The Siemens 3SK1121-1CB41-1AA0 is a SIRIUS safety relay rated for 24 VDC control voltage, with SIL 3 (IEC 61508 / IEC 62061) and PL e (ISO 13849-1) capability — the highest integrity tier for guarding hazardous machinery. It switches loads up to 3 A at 24 V, 115 V, or 230 V, so it handles solenoid valves, contactor coils, and indicator lamps directly without an interposing relay. The 22.5 mm wide housing snaps onto DIN rail (screw and snap-on mounting) and accepts screw-type terminals for 0.5–2.5 mm² conductors.
Safety integrity and monitoring features
With a safe failure fraction of 99 % and a maximum operating frequency of 360 cycles per hour, this relay suits high-demand safety applications like press guarding, robot cell access gates, and conveyor emergency stops. It includes a start input, feedback input, and monitoring of floating sensors — all required for a Category 4 / PL e safety circuit. The sensor input minimum pulse duration is 75 ms, which filters out contact bounce from mechanical safety switches. Cascading is supported, so multiple 3SK1121 units can chain together for zone control without extra logic.
Thermal derating and environmental limits
Total current through the relay drops from 12 A at 40 °C to 6 A at 60 °C — a straight 50 % derating curve that matters when packing multiple units in a closed panel. Shock resistance is 10 g for 11 ms, typical for machine-mounted enclosures. Power dissipation is 2.5 W maximum. Mounting position is any orientation, but the derating must be observed when units are side-by-side without air gap.