Integrator's note on the 5SM9121-6KK
Rated at 16 A continuous on a 230/400 V AC supply at 50 Hz, it uses instantaneous tripping — no intentional delay — so it clears ground faults as fast as the electronics can fire, making it a fit for circuits where fast disconnection is priority over selectivity.
Thermal derating curve matters
This unit holds its full 16 A rating up to 45 °C ambient (14.88 A at both 40 °C and 45 °C — the datasheet lists the same value for both, which is the rated current at those conditions). Above 45 °C it steps down: 14.4 A at 50 °C, 13.92 A at 55 °C, 13.6 A at 60 °C, 13.28 A at 65 °C, and 12.8 A at 70 °C. If your panel ambient runs over 45 °C, size the branch circuit for the derated number, not the 16 A nameplate.
The 10 mA trip level makes it suited for circuits where leakage must be kept very low — think wet-location socket outlets or circuits feeding hand-held equipment, where a 30 mA RCD might not catch a dangerous path through a person.