Red Seal Electrician Practice
Motor controlsQuestion 4 of 1506

Why does a large industrial circuit breaker need a protective relay?

  1. The relay supplies the extra current needed to push the large breaker contacts apart when they trip
  2. The relay converts the AC supply to DC so the breaker mechanism can latch shut
  3. The relay replaces the breaker contacts whenever they become worn from use
  4. The breaker cannot sense overcurrent, so an outside device must tell it to trip
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Correct answer: The breaker cannot sense overcurrent, so an outside device must tell it to trip

Unlike small breakers with a bimetal strip, large breakers must be told to open by an external device.

Source: Lessons in Electric Circuits IV: Digital (Kuphaldt), 5.4: Protective Relays

Practice motor controls

Questions last reviewed: 18 October 2026