Siemens 3SK1211-2BB40 Safety Relay

Siemens 3SK1211-2BB40 is a SIRIUS safety relay that functions as an output expansion module for safety circuits, delivering four normally open safety outputs plus a dedicated signaling channel with one normally closed contact, ideal for expanding safety logic in machines that require diagnostic-safe feedback. It is powered by a 24 V DC supply (Us = 24 V DC) and is designed for DIN rail mounting with spring-type push-in terminals, enabling quick, tool-free connections. The four NO contacts provide independent safety switching paths, while the NC signaling contact supports fault signaling or interlock feedback in the safety chain. As part of Siemens’ SIRIUS family, the device targets high-integrity safety applications, with safety ratings including SIL 3 and PL e, PFHD 1.7×10^-9 1/h for high-demand use, and a low-demand PFDavg of 1×10^-6, with a safe-failure fraction around 99%. The relay offers a mechanical service life of about 10,000,000 operating cycles, ensuring long-term reliability in repetitive safety operations. This compact module is well-suited for emergency-stop circuits, safety gates, and interlock-based protective circuits, providing reliable, diagnosable output expansion without altering the core safety architecture of the control system.