What is a megohmmeter used for, and what is a typical acceptance test for insulation resistance?

• A. A megohmmeter is used to measure resistance in active circuits under load.
• B. A megohmmeter is used to test voltage drop across a resistor.
• C. A megohmmeter is used to measure current flow.
• D. A megohmmeter is used to test insulation resistance; acceptance often requires several MΩs of resistance between conductors and ground for the tested insulation.

Answer: (D)

A megohmmeter measures insulation resistance by applying a high DC test voltage and gauging the leakage current to calculate resistance. The healthier the insulation, the smaller the leakage current and the higher the resistance, typically in the megohm range. In practice, acceptance tests look for several megohms of resistance between conductors and ground for the insulation being tested, indicating good insulation quality and low leakage.

This instrument is not meant to measure resistance in active circuits under load, nor to test voltage drop across a resistor, nor to measure circuit current flow. Those are functions of other instruments like voltmeters or ammeters; the megohmmeter specifically assesses how well insulation resists leakage.