Red Seal Electrician Practice
Wiring and componentsQuestion 844 of 1506

A 1 mm layer of an insulation breaks down at 10 kV. For a rough estimate, what is the breakdown voltage of a layer of the same material that is 3 mm thick?

  1. A little under 30 kV
  2. Exactly 10 kV
  3. A little over 30 kV
  4. About 3.3 kV
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Correct answer: A little under 30 kV

Thicker insulation withstands more voltage, but not quite in direct proportion. Three times the thickness gives slightly less than three times the voltage.

Source: Lessons in Electric Circuits I: DC (Kuphaldt), 12.8: Insulator Breakdown Voltage

Practice wiring and components

Questions last reviewed: 18 October 2026