Troubleshooting:Safety Precautions

It is not a question of if a control circuit will eventually fail, but when will it fail. One of the main jobs of an industrial electrician is to troubleshoot and repair a control circuit when it fails. In order to repair or replace a faulty component, it is ﬁrst necessary to determine which component is at fault. The three main instruments used by an electrician to troubleshoot a circuit are the voltmeter, ohmmeter, and ammeter. The volt- meter and ohmmeter are generally contained in the same meter (Figure 49 – 1). These meters are called multimeters because they can measure several different electrical quantities. Some electricians prefer to use plunger type voltage testers because they are not susceptible to ghost voltages. High impedance voltmeters often give an indication of some amount of voltage, caused by feedback and induction. Plunger type volt- age testers are low impedance devices and require several milliamperes to operate. The disadvantage of plunger type voltage testers is that they cannot be used to test control systems that operate on low voltage, such as 24 volt systems.

Ammeters are generally clamp-on type (Figure 49 – 2). Both analog and digital meters are in common use. Clamp-on type ammeters have an advantage in that the circuit does not have to be broken to insert the meter in the line.

Safety Precautions

It is often necessary to troubleshoot a circuit with power applied to the circuit. When this is the case, safety should be the ﬁrst consideration. When de- energizing or energizing a control cabinet or motor control center module, the electrician should be dressed in ﬂame retardant clothing and wearing safety glasses, a face shield, and hard hat. Motor control centers employed throughout industry generally have the ability to release enough energy in an arc-fault situation to kill a person thirty feet away. Another rule that should always be observed

when energizing or de-energizing a circuit is to stand to the side of the control cabinet or module. Do not stand in front of the cabinet door when opening or closing the circuit. A direct short condition can cause the cabinet door to be blown off.

After the cabinet or module door has been opened, the power should be checked with a voltmeter to make certain the power is off. A procedure called check, test, check, should be used to make certain that the power is off:

1. Check the voltmeter on a known source of voltage to make certain the meter is operating properly.

2. Test the circuit voltage to make certain that it is off.

3. Check the voltmeter on a known source of voltage again to make certain that the meter is still working properly.

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