Which instrument is used to measure insulation resistance, and what does a high reading indicate?

• A. Multimeter; high reading indicates good insulation
• B. Megohmmeter; high reading indicates good insulation
• C. Clamp meter; high reading indicates good insulation
• D. Insulation tester; high reading indicates poor insulation

Answer: (B)

Testing insulation resistance is done with a megohmmeter. This instrument applies a high DC voltage across the insulation and measures the leakage current to calculate resistance, shown in megohms. A high reading means the insulation resists leakage well, indicating good insulation. If the resistance is low, leakage paths exist and insulation may be degraded or damaged. This is why a megohmmeter is used rather than a standard multimeter or a clamp meter, which are designed for different measurements. An insulation tester performs the same kind of function, but in practice the term megohmmeter is the standard name for this high-voltage insulation resistance measurement.