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

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

• A. Inductive circuits, high inrush current, and DC motors
• B. Inductive circuits only
• C. High inrush current only
• D. DC motors only

Answer: (D)

Switch ratings assume a certain type of load and a safe margin for interrupting current. DC motors present a particular challenge: at startup they draw a startup or inrush current that can be several times the running current, and when the contacts open there’s no natural zero crossing to help extinguish the arc. The motor’s inductance and back-EMF keep current flowing, creating an energetic arc that accelerates contact wear or can weld the contacts if the switch is used at its nominal rating. To prevent overheating and excessive arcing, the switch must be derated for DC motor loads. Inductive circuits and high inrush are related factors, but in this context the defining condition that necessitates derating is the DC motor startup/arc behavior.

Switch ratings assume a certain type of load and a safe margin for interrupting current. DC motors present a particular challenge: at startup they draw a startup or inrush current that can be several times the running current, and when the contacts open there’s no natural zero crossing to help extinguish the arc. The motor’s inductance and back-EMF keep current flowing, creating an energetic arc that accelerates contact wear or can weld the contacts if the switch is used at its nominal rating. To prevent overheating and excessive arcing, the switch must be derated for DC motor loads. Inductive circuits and high inrush are related factors, but in this context the defining condition that necessitates derating is the DC motor startup/arc behavior.