The Siemens 3SK1121-2CB41 is a SIRIUS safety relay designed for enabling circuits in safety-related applications up to SIL 3 (IEC 61508) and PL e (ISO 13849-1). It operates on a 24 VDC supply and switches 3 A across 24 V, 115 V, and 230 V circuits.
The SIL 3 / PL e rating means this relay meets the highest single-device safety integrity level for machinery guarding per IEC 62061 and ISO 13849-1 — suitable for risk reduction where a fault could cause serious injury. The 99% safe failure fraction (SFF) confirms the diagnostic coverage is thorough enough for redundant architectures without external monitoring. The 3 A switching current at 24 V, 115 V, and 230 V is consistent across all three voltages, so the relay handles the same contact load regardless of the control voltage. The 12 A total current maximum is the package limit across all contacts; stay under that when summing the loads on multiple channels. Response times: 30 ms typical, 40 ms maximum. Verify stopping-performance calculation includes this delay plus downstream contactor dropout. The start input and feedback input are both present, so the relay can monitor external contactors (EDM) and require a manual reset after a safety event — standard for Category 3/4 architectures.
Screw and snap-on mounting on DIN rail. 22.5 mm width. Any mounting position allowed. Solid wire: 1x (20–16 AWG) or 2x (20–16 AWG). Stranded wire: same range. With ferrule: 1x (0.5–1.0 mm²) or 2x (0.5–1.0 mm²).
Operating temperature: -25 to +60 °C. Storage: -40 to +80 °C. Shock resistance: 10g / 11 ms. EMC emitted interference meets IEC 60947-5-1, Class A (industrial environment). Power loss is 2.5 W maximum — negligible for panel cooling calculations, but worth noting if the relay sits next to heat-sensitive components in a sealed enclosure. Cascading is supported, so multiple 3SK1121 units can be linked to expand the number of safety zones without adding separate logic.