What instrument is used to perform an insulation-resistance test on 480-volt feeder conductors?

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

Answer: (A)

Measuring insulation resistance requires an instrument that can apply a high DC voltage to the insulation and measure very large resistances. A megohmmeter does exactly this: it applies a high DC voltage to the feeder insulation and measures the leakage current to determine resistance in the megohms. For a 480-volt feeder, you test at a standard high DC voltage (around 500 V or higher) and look for a high megohm reading, which indicates good insulation; a low reading signals insulation deterioration or moisture. Other tools aren’t suitable: a multimeter provides low-voltage resistance measurements and isn’t designed to test insulation under operating conditions; an ohmmeter likewise measures low resistances and isn’t used for insulation integrity; a cable tester checks continuity or faults along the run, not insulation resistance under DC stress.