410 Electrical Systems Th e insulation in coil, transformer, and motor windings is relatively thin in order to conserve space. For this reason, the insulation rating is only marginally greater than the working voltage. In an overvoltage situation, the overvoltage present is greater than the insulation voltage rating of the wire and an arc-over may occur. When an arc-over occurs, a carbon path is formed. Th is carbon path presents a relatively high resistance, which is unaff ected by the low voltage of a DMM. How- ever, the carbon path presents a signifi cant problem at normal operating voltages. Another scenario occurs when insulated wire comes in contact with the bare metal of the stator of a motor. Vibration eventually wears through the insulation of the winding and causes a high resistance circuit to ground. Again, this is relatively unaff ected by the lower voltage output of the DMM in the ohmmeter mode. In either scenario, when measured at operating volt- age, substantial current fl ow may occur. By testing with the higher output voltage of a megohmmeter, you can identify high-resistance shorts such as those described. 21.3.7 LCR Meter An LCR meter measures inductance (L), capacitance (C), and resistance (R). While a DMM is capable of making both resistance and capacitance measurements, an LCR meter is much more accurate. An LCR meter is also capable of mak- ing inductance measurements, which a DMM cannot. An example of an LCR meter is shown in Figure 21-19. An LCR meter is more accurate than a DMM because an LCR meter has a higher measurement resolu- tion and a variable frequency source. Taking capacitance and inductance measurements using a frequency at or near the operating frequency provides a more accurate measurement. Inductance measurements are quite valuable when trying to diagnose a transformer, coil, or motor winding if a shorted turn is suspected. When a coil has a shorted turn, it will exhibit a substantially lower inductance than that of a coil without a shorted turn. 21.3.8 Oscilloscopes An oscilloscope is a device that is capable of graphi- cally representing a voltage waveform. Th is device is extremely valuable in troubleshooting problems that are otherwise invisible. A modern digital oscilloscope is shown in Figure 21-20. T E C H T I P Oscilloscopes Some industrial maintenance technicians say, “I don’t need an oscilloscope.” This is often the case when the technician does not know how to use one. The more you learn about using an oscilloscope, the more indispensable this diagnostic tool becomes. Divisions Th e oscilloscope screen is divided into a grid. Refer to Figure 21-21. Th e Y (vertical) axis depicts voltage or amplitude of the incoming signal. If the trace defl ects upward, it depicts a positive input voltage. If the trace defl ects downward, it depicts a negative value. Th e X (horizontal) axis depicts time. Each little “box” on the oscilloscope grid is referred to as a division. On the Y axis, the value of each division is in volts (volts/division). On the X axis, the value of each division is in time (time/division). Two controls on the Goodheart-Willcox Publisher Figure 21-19. An LCR meter. Notice that there are various probes to more easily make component measurements. This unit comes with a probe that resembles a pair of tweezers for measuring tiny surface mount components. In place of a mode selector switch, this model uses buttons to navigate on-screen menus. Copyright Goodheart-Willcox Co., Inc.