave to my friend. Turns out to be a nightmare intermittent low voltage control problem. The original board was burned, and had an oversized 10A fuse in it. Installed a new board, and it would blow the 5A fuses every 4-5 hrs.

Long story short, the original LV wire to the compressor had the problem. He replaced the wire, but told me the original wire on the contactor coil was using only one wire from the board, and the other side of the coil was from the metal frame ??? Using the metal as a neutral, but the voltage is only 24V from a different source ??? Odd how that even worked ?
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Old 08-10-2014, 09:08 PM   #2
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If the transformer had a grounded secondary and one side of the board was connected to the ungrounded conductor the other side of the board was grounded, it would work.
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Old 08-10-2014, 09:12 PM   #3
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If the transformer had a grounded secondary and one side of the board was connected to the ungrounded conductor the other side of the board was grounded, it would work.
AC voltage though and at a different voltage So the secondary of the LV transformer would not be grounded right ? So how could it get back to it's source ?
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Old 08-10-2014, 09:57 PM   #4
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AC voltage though and at a different voltage So the secondary of the LV transformer would not be grounded right ? So how could it get back to it's source ?
It had to be grounded either intentionally or accidentally.
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Old 08-10-2014, 10:05 PM   #5
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You know that single conductor circuit can't work, so follow your instincts. Get an ohmmeter and start ringing out paths to ground. If they're using the equipment frame as a return that complete circuit has to be there somewhere.

Is one side of the secondary hard grounded?
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Old 08-10-2014, 10:12 PM   #6
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YeahBad, has to be grounded. I didn't think those really small control transformers were internally grounded on the secondary is all.

Big John, We finished yesterday, I was just pondering
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Old 08-10-2014, 11:03 PM   #7
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YeahBad, has to be grounded. I didn't think those really small control transformers were internally grounded on the secondary is all.

Big John, We finished yesterday, I was just pondering
They aren't internally grounded. Someone had to jump one lead to ground.
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Old 08-10-2014, 11:10 PM   #8
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Like if they grounded the conductor at the transformer in the air handler the path would be to the main panel and then out to the condensing unit. It could/would also travel through the copper refrigerant lines between both units.
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Old 08-10-2014, 11:15 PM   #9
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I bet the trick of using the metal frame is a last ditch effort when you lose all but one conductor in your control wires to the compressor. Over 150' away, it was a real PIA to run. We just ran a temporary across the roofs of these condos to test if this was the problem.
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Old 08-10-2014, 11:58 PM   #10
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Carrier, Rheem and their associated brands have grounded secondaries on their control transformers in the air handlers.
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