What is insulation resistance testing (megger test) and what indicates good insulation?

• A. High resistance (in megohms) indicates good insulation; values depend on voltage and equipment
• B. Low resistance indicates good insulation
• C. No reading indicates good insulation
• D. Moderate resistance indicates good insulation

Answer: (A)

Insulation resistance testing using a megohmmeter applies a high DC voltage to the insulation—between conductors or between a conductor and earth—and measures how much leakage current flows. Since insulation is supposed to resist current, the device reports a resistance value in megohms. The higher that resistance, the less leakage there is, which means the insulation is in good condition. The key take-away is that good insulation shows a very high resistance; a low resistance suggests leakage paths from moisture, contamination, damage, or aging.

Keep in mind that what counts as “good” isn’t a single universal number. The acceptable resistance depends on the equipment’s voltage rating, insulation type, and the standards you’re following. Temperature, humidity, and time after applying voltage can also affect readings, so you often compare the measured value to the manufacturer’s spec or a standard chart and look for a stable, high reading rather than a low or rapidly changing one.