The Siemens 5SU1624-1KK82 is a 2-pole SENTRON RCBO (residual current circuit breaker with overcurrent protection) rated for 125 A at 30 °C, with a 300 mA residual trip and a 10 kA breaking capacity per EN 60898. The C characteristic means it handles moderate inrush — typical for motor and lighting circuits in commercial or industrial distribution boards. The 300 mA residual threshold is an equipment-protection level, not personnel protection; it guards against arcing faults and leakage that could damage wiring or start a fire.
The 125 A rating at 30 °C is the headline number, but the thermal derating curve matters in a warm panel: at 40 °C it's 115.62 A, at 50 °C it's 106.25 A, and at 60 °C it drops to 96.25 A. If your enclosure runs hot, size the upstream protection accordingly — don't spec a 125 A bus if the RCBO sits in a 50 °C cabinet. The 300 mA residual trip is not a GFCI for personnel — that's 30 mA or lower. This is an equipment-protection RCBO, often used behind a main switch or as a feeder protection in a distribution board where the downstream circuits have their own 30 mA RCDs. The AC fault-current type means it detects sinusoidal AC leakage only; if your load has DC components (VFDs, UPS, rectifiers), you'd need a Type A or B device.
The IP20 rating applies only when the distribution board is installed with conductors connected — it's not a standalone outdoor device. Terminal capacity accepts 6 to 50 mm² solid or stranded copper, so it handles large feeder cables without needing a lug adapter.