Safety relay for enabling circuits — 22.5 mm wide, SIL 3 / PL e
The Siemens SIRIUS 3SK1121-1CB44 is a safety relay for relay enabling circuits, with a safe failure fraction of 99 %.
SIL 3 / PL e is the highest integrity level for single-channel safety relays under these standards — it means the relay can be used in safety functions up to Category 4 architecture, suitable for guarding hazard zones on machinery where a failure could cause serious injury. The 99 % safe failure fraction (SFF) indicates that 99 % of possible dangerous failures are detected or are inherently safe. Response time is 30 ms typical, 40 ms maximum, with a DC maximum of 110 ms — fast enough for most emergency-stop and light-curtain applications, but verify against your safety distance calculation if the stopping time is critical. The sensor input minimum pulse duration of 75 ms and maximum operating frequency of 360 cycles per hour define the fastest cycle rate — useful when the relay follows a high-speed sensor or light curtain in a pick-and-place cell.
Where this class of safety relay is used
The 3SK1121-1CB44 includes a start input, feedback input, and cascading capability — so it can be wired into a multi-zone safety system where several relays monitor different hazard points and share a common start/reset circuit. Its EMC emission meets IEC 60947-5-1 Class A, meaning it is suitable for industrial environments without additional filtering in most panels — though verify if your installation has sensitive electronics nearby that require Class B.
Panel integration — DIN rail fit and wiring
At 22.5 mm wide, 100 mm high, and 121.6 mm deep, it occupies a single DIN-rail module — standard for compact control panels where space is tight. Screw and snap-on mounting lets you clip it onto the rail or bolt it to a backplate; the mounting position is unrestricted. Wire sizes accepted: solid 1x (20–14 AWG) or 2x (18–16 AWG), stranded 1x (20–16 AWG) or 2x (20–16 AWG), and with ferrule 1x (0.5–2.5 mm²) or 2x (0.5–1.0 mm²) — covers most panel wiring from 14 AWG motor leads down to 20 AWG sensor cables.