Under what conditions must a switch be derated from its nominal rating?

A switch must be derated from its nominal rating primarily when used with inductive circuits and high in-rush current. Inductive loads, such as motors, transformers, and solenoids, can generate back electromotive force (back EMF) when the switch is opened. This back EMF can cause voltage spikes that exceed the switch's nominal rating, potentially damaging it or leading to failure. Additionally, high in-rush current can occur when a device first starts or when there is a sudden application of power, leading to currents that are significantly higher than the steady-state running current. Therefore, to ensure reliability and longevity, switches are derated in these situations to accommodate these higher demands and protect the electrical components involved.

In contrast, derating may not be necessary for switches used exclusively with resistive loads, where currents are more predictable and manageable. Similarly, while AC circuits do require specific considerations regarding switch ratings due to their unique characteristics, derating is especially essential with inductive circuits due to the additional challenges presented by inductance and in-rush conditions. DC applications do not inherently necessitate derating but do require consideration of load characteristics, particularly if inductive elements are present.