Several Practical Questions About Megohmmeters
1. What precautions should be taken when using a megohmmeter to measure insulation resistance?
(1) When measuring the insulation resistance of equipment, the power must be cut off first. For equipment with large capacitance (such as capacitors, transformers, motors, and cable lines), discharge must be performed first;
(2) The megohmmeter should be placed in a horizontal position. Before connecting wires, shake the megohmmeter to see if the pointer is at the "∞" position, then short-circuit the (L) and (E) terminals and slowly shake the megohmmeter to see if the pointer points to "zero". For semiconductor-type megohmmeters, short-circuit calibration is not suitable;
(3) The leads of the megohmmeter should be multi-strand soft wires with good insulation;
(4) For double-circuit overhead lines and busbars that cannot be completely de-energized, when the induced voltage in the measured circuit exceeds 12 volts, or during thunderstorms, measurement on overhead lines and electrical equipment connected to overhead lines is prohibited;
(5) When measuring capacitors, cables, large-capacity transformers, and motors, a certain charging time is required. The larger the capacitance, the longer the charging time should be. Generally, the reading after one minute of rotation is taken as the standard;
(6) When measuring insulation, the megohmmeter should be kept at the rated speed, generally 120 revolutions per minute. When the measured object has a large capacitance, to avoid pointer oscillation, the speed can be appropriately increased (e.g., 130 revolutions per minute);
(7) The surface of the measured object should be wiped clean and free of dirt to prevent leakage current from affecting measurement accuracy.
2. Why is the measurement time specified as 1 minute when measuring insulation with a megohmmeter?
When measuring insulation resistance with a megohmmeter, it is generally specified to take the reading after one minute of rotation as the standard. This is because when a DC voltage is applied to an insulator, the current flowing through the insulator (absorption current) gradually decreases over time. The DC resistivity of insulation is determined based on the steady-state conduction current, and the decay time of the insulation absorption current varies for different materials. However, tests have shown that for the vast majority of materials, the insulation absorption current tends to stabilize after one minute. Therefore, it is specified that the insulation resistance value after one minute of applied voltage is used to determine the quality of insulation performance.
3. How to select a megohmmeter?
The selection of a megohmmeter mainly involves choosing its voltage and measurement range. High-voltage electrical equipment requires a megohmmeter with a higher voltage, while low-voltage electrical equipment requires one with a lower voltage. The general selection principles are: for electrical equipment below 500 volts, use a 500-1000 volt megohmmeter; for porcelain insulators, busbars, and disconnecting switches, choose a megohmmeter above 2500 volts.
The principle for selecting the measurement range of a megohmmeter is to make the range suitable for the measured insulation resistance value to avoid large reading errors. For example, some megohmmeters have readings that do not start from zero but from 1 megohm or 2 megohms. Such meters are not suitable for measuring the insulation resistance of low-voltage electrical equipment in humid environments, because the insulation resistance of such equipment may be less than 1 megohm, causing the meter to give no reading, which can easily be mistaken for zero insulation resistance and lead to incorrect conclusions.
