The Siemens 3SK1121-2CB44 is a SIRIUS safety relay designed for enabling circuits in safety-related applications up to SIL 3 (IEC 61508, IEC 62061) and PL e (ISO 13849-1). It operates on a 24 VDC rated supply and switches up to 3 A at 24 V, 115 V, or 230 V.
Safe failure fraction of 99 % — this relay meets the highest single-device integrity level for machinery safety functions. The 75 ms minimum sensor input pulse sets the shortest signal the relay will accept from a safety switch or light curtain; shorter pulses are filtered as noise. The 360 1/h maximum operating frequency limits mechanical cycling.
At 22.5 mm wide, 100 mm high, and 121.6 mm deep, this relay fits a standard single-module DIN-rail slot. Screw terminals accept 1x or 2x solid/stranded conductors from 20 to 16 AWG; ferruled conductors from 0.5 to 1.0 mm². The relay includes start input, safety switch input, and feedback input for monitoring external contactors. Cascading is supported for multi-zone safety circuits.
Electrical characteristics and approvals
Total current maximum is 12 A across all contacts, with a maximum power loss of 2.5 W. The relay meets EMC emitted interference Class A per IEC 60947-5-1. Shock resistance is 10g / 11 ms. The 0.8... 1.2 at DC refers to the operating voltage tolerance range. Typical response time is 30 ms, maximum 40 ms; at DC maximum the release time is 110 ms.