Red Seal Electrician Practice
Motor controlsQuestion 8 of 1506

What do all protective relays have in common, even if they monitor different quantities?

  1. They all measure only line current, using a current transformer, and nothing else
  2. They all open the main breaker contacts directly, so they need no trip coil
  3. They give a contact closure that can switch power to a trip coil or an alarm
  4. They all need a 480 V AC supply in order to operate their internal parts
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Correct answer: They give a contact closure that can switch power to a trip coil or an alarm

Whatever they measure, protective relays give a contact closure signal.

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

Practice motor controls

Questions last reviewed: 18 October 2026