Checking Fuses
See figure below. Measure voltage from the line side of L1, to the load side of L2. If 240 volts is read, the fuse in L2 is good. If no voltage is measured, the fuses in L2 is bad. Measure from the line side of L2 to the load side of L1. If 240 volts is read, the fuse in L1 is good. Next, measure from the line side of L1 to the load side of L3. The voltmeter will measure 240 volts if the if the fuse in L3 is good.
Checking the Magnetic Starter
When it has been determined that the supply power is available to the line and load side of the fuses and the fuses are good, the control circuit must be checked. See figure below. The first step is to measure the voltage to L1 and L2. If 240 volts is measured between L1 and L2, the control circuit voltage to the controls is correct.
Leave the probe on L1, and check the line and the load side of the overload contacts until the coil (C). This check will verify if any one of the overload contacts is open in the control circuit from L2. If the overload contacts are open, reset the overload and check for the correct voltage. An overload condition on the motor windings can trip open the overload contacts.
If the inlets and outlets on the motor enclosure are blanked with lint or dirt, this condition can create an overload. A stuck bearing on a motor or driven load can cause the motor windings to be overloaded. NOTE: if the overload contacts are not open, check for a loose connection or broken wire. Loose connections are common causes of trouble in motor control circuits. A visual inspection will sometimes detect loose, discolored, or burned wires, or other malfunctions.
