Safety relay for guarding circuits — SIL 3 / PL e, 24 VDC
The Siemens 3SK1121-1CB42-1AA0 is a SIRIUS safety relay rated for safety-related circuits up to SIL 3 (IEC 61508, IEC 62061) and PL e (ISO 13849-1) — the highest integrity levels for machine guarding, emergency stop, and light-curtain monitoring. It operates on a 24 VDC supply and switches loads up to 3 A at 24 V, 115 V, or 230 V. Depth is 121.6 mm, height 100 mm. It includes both a start input and a feedback input, supports monitoring of floating sensors, and allows cascading of multiple units — typical for multi-zone safety architectures.
Screw terminals, conductor ranges, and total current derating
Connection is via screw-type terminals accepting solid conductors 1x (20–14 AWG) or 2x (18–16 AWG), and stranded 1x (20–16 AWG) or 2x (20–16 AWG). With core end processing, accept 1x (0.5–2.5 mm²) or 2x (0.5–1.0 mm²). Total current through the device is limited to 12 A at 40 °C and derates to 6 A at 60 °C — plan your load distribution accordingly. Maximum power loss is 2.5 W.
Environmental and EMC ratings
Shock resistance is 10g / 11 ms. EMC emitted interference meets IEC 60947-5-1 Class A.