What it is and what it does
The Siemens 5SM2845-2 is a 4-pole selective residual-current device (RCD) from the SENTRON series, designed as an add-on unit for the 5SY miniature circuit breaker range. It's rated 63 A at 400 V AC, with a fixed 1000 mA trip threshold, type AC (sensitive to sinusoidal AC residual currents). Selective design (type S) means it incorporates a short time delay so that downstream RCDs trip first during a fault, keeping this unit as the backup — critical for panel coordination in distribution boards where you want only the branch circuit to drop, not the whole submain.
Key ratings and what they mean for fit
Rated at 63 A continuous. The 1000 mA residual trip threshold is for fire or equipment protection, not personnel protection. Rated insulation voltage (Ui) is 460 V, surge voltage resistance is 4 kV, and overvoltage category III — all consistent with 400 V three-phase distribution panels where it's installed downstream of the main switch. Operating temperature range spans -40 °C to 75 °C, so it's fine for unheated enclosures in cold climates or warm machine cabinets. The IP20 rating (with conductors connected, in an installed distribution board) is standard for enclosed panel mount — no washdown rating here. Mechanical life is 10,000 operating cycles — typical for a protective device that sees infrequent switching.
Mounting and integration
Occupies 4 modular width units. Installation depth is 70 mm. Mounting position is any; supply can enter from top or bottom. Designed as a clip-on RCD unit for the 5SY MCB series — it's not a standalone device; it mounts to the right of the corresponding 5SY breaker and connects via the busbar and a mechanical trip link. If you're replacing a failed unit in an existing 5SY panel, this drops in without rewiring the load side. The OFF-delay is not adjustable, and the rated fault current is fixed at 1000 mA — no field tweaks. False-tripping protection is built in (suppression of nuisance trips from transient surges).