What it is and what it does
The Siemens SENTRON 5SM3645-8 is a 4-pole selective residual current circuit breaker (RCCB), type A, rated at 125 A and 300 mA residual sensitivity on a 400 V AC, 50 Hz network. The 'selective' design means it incorporates a short-time delay so that downstream RCCBs trip first during a ground fault — this unit stays in, keeping upstream circuits live and narrowing the outage to the faulted branch. It's sized for main or submain protection in a distribution board where coordination with downstream 30 mA or 100 mA devices is required.
Key ratings and what they mean for fit
Rated at 125 A continuous and 400 V AC phase-to-phase, this RCCB handles the full load current of a large submain or a main switch in a commercial panel. The 300 mA residual sensitivity is intentionally higher than the 30 mA used for personal shock protection — it's meant for equipment protection and fire prevention on circuits with natural leakage, not for direct human contact. The short-circuit withstand is 10 kA per IEC 61008-1 and 1.25 kA per EN 60898; the 10 kA figure governs its installation behind a suitable upstream overcurrent device (MCB or fuse) rated to clear faults above that level.
Panel fit and mounting
Occupies 4 modular width units (72 mm) on a DIN rail, with a depth of 77 mm and height of 90 mm. The 70 mm installation depth is the space needed behind the panel door — check gland plate clearance if the enclosure is tight. Mounts in any position, and supply can enter from top or bottom. Rated IP20 only when installed in a distribution board with conductors connected; the enclosure provides the environmental seal, not the device itself.
Sourcing and lifecycle
Listed as current-production. The SENTRON series is Siemens' standard distribution protection line, widely specified across European and Middle Eastern panels. This part is sourced and quoted to order against an RFQ through independent distribution channels — availability and current pricing confirmed at quote time.
Installation notes
Silicon-free construction suits environments where silicone outgassing can contaminate contacts or optical surfaces — relevant for automotive paint lines or clean-room adjacent panels. The design accepts supplementary devices for alarm signaling or remote tripping. Operating temperature range is -25 °C to 45 °C; storage down to -40 °C. Power dissipation is 8.9 W per pole at rated current in hot operating state — factor this into enclosure thermal calculations if the panel is densely populated.