Why should a hermetic motor be energized or tested with a megger while in deep vacuum?

• A. The Vacuum Pump Could Be Damaged
• B. The Motor Could Draw Too Much Amperage
• C. Electric Arcs Can Contaminate Refrigerant in the System
• D. The Dielectric Strength of the Motor's Insulation Would Be Greatly Reduced

Answer: (D)

In deep vacuum, the way insulation responds to high voltage changes dramatically because there is no gas surrounding the winding to help prevent arcing. The dielectric strength of solid insulation can effectively be lowered in vacuum since discharges can initiate and travel along surfaces or through microscopic voids more easily when there is no medium to quench or limit the discharge. When you apply megger voltage to a hermetic motor in vacuum, the insulation is at greater risk of breakdown or flashover at a lower voltage than it would in air, which can quickly damage the windings. That is why this environment makes the insulation more vulnerable, and it’s the reason the statement about dielectric strength being greatly reduced is the best choice. The other options don’t capture this fundamental risk: the pump itself isn’t the primary concern of the insulation test, the current draw under a megger is not the key issue here, and contamination of refrigerant isn’t the central effect in a vacuum testing scenario.