Document 93K3bv0NQybjoGrQOpoKoKMvL

Fic 33-10 -- An electrician checking equipment with an in sulation resistance tester His findings will be recorded on a chart similar to that shown in Fig 33-11 Courtesy Associated Research, Inc down voltage The megohmmeter ("megger") is a highscale ohmmeter which gives the value of in sulation resistance Almost everv wire in a plant (including the wires on motors, genera tors, and transformers) is wrapped in electri cal insulation Not all electrical insulation provides a per fect barrier against dirt and dampness In fact, some tvpes may become porous with age and absorb dangerous quantities of dirt and moisture In that event, electrical resistance drops to the point that leaking current burns a hole through the insulation to ground Sometimes this drop in insulation resist ance is sudden, especially if the equipment is flooded, but generally the insulation resist ance drops slowly, so that tests can be made, records kept, and failures predicted The megohmmeter can be used to check new electrical equipment delivered to the plant to see whether the electrical insulation meets the manufacturer's specifications or whether it has become damaged while in shipment or has acquired dampness and dirt This factor is especially important in highvoltage equipment To check the insulation of an ordinary al ternating- or direct-current motor, the motor safety switch is pulled, and one lead of the testing equipment is hooked to the switch blade and the other lead to ground (Fig 3310) When the megohmmeter is operated, a small current is sent through and over the sur face of the insulation at a rated test voltage (usuallv 500 or 1000 volts dc) and the resist ance of the insulation in megohms (millions of ohms) can be read from the instrument's scale When the test is completed, all that remains to be done is to remove the test leads and put the motor back into service A few minutes every six months is all the time required for an accurate checkup of elec trical insulation to prevent equipment failure, short circuits, fires, and possible shock haz ards to operating personnel Periodic read ings, recorded as shown in Fig 33-11, are an insulation history of the machine These records, for each piece of electrical equip ment, make it possible to determine at any time whether the insulation resistance is dropping or holding its own Ground fault indicators and locators An ungrounded electrical distribution system is 997