This 3D video will show the process of diagnosing a grounded compressor in a split residential HVAC system using isolation diagnosis. We may also call these compressors “shorted to ground.”

When you find a tripped breaker, avoid resetting it; it may be tempting to reset the breaker and see what happens, but the breaker is tripped for a reason; closing the circuit again may cause the overcurrent condition to create a major arc or spark. Shorts at the compressor may also cause carbon and acid to enter the refrigerant and contaminate the system.

Our diagnosis should start off with a solid visual inspection of high-voltage wires, terminals, contactor, capacitor, crankcase heater, breaker, and disconnect. At this time, we must take extra care to note wire rubouts or abrasions, signs of arcing, and loose connections.

Then, we’ll inspect the compressor leads and terminals, wearing appropriate PPE. Before removing the compressor wiring, take a picture of the terminals so that you can remember how the compressor was wired when it’s time to put it all back together.

With the wires pulled off, you can measure the resistance to ground at each terminal with a megohmmeter, using the suction or discharge line as your ground. The quality of the reading will depend on the quality of the meter you use. Low resistance values indicate a short, though a short may also be present even if your resistance values are within the acceptable range.

Abrasions and black stains on the windings are also signs of a short. However, stains may also contain enough carbon to insulate the shorted area, causing a regular resistance reading. When high voltage is applied, the low resistance will show up when reading in the megohm scale.

If you have evidence that the compressor is shorted to ground, you can isolate the compressor by taping or plugging the terminals so that they don’t touch anything. Then, you can reassemble the unit and run it without the compressor. If the breaker doesn’t trip, and everything runs correctly, you can be sure that the compressor is grounded. If the breaker still trips, then you can confirm that a grounded compressor is not the issue.

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