Use, maintenance and selection of megohmmeter

If a multimeter is used to measure the insulation resistance of a device, then only the insulation resistance value under low voltage is measured, which cannot truly reflect the insulation performance when working under high voltage conditions. The difference between a megohmmeter and a multimeter is that a megohmmeter has a higher voltage power supply, which is generally generated by a hand-cranked DC generator or a transistor converter, with a voltage of 500 to 5000V. Therefore, using a megohmmeter to measure the insulation resistance can obtain an insulation resistance value that meets the actual working conditions.

1. Use and maintenance of megohmmeter

(1) Before measurement, the power supply of the device under test must be cut off, and the conductive parts of the device must be connected to the ground for full discharge to ensure safety. Electrical equipment measured with a megohmmeter must also be grounded and discharged in time before it can be measured again.

(2) Before measurement, check whether the megohmmeter is intact. That is, before the megohmmeter is connected to the object under test, shake the handle to make the generator reach the rated speed (120r/min) and observe whether the pointer points to the "∞" position on the scale. Short-circuit the terminal "line (L) and ground (E)", slowly shake the handle, and observe whether the pointer points to the "0" position of the scale. If the pointer cannot point to the position it should point to, it indicates that the megohmmeter is faulty and should be repaired before use.

(3) The wiring must be correct. There are generally three terminals on the megohmmeter, marked L (line), E (ground) and G (shield). L is connected to the conductor part of the object being measured that is insulated from the earth, E is connected to the shell of the object being measured or the earth, and G is connected to the shield of the object being measured or the part that does not need to be measured.

Terminal G is used to shield surface current. When measuring the insulation resistance of a cable, the leakage current on the surface of the insulating material will make the measurement result inaccurate, especially in places with high humidity and when the cable insulation surface is not clean, which will cause a large measurement error. To avoid the influence of surface current, add a metal shielding ring to the surface of the object being measured and connect it to the "shield" terminal of the megohmmeter. In this way, the surface leakage current IB starts from the positive pole of the generator and flows back to the negative pole of the generator through terminal G to form a loop. IB no longer passes through the measuring mechanism of the megohmmeter, thus fundamentally eliminating the influence of surface leakage current.

(4) The wires connecting the terminal and the device under test cannot be double-stranded insulated wires or twisted wires. They should be connected separately with single-stranded wires to avoid errors caused by poor insulation of the twisted wires. To obtain correct measurement results, the surface of the device under test should be wiped clean with a clean cloth or cotton yarn.

(5) The handle should be shaken from slow to fast. If the pointer is found to be zero, it means that the insulation under test may have a short circuit. At this time, the handle should not be shaken to prevent the coil in the meter from heating and being damaged. The hand-cranked generator should maintain a constant speed and should not be fast or slow so that the pointer keeps swinging. Usually the most suitable speed is 120r/min.

(6) When measuring the insulation resistance of equipment with large capacitance, the megohmmeter cannot be stopped immediately after reading the reading, otherwise the charged capacitor will discharge the megohmmeter, which may burn the megohmmeter. After reading the reading, the handle speed should be reduced on the one hand, and the grounding terminal wire should be removed on the other hand. Before the megohmmeter stops rotating and the object under test is fully discharged, the conductive part of the device under test cannot be touched by hand.

(7) When measuring the insulation resistance of the equipment, the temperature, humidity, and relevant conditions of the test object should also be recorded during the measurement, so as to facilitate the analysis of the measurement results.

2. Selection of megohmmeter

The selection of megohmmeter mainly involves selecting its voltage and measurement range. The insulation resistance requirements of high-voltage electrical equipment are high, so a high-voltage megohmmeter must be used for testing; the voltage that the internal insulation material of low-voltage electrical equipment can withstand is not high. In order to ensure the safety of the equipment, a low-voltage megohmmeter should be selected.

The principle of selecting a megohmmeter is not to make the measurement range exceed the value of the insulation resistance being measured too much, so as to avoid large reading errors due to coarse scales. In addition, it should be noted that the starting scale of some megohmmeters is not zero, but 1MΩ or 2MΩ. This kind of megohmmeter is not suitable for measuring the insulation resistance of low-voltage electrical equipment in a humid environment, because the insulation resistance of the equipment in this environment is small, and it may be less than 1MΩ. No reading can be read on the meter, and it is easy to mistakenly believe that the insulation resistance is 1MΩ or zero.