SERVICE CALL 4

An insurance office customer calls. There is no air-conditioning. This system has an electric furnace with the air handler (fan section) mounted above the suspended ceiling in the stockroom. The condensing unit is on the roof. The low-voltage power supply is at the air handler.

After politely introducing himself, the service technician puts on shoe covers to protect the customer's floors. After carefully listening to the customer's account of the problem and answering any questions, the technician goes to the room thermostat. It is set on cooling, and the indoor fan is running. Before going out of the building and onto the roof, the service technician informs the customer why he is leaving. The technician goes to the roof and discovers the breaker is tripped. Before resetting the breaker, the technician decides to find out why the breaker tripped. The cover to the electrical panel to the unit is removed. A voltage check shows that all the voltage to the breaker is off. The contactor's 24-V coil is the only thing energized because its power comes from downstairs at the air handler.

The technician checks the motors for a ground circuit by placing one lead on the load side of the contactor and the other on the ground with the meter set on R 3 10,000. The meter shows a short circuit to ground (no resistance to ground). See Figure 20.23 (below) in Reading 20.9 for an example of a motor ground check. From this test, the technician does not know if the problem is the compressor or the fan motor. The wires are disconnected from the line side of the contactor to isolate the two motors. The compressor shows normal, no reading. The fan motor shows 0 resistance to ground.

What is the problem and the recommended solution?