Common Problems When Using a Megohmmeter

Common Problems When Using a Megohmmeter

1. What should you pay attention to when using a megohmmeter to measure insulation resistance?

When measuring the insulation resistance of equipment, the power supply must be cut off first. Equipment with large capacitance (such as capacitors, transformers, motors and cable lines) must be discharged first; ⑵. The megohmmeter should be placed in a horizontal position. Before connecting, shake the megohmmeter to see if the pointer is at "∞", then short-circuit the two terminals (L) and (E), and slowly shake the megohmmeter to see if the pointer is at "zero". For semiconductor megohmmeters, short-circuit calibration is not suitable; ⑶. The megohmmeter leads should be multi-strand soft wires, and should have good insulation; ⑷. For double-circuit overhead lines and busbars that cannot be completely shut down, when the induced voltage of the measured circuit exceeds 12 volts, or when thunderstorms occur, overhead lines and electrical equipment connected to overhead lines are prohibited from measurement; ⑸. When measuring capacitors, cables, large-capacity transformers and motors, a certain amount of charging time is required. The larger the capacitance, the longer the charging time should be. Generally, the reading after the megohmmeter rotates for one minute shall prevail; ⑹. When shaking the insulation, the megohmmeter should maintain the rated speed. Generally, it is 120 rpm. When the capacitance of the measured object is large, in order to avoid the pointer swinging, the speed can be appropriately increased (such as 130 rpm); ⑺. The surface of the measured object should be wiped clean and free of dirt to prevent leakage from affecting the accuracy of the measurement.

2. When using a megohmmeter to measure insulation, why is the shaking test time set to 1 minute?
When using a megohmmeter to measure insulation resistance, it is generally stipulated that the reading after shaking for one minute is used as the preparation. Because after applying a DC voltage to the insulator, the current (absorption current) flowing through the insulator will gradually decrease with the increase of time. The DC resistivity of the insulation is determined based on the steady-state conduction current, and the insulation absorption current decay time of insulators of different materials is also different. However, experiments have shown that the insulation absorption current of most materials has stabilized after one minute, so it is stipulated that the insulation resistance value after one minute of pressure application is used to determine the insulation performance.

3. How to choose a megohmmeter?
The selection of a megohmmeter mainly depends on its voltage and measurement range. High-voltage electrical equipment requires a high-voltage megohmmeter. Low-voltage electrical equipment requires a low-voltage megohmmeter. The general selection principle is: electrical equipment below 500 volts should use a 500-1000 volt megohmmeter; porcelain bottles, busbars, and knife switches should use a megohmmeter of 2500 volts or more. The principle of selecting the measurement range of a megohmmeter is: to make the measurement range adapt to the value of the measured insulation resistance to avoid large errors when reading. For example, the readings of some megohmmeters do not start from zero, but from 1 megohm or 2 megohm. This kind of meter is not suitable for measuring the insulation resistance of low-voltage electrical equipment in a humid environment. Because the insulation resistance of such equipment may be less than 1 megohm, the meter cannot get a reading, and it is easy to mistakenly believe that the insulation resistance is zero, and draw a wrong conclusion.