Red Seal Electrician Practice
TransformersQuestion 1140 of 1506

Why are overcurrent devices for transformer primaries usually slow-acting?

  1. They must trip as soon as any current above rated flows.
  2. They must open on every harmonic that the supply carries.
  3. They are used only on DC.
  4. They must carry the inrush surge without opening.
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Correct answer: They must carry the inrush surge without opening.

Energizing causes a short surge above normal current. Slow devices ride through it.

Source: Lessons in Electric Circuits II: AC (Kuphaldt), 10.8: Practical Considerations - Transformers

Practice transformers

Questions last reviewed: 18 October 2026