What this RC unit does and where it fits
The Siemens 5SM9645-0KK is a 4-pole residual current unit (RC unit) from the SENTRON 5SM9 series, designed for type AC fault currents with a rated residual tripping current of 300 mA and a rated current of 63 A at 230/400 V AC. It's an instantaneous-trip electronic unit — no intentional time delay, so it clears ground faults as fast as the electronics can fire, which matters when you're coordinating downstream protection in a distribution board. The 4-pole construction means it switches all three phases plus neutral, making it suitable for three-phase circuits with a neutral conductor that needs monitoring for leakage.