The Siemens 5SU9104-1KK45 is a 1P+N RCBO — an RCD-operated circuit breaker from the SENTRON 5SU9 series. It combines overcurrent protection and residual current detection in a single 18 mm wide module, rated 45 A at 30 °C with a C tripping characteristic and 10 kA breaking capacity per EN 60898 and IEC 60947-2. This is the part you reach for when you need to protect a final subcircuit against both overload/short-circuit and earth leakage, all in one DIN-rail slot.
The 45 A rating at 30 °C is the headline number, but the derating curve matters more in a warm panel: at 40 °C it's 42.3 A, at 45 °C it's 40.95 A, and at 55 °C it drops to 36.45 A. If your enclosure ambient sits at 45 °C and the load pulls 41 A, this RCBO is already at its limit — size up or ventilate. The residual current side is instantaneous (design of the product: instantaneous) and AC-type (type of fault current: AC), meaning it detects sinusoidal AC earth faults only — not pulsating DC or smooth DC. That's fine for general-purpose final circuits feeding resistive, inductive, or electronic loads with a simple bridge rectifier, but if the downstream equipment has variable-speed drives or switched-mode power supplies that generate pulsed DC leakage, you'd need a Type A or Type F RCBO instead. This one is for the straightforward AC residual current job.
Where it lives in the panel
It snaps onto a DIN rail (mounting position any) and occupies one width unit (18 mm wide, 125 mm tall, 77 mm deep, 70 mm installation depth). The supply can come from either the top or bottom terminals (position of power supply cord: either top or bottom), which simplifies busbar routing in a crowded enclosure.