Safety relay for guarding circuits — SIL 3 / PL e, 24 V DC
The Siemens 3SK1122-1AB40 is a SIRIUS safety relay with solid-state enabling circuits, rated for 24 V DC control voltage. It carries SIL 3 per IEC 61508 and IEC 62061, and PL e per ISO 13849-1, with a safe failure fraction of 99 %. That means it is qualified for the highest single-integrity safety functions in machine guarding — e-stop chains, light curtain interfaces, and two-hand control circuits — where a failure must not lead to loss of the safety function.
Screw-type terminals accept solid wire 1x (20–14 AWG) or 2x (18–16 AWG), and stranded wire 1x or 2x (20–16 AWG). With ferrule processing, the range is 1x (0.5–2.5 mm²) or 2x (0.5–1.0 mm²). The insulation voltage rating is 50 V, so this relay is used in the control circuit, not the power circuit.
Timing and input configuration
The sensor input minimum pulse is 60 ms, with a maximum operating frequency of 2000 cycles per hour. The start input and feedback input are both present, and the relay can monitor floating sensors. Cascading is supported for multi-zone safety architectures. The typical off-delay is 6500 ms, with a maximum of 85 ms at DC.
Environmental and EMC
Shock resistance is 10 g for 11 ms. EMC emitted interference meets IEC 60947-5-1, Class A, suitable for industrial environments. Maximum power loss is 2 W.