Electrical Power Systems Technology

Power Measurement Equipment 75

ground-fault interrupters are discussed in Chapter 10.

A ground-fault indicator may be used to check for faulty grounding

at various points in an electrical power system. Several conditions can ex-

ist that might be hazardous. These faulty wiring conditions include: (1)

hot and neutral wire reversed, (2) open equipment ground wire, (3) open

neutral wire, (4) open hot wire, (5) hot and equipment ground wires re-

versed, or (6) hot wire on the neutral terminal and the neutral is uncon-

nected. Each of these conditions would present a serious problem in the

electrical power system. Although proper wiring eliminates most of these

problems, a periodic check with a groundfault indicator will ensure that the

electrical wiring is safe and efficient.

MEGOHMMETERS

Megohmmeters are used to measure high resistances that are beyond

the range of a typical ohmmeter. These indicators are used primarily for

checking the quality of insulation on electrical power equipment (mainly

motors). The quality of equipment insulation varies with age, moisture

content, and the applied voltage. A megohmmeter is similar to a typical

ohmmeter, except that some types use a hand-cranked permanent-mag-

net DC generator as a voltage source, rather than a battery. The opera-

tor cranks the DC generator while making an insulation test. Figure 3-12

shows a diagram of a megohmmeter circuit. This circuit is essentially the

Figure 3-12. Circuit diagram of a megohmmeter, analog type