The MS450-40 is a manual motor protector rated 40 A, with an adjustable full-load amp (FLA) setting range of 28 to 40 A. That adjustability is the key spec: you set the dial to match the motor nameplate FLA, and the device provides both overload protection and short-circuit protection in one package — no separate contactor or fuse block needed for the branch circuit. UL508 listing means this unit is accepted for US industrial control panels without additional agency review. The dual Trip Class 10/20 gives you a choice: Class 10 for standard motor starting (faster trip on overload, better protection for the motor winding) or Class 20 for high-inertia loads like fans and flywheels that need a longer acceleration window before the overload trips.
Built-in phase failure detection — why it matters for three-phase motors
Phase failure detection is integrated into the MS450-40. If one phase drops out, the protector trips on the remaining single-phase current — which can be 1.73× the rated current and will overheat the motor winding in seconds.