What instrument is used to measure insulation resistance?

• A. A megohmmeter (megger).
• B. A multimeter.
• C. A tone generator.
• D. An ammeter.

Answer: (B)

Insulation resistance is tested by applying a high DC voltage and measuring how much current leaks through the insulation, which translates into a resistance value in megohms. The instrument designed for this is a megohmmeter (megger). It can apply the necessary test voltage and accurately read very high resistances, which is essential because good insulation should show extremely high resistance and little leakage.

A multimeter is built for general circuit work and typically measures resistance only up to a few megaohms at most, with lower test voltages. That makes it unreliable for insulation-resistance testing, where leakage paths can require much higher resistance readings to judge properly. A tone generator helps locate and identify cables by signaling a tone, not by measuring insulation or leakage. An ammeter measures current flow, not the resistance of insulation.

So, to assess insulation integrity effectively, you use a megohmmeter, which is specifically suited for measuring insulation resistance in megohms.