The Siemens 3RK3251-2AA10 is a SIRIUS modular safety expansion module rated for 4/8 F-RO (fail-safe relay outputs). It evaluates safety functions up to EN 954-1 category 4, meaning it can handle the highest fault-tolerant safety circuits — dual-channel, cross-monitored — for guarding applications like press brakes, robot cells, and conveyor zones where a single fault must not lose the safety function. It snaps onto standard DIN rail (35 mm), measures 124 mm deep by 45 mm wide by 113 mm tall, and uses spring-loaded terminals accepting 0.25–1.5 mm² solid or stranded wire (with or without ferrule).
Category 4 per EN 954-1 is the deciding rating for safety buyers: it requires redundant architecture and continuous self-test so that a single component failure never defeats the safety function. That makes this module suitable for stop categories 0 and 1 on machinery where risk assessment demands the highest structural integrity. The 4/8 F-RO designation means four fail-safe relay output channels, each with two contacts (normally open) — effectively eight switching paths. The outputs are parameterizable, so you can configure them as delayed or immediate off, depending on the safety function (e.g., delayed drop for a controlled stop vs. immediate for an EMERGENCY OFF). Surge voltage resistance is rated at 4000 V, and insulation voltage at 300 V — this module is built for 400 VAC line environments with adequate clearance. Operating temperature spans -20 to +60 °C, with storage/transport range of -40 to +85 °C. Shock resistance of 15 g for 11 ms covers most machine vibration profiles.
The spring terminals are tool-less — push in with a screwdriver to release. Wire strip length is 8–10 mm; use ferrules for stranded wire to avoid whiskers. The module is an expansion to a SIRIUS safety base unit (e.g., 3RK3... base), not a standalone controller — it extends the number of safety outputs on the same backplane bus.