What this SIRIUS safety relay does
The Siemens 3SK1220-1AB40 is a SIRIUS safety relay configured as a sensor extension module. It sits between your safety sensors (light curtains, interlock switches, e-stops) and the safety controller, handling the wiring and logic for up to two channels. It monitors floating or non-floating sensors, detects cross-circuits, and supports antivalent sensors and two-hand switch inputs — all configurable without software, just by setting the onboard parameters. Runs on 24 VDC nominal, with a tolerance band of 0.8 to 1.2 times the rated value — so it stays happy between about 19.2 V and 28.8 V. The supply is monitored internally; if it dips below dropout, the relay drops its outputs and the machine stops safely.
Width is 17.5 mm — one standard DIN-rail module. Depth is 121.6 mm, height 100 mm. Mounts in any position via screw and snap-on mounting. That 17.5 mm width matters when you're packing multiple safety functions into a crowded panel; you can fit a two-channel sensor interface in the same footprint as a single miniature circuit breaker. Screw-type terminals accept solid conductors: 1x (20…14 AWG) or 2x (18…16 AWG). With ferruled stranded wire, it takes 1x (0.5…2.5 mm²) or 2x (0.5…1.0 mm²). Strip length is standard for the terminal block family — the spring pressure is consistent across the SIRIUS range, so a wireman who's landed a 3SK1 before will feel at home here.
Configurable safety logic — no software needed
The 3SK1220-1AB40 is parameterizable for: sensor floating or non-floating, monitored start-up or automatic start, 1-channel or 2-channel sensor connection, cross-circuit detection, startup testing, antivalent sensors, and two-hand switches. That's a lot of flexibility from a module with no programming port — you set it via the input wiring and the start input design. The monitoring of floating sensors and non-floating sensors means it works with mechanical contacts, solid-state outputs, and proximity switches alike. Response time is fixed: the sensor input minimum is 60 ms, and the ON pushbutton input minimum is 0.15 s. No delayed switching on the safety path — it's instantaneous (0 ms delayed). That keeps the safety reaction predictable for risk assessment.
Humidity tolerance 10 to 95% RH during operation. Shock resistance 10g / 11 ms. Degree of pollution 3, overvoltage category 3 — that's the industrial environment rating, meaning it's designed for conductive pollution and transient overvoltages common in control panels. UL approved. EMC emissions per IEC 60947-5-1, Class A — suitable for industrial environments, not residential. Power loss max 1.2 W, so no derating concerns in a dense panel.