The Siemens 3SK1121-1CB41 is a SIRIUS safety relay designed for relay enabling circuits, rated SIL 3 per IEC 61508 and IEC 62061, and PL e per ISO 13849-1. It operates on a 24 VDC supply and switches up to 3 A at 24 V, 115 V, or 230 V.
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 — it is suitable for safety circuits up to Category 4 / PL e, such as two-hand controls, light curtains, and emergency stop chains. The safe failure fraction (SFF) of 99 % indicates that 99 % of dangerous failures are detected, which is the threshold for SIL 3 capability. The 3 A switching rating at 24 V, 115 V, and 230 V is consistent across all three voltage levels, so the contact rating does not derate with higher voltage — useful for mixed-voltage control circuits. The total current maximum of 12 A applies to the sum of all contacts, so parallel wiring of outputs must stay under that package limit. Response times: typical turn-off is 30 ms, maximum 40 ms at AC, and up to 110 ms at DC. The sensor input minimum pulse duration is 75 ms, which means the relay filters out shorter noise pulses — important when interfacing with solid-state safety sensors that may have fast edges.
Deployment Context
The 22.5 mm width and screw/snap-on mounting let it drop into a standard DIN-rail enclosure alongside other SIRIUS control gear. Operating temperature range is -25 to +60 °C, storage -40 to +80 °C, with pollution degree 3 and overvoltage category III — rated for industrial environments with conductive dust and transient surges. It includes both a start input and a feedback input, so it can monitor external contactors or valve solenoids for weld-short detection, and requires a manual or automatic start signal per the safety application. Cascading is supported, allowing multiple 3SK1 relays to be linked for expanded I/O without additional safety logic.