Safety relay for enabling circuits — SIL 3 / PL e at 24 V DC
The Siemens 3SK1121-1CB42 is a SIRIUS safety relay designed for relay enabling circuits in safety-related applications, rated SIL 3 per IEC 61508 and IEC 62061, and PL e per ISO 13849-1. It operates on a 24 V DC supply and switches up to 3 A at 24 V, 115 V, or 230 V.
SIL 3 and PL e are the highest single-device safety integrity levels for this class — the relay can be used in safety functions up to Category 4 architecture per EN 13849-1, with a safe failure fraction of 99 %. The 3 A switching current at 24–230 V AC/DC covers most pilot-duty loads: contactor coils, solenoid valves, and indicator lamps. The total current through the relay must not exceed 12 A. Response time is 30 ms typical, 40 ms maximum on pickup, and 110 ms maximum on DC dropout — fast enough for emergency-stop chains but not for light-curtain blanking (which needs sub-ms). The sensor input minimum pulse duration is 75 ms, meaning the safety input must see a valid signal for at least that long before the relay responds — this filters contact bounce on E-stop buttons but also means the relay cannot follow a fast-opening guard door switch without a delay. Operating frequency is capped at 360 cycles per hour, or one cycle every 10 seconds, which is fine for machine cycle start/stop but not for high-speed indexing.
Mounting and wiring in the panel
The 22.5 mm width matches the SIRIUS compact footprint — three units fit in a standard 70 mm wide slot. Wire termination accepts solid conductors 1x (20–14 AWG) or 2x (18–16 AWG), stranded 1x (20–16 AWG) or 2x (20–16 AWG), and ferruled 1x (0.5–2.5 mm²) or 2x (0.5–1.0 mm²). Strip length 8 mm for spring-cage terminals.
Shock resistance 10g / 11 ms. EMC emission meets IEC 60947-5-1 Class A (industrial limits). Power loss is 2.5 W maximum, so no forced cooling needed in a standard cabinet.