Insulation resistance tester
An insulation resistance tester, also called a megohmmeter, checks insulation quality. It applies a known voltage, measures the leakage current and works out the resistance from them. The device under test must be de-energized, and equipment must be disconnected from other circuits before you connect the instrument. Depending on the instrument and its purpose, test voltages range from 50 Vdc to 5000 Vdc. The test current is usually driven by an applied voltage of 500 volts or more. The instrument may be powered by an external source, internal batteries or a hand crank. It reads insulation resistance directly in ohms or megohms, and good insulation usually reads in the megohm range. The test does not damage the insulation. Readings change with the temperature and moisture of the insulation, so record those conditions and compare results over time. The tester measures any resistance connected between its terminals, including series or parallel leakage paths through the insulation or across its surface. Handheld insulation multimeters combine insulation testing with voltage, resistance and continuity measurements. Some multi-function models are made specifically for testing PV arrays.
<!-- review notes
- sentence 5: support check: partial - Nondestructive and temperature/moisture dependence plus record-keeping are stated, but the instruction to record those conditions and compare over time is only loosely implied (record-keeping, relative values; u26 form has temperature/humidity fields). -->