Why should a megohmmeter not be used on a live circuit?

Using a megohmmeter, which is specifically designed for testing insulation resistance by applying a high voltage, on a live circuit poses significant risks. The primary concern is that the megohmmeter's internal components and the voltage it generates are not compatible with active electrical circuits. When used on a live circuit, the high voltage from the megohmmeter can interfere with the circuit's operation, potentially leading to damage of the meter itself due to unexpected current paths or overload conditions.

Moreover, operating a megohmmeter on a live circuit can result in inaccurate measurements because the presence of live voltage can skew the results. The device is engineered to measure resistance and insulation integrity in a controlled, de-energized environment where it can apply its test voltage safely, ensuring valid and reliable readings. Therefore, it is critical for safety and accuracy to only use a megohmmeter on de-energized circuits.