A commercial customer calls. There is no air-conditioning in the upstairs office. This i...

A commercial customer calls. There is no air-conditioning in the upstairs office. This is a three-story building with an air handler on each floor and a chiller in the basement.

The service technician arrives and politely introduces himself to the customer. After handing the customer a business card and describing the service he can provide, the technician asks some pertinent questions using everyday language the customer can understand. The technician then carefully listens and makes the customer feel like part of the solution to the service call problem. Once the technician has permission to enter the building, he goes to the fan room where the complaint is. There is no need to go to the chiller or the other floors because there would be complaints if the chiller was not furnishing cooling to them.

The chilled water coil piping feels cold; the chiller is defi nitely running and furnishing cold water to the coil. The fan motor is not running. SAFETY PRECAUTION: Because the motor may have electrical problems, the technician proceeds with caution by opening, locking, and tagging the electrical disconnect.• The technician pushes the reset button on the fan motor starter and hears the ratchet mechanism reset. The motor will not try to start while the technician is pushing the reset button because the disconnect switch is open. The unit must have been pulling too much current for the overload to trip.

Using an ohmmeter, the technician checks for a ground by touching one lead to a ground terminal and the other to one of the motor leads on the load side of the starter. The meter is set on R × 10,000 and will detect a fairly high resistance to ground. See Figure 20.23 for an example of a motor ground check. SAFETY PRECAUTION: Any movement of the meter needle when the ohmmeter’s leads are connected between ground and the motor lead would indicate a ground—so caution is necessary. If the meter needle moves as much as one-fourth of its scale, the circuit should not be energized until the ground is cleared up or physical damage may occur.• The motor is not grounded. The resistance between each winding is the same, so the motor appears to be normal. The technician then turns the motor over by hand to see whether the bearings are too tight; the motor turns normally.

The technician shuts the fan compartment door and then fastens the clamp-on ammeter to one of the motor leads at the load side of the starter. The electrical disconnect is then closed to start the motor. When it is closed, the motor tries, but will not start and pulls a high amperage. The motor seems to be normal from an electrical standpoint and turns freely, so the power supply is now suspected.

SAFETY PRECAUTION: The technician quickly pulls the disconnect to the off position and gets an ohmmeter.• Each fuse is checked with the ohmmeter. The fuse in L2 is open. The fuse is replaced and the motor is started again. The motor starts and runs normally with normal amperage on all three phases. The question is, why did the fuse blow?

What is the problem and the recommended solution?