Derating of a switch is required for which circuit conditions?

• A. High inrush current circuits
• B. DC circuits only
• C. Purely resistive AC circuits
• D. Circuits with a high inrush current and inductive loads such as coils and relays

Answer: (D)

The key idea is that a switch’s rating covers steady, continuous current, not the surge that can occur when a circuit is switched. Inrush current spikes happen when devices with a large startup draw (like motors, transformers, or capacitor banks) are energized. Inductive loads—coils and relays—also produce back-EMF, which creates a peak current and voltage spike at switching. These peaks heat the contacts and can cause arcing or premature wear, so choosing a switch with a higher rating or a higher surge-capability (derating) helps ensure it can handle the turning-on and turning-off stresses safely. That’s why the best answer involves circuits that combine a high inrush with inductive loads such as coils and relays. The other options don’t capture both the inrush and inductive aspects that drive the need for derating.