Why should a hermetic motor not be energized or tested with a megger when in a deep vacuum?

Why should a hermetic motor not be energized or tested with a megger when in a deep vacuum?

• A. The dielectric strength would be greatly reduced
• B. It would overheat
• C. It would be contaminated by air
• D. Nothing would happen

Answer: (A)

The key idea here is how dielectric strength depends on the surrounding medium and pressure. A megger applies a high DC voltage to insulation to see if any leakage path exists, relying on the medium between conductors to help prevent breakdown and to carry leakage current safely. In deep vacuum, there’s essentially no gas around the insulation. That absence of gas changes how breakdown can occur. Tiny surface imperfections, contaminants, or outgassed molecules can start field emission and form a conducting plasma path more easily when a high voltage is applied. With almost no medium to limit or quench a developing arc, the insulation can break down at voltages that would be tolerated in normal atmospheric or oil-filled conditions. So the dielectric strength effectively drops, making the high-voltage test with a megger dangerous to the winding and insulation. Overheating is unlikely since megger tests involve little current; contamination by air isn’t a concern in vacuum; and saying nothing would happen ignores the real risk of breakdown.

The key idea here is how dielectric strength depends on the surrounding medium and pressure. A megger applies a high DC voltage to insulation to see if any leakage path exists, relying on the medium between conductors to help prevent breakdown and to carry leakage current safely.

In deep vacuum, there’s essentially no gas around the insulation. That absence of gas changes how breakdown can occur. Tiny surface imperfections, contaminants, or outgassed molecules can start field emission and form a conducting plasma path more easily when a high voltage is applied. With almost no medium to limit or quench a developing arc, the insulation can break down at voltages that would be tolerated in normal atmospheric or oil-filled conditions. So the dielectric strength effectively drops, making the high-voltage test with a megger dangerous to the winding and insulation.

Overheating is unlikely since megger tests involve little current; contamination by air isn’t a concern in vacuum; and saying nothing would happen ignores the real risk of breakdown.