What this RCD unit is and where it fits
The Siemens 5SM2633-6 is a SENTRON residual-current device (RCD) designed for 3-pole AC circuits at 50 Hz, rated 40 A with a 300 mA Type A tripping characteristic. It's an instantaneous unit — no adjustable time delay — so it trips as soon as the residual current exceeds threshold. That 300 mA rating makes it suitable for equipment protection rather than personnel shock protection (which typically calls for 30 mA). The Type A designation means it detects pulsating DC fault currents on top of sinusoidal AC, covering rectifier-fed loads like VFDs, switching power supplies, and LED drivers.
Thermal derating and real-world current capacity
The 40 A rating holds at 40 °C ambient (37.2 A at both 40 °C and 45 °C). From there it steps down: 36 A at 50 °C, 34.8 A at 55 °C, 34 A at 60 °C, 33.2 A at 65 °C, and 32 A at 70 °C. If your panel runs hot — say a packed enclosure near a drive cabinet — the 32 A at 70 °C figure is the one to design around, not the 40 A headline. Insulation voltage is rated 460 V, overvoltage category III, pollution degree 2.
Physical fit and mounting
Occupies 3 modular width units on a DIN rail (105 mm wide, 90 mm tall, 70 mm deep). Fastening method is standard REG (regular) snap-on; mounting position is any orientation. Supply can enter from top or bottom. The IP20 rating applies only when the distribution board is installed with conductors connected — it's a panel-internal device, not a standalone outdoor unit. Not sealable.