When testing conductor insulation with a megger, it is desirable to have the instrument's voltage?

When testing conductor insulation with a megger, it is desirable to have the instrument's voltage?

• A. Slightly higher than the peak value of the rated AC voltage
• B. The same as the rated AC voltage
• C. Slightly lower than the peak value of the rated AC voltage
• D. No more than 80% of the AC voltage

Answer: (A)

The test aims to reveal insulation weaknesses by stressing the insulation beyond normal operating conditions. For AC equipment, the insulation must withstand the peak voltage it could experience, which is the RMS rating multiplied by √2. Using a megger voltage just above that peak provides a small safety margin that pushes leakage paths enough to show up problems, without exceeding what the insulation can safely handle. If you apply the exact rated AC voltage, marginal weaknesses may not reveal themselves; applying a voltage that’s too high risks damage. So, a voltage slightly higher than the peak value of the rated AC voltage is the best choice to effectively test insulation.

The test aims to reveal insulation weaknesses by stressing the insulation beyond normal operating conditions. For AC equipment, the insulation must withstand the peak voltage it could experience, which is the RMS rating multiplied by √2. Using a megger voltage just above that peak provides a small safety margin that pushes leakage paths enough to show up problems, without exceeding what the insulation can safely handle. If you apply the exact rated AC voltage, marginal weaknesses may not reveal themselves; applying a voltage that’s too high risks damage. So, a voltage slightly higher than the peak value of the rated AC voltage is the best choice to effectively test insulation.