The Siemens 5SM3442-0 is a 2-pole, type AC residual current operated circuit breaker (RCCB) from the SENTRON family, rated 25 A at 400 V AC with a 100 mA trip threshold. It snaps onto a DIN rail (REG) and occupies 4 width units (72 mm) in the panel — a standard footprint for a 2-pole block in a distribution board.
The 100 mA residual tripping current is the key selection parameter: it is sized for equipment protection (fire prevention) rather than personnel shock protection, which typically calls for 30 mA or lower. On a 25 A circuit feeding a pump, compressor, or lighting distribution board, the 100 mA threshold provides ground-fault detection without nuisance tripping from normal leakage on long cable runs or motor windings. The 10 kA short-circuit current rating (SCCR) per EN 60898 and IEC 61008-1 means the device can safely interrupt a fault up to 10 kA at 400 V — adequate for most commercial and light industrial distribution boards where the prospective fault current stays under that level. Overvoltage category III confirms it is rated for fixed-installation use downstream of the main incoming supply.
Depth is 77 mm, installation depth 70 mm — verify enclosure depth if retrofitting into a shallow panel. The top or bottom power supply cord position gives flexibility in busbar routing. Finger and back-of-hand safe touch protection is built in. Silicon-free construction is noted for applications where outgassing must be avoided (e.g., adjacent to sensitive electronics or in clean-room adjacent zones).