Red Seal Electrician Practice
Motor controlsQuestion 23 of 1506

Power system engineers usually make protective relays send a closed contact (power applied) to trip a large breaker. What is the reason?

  1. Closed contacts carry less current than open contacts, so they last longer
  2. An open wiring fault then trips the breaker and warns the operator at once
  3. An open wire fault leaves the breaker closed, not a blackout
  4. A closed contact signal never fails in service, so no fault can ever go unnoticed
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Correct answer: An open wire fault leaves the breaker closed, not a blackout

Unplanned trips could cut power to hospitals and water plants. The cost is that an open fault stays hidden until needed.

Source: Lessons in Electric Circuits IV: Digital (Kuphaldt), 6.5: Fail-safe Design

Practice motor controls

Questions last reviewed: 18 October 2026