Copyright Goodheart-Willcox Co., Inc. 388 Motors and Electric Control Systems the line voltage powering the equipment and components. Third, test specific devices and components within the electrical circuit for fault. 17.3.1 Voltage Testing Voltage testing involves testing the voltage being provided to the circuit and the voltage across switching devices and load devices. The following are some guidelines for voltage testing while troubleshooting: • Always apply the information gained during the customer interview and initial inspection. If you can identify some potential causes based on the information, begin your troubleshooting on those potential causes. • Voltage testing involves comparing voltage measurements to values expected when the unit is operating properly. When you obtain a measurement that differs from the expected value, you have identified a potential problem that requires more investigation. • Begin by checking the voltage at the terminals of the disconnect. Verify the measurement is within the acceptable range for the unit. If there is no voltage at the disconnect, check the overcurrent protection and the electrical service. • Refer to the ladder diagram to keep your checking methodical. If there is no ladder diagram, you can draw your own to help with troubleshooting. • Check for loose connections while testing. • Checking the voltage at primary and secondary terminals of the transformer is a good way to check the circuit voltage. If the primary voltage is good but the secondary terminals measure 0 V, the transformer is most likely bad. • A switching device is open if you measure voltage across the device. See Figure 17-19. If the switching device is open and is supposed to be closed, test the device. • If you measure 0 V across a switch, it may be closed, or it may be open with equal electrical potential on either side. • An open may be caused by a broken wire. • If a load device is not operating and the voltage across it is correct, the load device is the problem. The device may have an internal switch, such as a thermal overload, that is open or an open elsewhere in the device. • If a load device is not operating and the voltage across the device is 0 V, the problem could be in the switching devices in the circuit. The problem could also be a short in the load device. A short in a load device may cause the circuits overcurrent protective device to open. See Figure 17-20. 17.3.2 Device and Components Testing When you have identified a switching device or a load device as a potential problem, the next step is to test the component. Testing procedures for specific devices are discussed in other chapters, but the following are some general guidelines for device testing: • De-energize the circuit before beginning device testing. Whenever you de-energize a circuit, verify that the circuit is de-energized by taking multiple voltage measurements at different locations in the 0 V OFF A COM V Ω A V Ω 120 V OFF A COM V Ω A V Ω Switch Operating Correctly (Closed) Switch Faulty (Open) Goodheart-Willcox Publisher Figure 17-19. Ladder diagram for a fan motor circuit. The switch is closed but the fan motor is not energized. In the first example, the 0 V measurement across the switch indicates that the switch is functioning properly (closed). In the second example, the 120 V measurement across the switch indicates that the switch is open when it is expected to be closed. Thus, the switch is likely faulty.