Safety relay for guarding circuits
The Siemens 3SK1121-2CB42 is a SIRIUS safety relay for enabling circuits, with a Safe Failure Fraction of 99%. The 22.5 mm wide housing mounts via screw or snap-on on a DIN rail, fitting into standard control panels with a depth of 121.6 mm and height of 100 mm.
SIL 3 / PL e — what it means for guarding
SIL 3 (IEC 61508) and PL e (ISO 13849-1) are the highest single-device safety integrity levels, meaning this relay is suited for guarding hazardous machinery where a failure could cause serious injury — e.g., press brakes, robotic cells, or conveyor interlocks. The 99% Safe Failure Fraction (SFF) indicates that 99% of dangerous failures are detected by the internal diagnostics, which drives the high SIL claim without requiring a redundant channel in many architectures. It includes start input, feedback input, and cascading capability, so it can be wired into multi-zone safety circuits or reset loops typical in Category 3/4 structures.
Rated for operation from -25°C to +60°C, storage from -40°C to +80°C, with shock resistance of 10g / 11 ms and overvoltage category III / pollution degree 3 — suitable for industrial control cabinets without climate control. EMC emission meets IEC 60947-5-1 Class A, so it is intended for industrial environments; verify compatibility in residential or light-commercial panels. Maximum switching frequency of 360 operations per hour and a minimum sensor input pulse of 75 ms define the safe response timing for monitoring guard-door switches or light curtains.