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The selection of a megohmmeter mainly involves selecting its voltage and measurement range. High voltage megohmmeters are required for high-voltage electrical equipment. Low voltage electrical equipment requires the use of low voltage megohmmeters. The general selection principle is to use a 500-1000 volt megohmmeter for electrical equipment below 500 volts; Porcelain bottles, busbars, and disconnectors should use megohmmeters with a voltage of over 2500 volts.
The principle for selecting the measurement range of a megohmmeter is to adapt the measurement range to the measured insulation resistance value without causing significant errors when reading. Some megohmmeters do not start from zero, but rather from 1 or 2 megohms. This type of meter is not suitable for measuring the insulation resistance of low-voltage electrical equipment in humid environments. Because the insulation resistance of this type of equipment may be less than 1 megaohm, making it difficult for the instrument to obtain a reading, it is easy to mistakenly assume that the insulation resistance is zero and draw incorrect conclusions.
