Document X7kKMozLrg94w2zXy4j7nE17G

ELECTRICAL HAZARDS perfect bamer against dirt and dampness In fact, some types 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 m insulation resist ance is sudden, especially if the equipment is flooded, but generally the insulation re sistance 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 m shipment or has acquired dampness and dirt This factor is especially important in highvoltage equipment To check the insulation of an ordinary alternating- or direct-current motor, the mo tor safety switch is pulled, and one lead of the testing equipment is hooked to the switch blade and the other lead to ground (Fig 31-8) When the megohmmeter is op erated, a small current is sent through and over the surface of the insulation at a rated test voltage (usually 500 or 1,000 volts dc) and the resistance of the insulation m 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 fig. 31-9. These record cards tell the history of a motor The first readings were taken when the motor was new. Note that in the left record, the gradual decrease of in sulation resistance was the result of normal aging and no trouble was indicated The 31-18