Safety relay for guarding applications — SIL 3 / PL e with solid-state enabling circuits
The Siemens 3SK1112-2BB40 is a SIRIUS safety relay designed for safety-related circuits in industrial guarding and hazard-zone applications. It achieves SIL 3 per IEC 61508 and PL e per ISO 13849-1, with a safe failure fraction (SFF) of 99 % — meaning it can detect and signal a dangerous fault with very high diagnostic coverage, suitable for safety functions up to the highest performance level. Rated at 24 VDC, the relay uses solid-state enabling circuits rather than electromechanical contacts for the safety path, which eliminates contact wear and bounce in high-cycle applications. The two instantaneous contacts provide the switching output for the safety function. Mounts on a DIN rail via screw and snap-on mounting — the 22.5 mm width fits standard enclosure rail spacing. The spring-loaded (push-in) terminals accept solid wire 1x (20…16 AWG) or 2x (20…16 AWG) and stranded with or without ferrules, so the panel builder can land the safety circuit without screwdriver torque checks.
Integration and wiring — cascading, feedback, and start inputs
The relay includes a start input, a feedback input, and a safety switch input, allowing it to be wired into a standard safety chain: E-stop or guard switch on the safety input, feedback from a contactor or motor starter, and a start/reset button on the start input. Cascading design is supported, so multiple 3SK1112-2BB40 units can be linked to expand the number of safety zones without adding a separate safety PLC. The sensor input minimum on-time is 60 ms — short enough for most mechanical safety switches, but long enough to filter contact bounce. Operating frequency is rated at 2 000 1/h maximum, which covers typical machine cycle rates without derating. Electrical connection uses spring-loaded terminals (push-in) — strip length and ferrule sizing per the listed cross-sections: with core end processing 1x (0.5…1.0 mm²) or 2x (0.5…1.0 mm²); without core end processing 1x (0.5…1.5 mm²) or 2x (0.5…1.5 mm²).
Environmental and compliance — pollution degree 3, overvoltage category 3, EMC Class A
Rated for pollution degree 3 and overvoltage category 3, so it is suited for industrial control panels where conductive dust or condensation may be present. Operating temperature range is -25 to +60 °C; storage range is -40 to +80 °C. Shock resistance is 10 g / 11 ms, and mounting position is any. EMC emitted interference meets IEC 60947-5-1, Class A — intended for industrial environments, not residential. Insulation voltage rated value is 50 V.