Siemens 3SK1121-2AB40 is a SIRIUS safety relay for relay enabling circuits. It mounts on DIN rail (screw and snap-on, any position) and occupies 22.5 mm width with 121.6 mm depth. Rated at 24 V with a DC tolerance of 0.8 to 1.2 times nominal, it switches 1 A at 24 V, 1.5 A at 115 V, and 1.5 A at 230 V — enough to drive contactors or smaller motor starters in a safety circuit. The total current through the relay must stay under 12 A, and power loss tops out at 2 W, so no special ventilation is needed in a typical panel.
This relay carries SIL 3 per IEC 61508 and IEC 62061, and PL e per ISO 13849-1. That's the highest performance level for a single-channel safety device under the current standards. It means the probability of dangerous failure per hour is below 10⁻⁷, and with a safe failure fraction of 99%, nearly every failure that occurs will be detected and force the safe state. For the buyer: this part is certified for Category 4 / PL e safety functions — the kind you need for light curtains, two-hand controls, or perimeter guarding where a single fault cannot lead to loss of safety. Response time is 30 ms typical, 50 ms maximum — fast enough to stop a servo-driven axis before a hand reaches the pinch point in most applications. The sensor input minimum pulse is 75 ms, so it filters out contact bounce from mechanical safety switches without extra external timers.
Terminal wiring and environment
Terminals accept solid wire from 20 AWG to 16 AWG (1x or 2x) and stranded wire in the same range. If you're using ferruled ends, the acceptable range narrows to 0.5 to 1.0 mm². Shock resistance is 10 g for 11 ms — enough to survive a forklift bump against the panel but not a direct hammer blow. The EMC emission standard is IEC 60947-5-1 Class A, so it's suitable for industrial environments without additional filtering.