The minimum acceptable DC megohm resistance of a 4,160-volt motor is approximately

• A. 0.42 Megohms
• B. 4.2 Megohms
• C. 42 Megohms
• D. 2.1 Megohms

Answer: (B)

When you test a motor’s insulation with a DC megger, you’re checking how well the windings are insulated from ground and from each other. A practical rule of thumb for motors is that the minimum insulation resistance, in megohms, should be about 1 megohm per kilovolt of rated voltage. So for a motor rated at 4.16 kV, the minimum acceptable resistance is roughly 4.16 megohms, which is typically rounded to about 4.2 megohms.

That makes 4.2 megohms the best choice because it matches the expected minimum based on the motor’s voltage rating. The other numbers don’t fit this standard: 0.42 megohms is far too low for a 4 kV class motor, 2.1 megohms is about half the target, and 42 megohms is much higher than the typical minimum (though still indicating good insulation, it’s not the standard threshold used here).