What it is and what it does
The Siemens 5SM3642-0 is a 4-pole SENTRON residual-current circuit breaker (RCCB) rated for 25 A at 400 V AC, 50 Hz, with a 300 mA residual trip threshold and a 10 kA short-circuit current rating. It's an instantaneous type (no short-time delay), designed for AC fault currents only. The 4-pole configuration means it switches all three phases plus neutral, common for three-phase distribution boards where neutral protection is required.
Mounting and panel fit
Snaps onto standard DIN rail (REG profile), occupying 4 width units (72 mm wide). Installation depth is 70 mm, overall depth 77 mm — that extra 7 mm is the terminal clearance. Accepts solid or stranded conductors from 1.5 to 25 mm². Mounting position is unrestricted, but ambient must stay between -5 and +45 °C. The IP20 rating applies only when the distribution board is installed with conductors connected — bare terminals are finger-safe per the finger and back-of-hand protection spec.
Thermal derating and real-world current
The 25 A rating holds at 40 °C and 45 °C. Above that, it derates linearly: 24 A at 50 °C, 23 A at 55 °C, 22 A at 60 °C, 21 A at 65 °C, and 20 A at 70 °C. If your panel runs hot — say, a packed enclosure near a furnace line — factor that 20 A ceiling at 70 °C. The storage range of -40 to +75 °C is broader; that's for shelf life, not running.