When is it recommended to megger a motor to obtain its lowest insulation readings?

Megging a motor to obtain its lowest insulation readings is typically recommended immediately after shutting down the equipment. This timing allows for a more accurate assessment of the insulation condition, as it is likely that the motor has been operating and generating heat, which can affect insulation resistance values. Cooling down after shutdown can also enhance readings, as any moisture or contaminants could have a chance to settle.

Performing the megger test at this point ensures that the readings reflect the true insulation condition without the influence of operational temperature or transient conditions. It provides a crucial check on the insulation integrity before it is placed back into service, helping to prevent potential failures due to insulation breakdown. This practice also aligns with safety protocols, as it ensures that testing occurs when the equipment is not energized, reducing the risk of electrical hazards.