5SU1644-8RC40 RCBO 40 A 4-pole D characteristic Type A 300mA

The Siemens 5SU1644-8RC40 is a 40 A, 4-pole RCBO (residual current circuit breaker with overcurrent protection) in a 7-module wide housing. It combines a D-characteristic MCB for overcurrent and short-circuit protection with a Type A residual current device rated at 300 mA, all in a single unit that snaps onto DIN rail. The D characteristic means it tolerates high inrush currents — think motor starting or transformer energisation — without nuisance tripping, while the 300 mA Type A RCD catches sinusoidal AC and pulsating DC fault currents to earth.