A megohmmeter, also known as an insulation tester, consists of a high-voltage hand-cranked generator and a magnetoelectric double-coil ratio meter. It features stable output voltage, accurate readings, low noise, easy cranking, and is equipped with a shielding device to prevent leakage current in the measuring circuit and independent terminals.
1. Before measurement, correctly select the specification of the meter so that its rated voltage matches the rated voltage of the electrical equipment under test. For equipment rated 500V and below, generally use a 500-1000V megohmmeter; for equipment above 500V, use a 2500V megohmmeter; for high-voltage equipment, use a 2500-5000V megohmmeter.
2. When using a megohmmeter, first check its condition. Before connecting the test object, crank the meter; the pointer should rise to the "∞" position. Then short-circuit the two terminals and slowly crank the megohmmeter; the pointer should point to "0". If it meets the above conditions, the megohmmeter is in good condition; otherwise, it cannot be used.
3. During use, it must be placed horizontally and kept away from external magnetic fields.
4. The two wires between the terminals and the test object must not be twisted together; they should be connected separately to prevent poor insulation of twisted wires from affecting the readings.
5. When measuring, turn the handle gradually from slow to fast and maintain a speed of 150 r/min. After the governor slides, a stable reading is obtained; generally, take the stable value after 1 minute. If the pointer indicates zero, do not continue cranking to avoid damaging the coil.
6. In the presence of lightning or nearby equipment with high-voltage conductors, it is prohibited to use the meter for measurement. Only when the equipment is de-energized and cannot be energized by induced electricity should measurement be performed.
7. Before and after measurement, the test object must be fully discharged to ensure the safety of equipment and personnel.
8. When measuring the insulation resistance of capacitive electrical equipment, after obtaining a stable reading, first remove the test leads, then stop turning the handle. Immediately after measurement, ground and discharge the tested equipment.
9. Avoid severe and prolonged vibration to prevent damage to the meter's pivot and jewel, which could affect scale indication.
10. When not in use, the meter should be stored in a fixed place. The ambient temperature should not be too hot or too cold, and it should not be placed on damp or dirty ground. Also, avoid placing it near corrosive air.
