Which instrument is commonly used to perform an insulation-resistance test on electrical feeders?

• A. Megohmmeter
• B. Multimeter
• C. Cable tester
• D. Voltage tester

Answer: (A)

Insulation-resistance testing relies on applying a high DC voltage to the insulation and measuring the leakage current to determine resistance in the megohm range. To do this accurately, you need an instrument that can generate that high voltage and read the resulting current, which is exactly what a megohmmeter (megger) does. A high insulation-resistance reading indicates solid insulation, while a low reading points to deterioration, moisture, or contamination that could lead to leaks or faults under fault conditions.

A multimeter is useful for basic voltage, current, and low-resistance checks, but it does not reliably test insulation resistance at the high levels and conditions required for feeders. A cable tester examines physical integrity and fault locations, not insulation resistance. A voltage tester only shows whether a conductor is live and does not measure insulation integrity.