Siemens 3SK1220-1AB40 Input expansions. Siemens 3SK1220-1AB40 is the exact printed commercial reference for a safety relay in the SIRIUS 3SK Safety Relays range. Siemens' official product description is: SIRIUS safety relay Input expansion Advanced Input expansion for an additional 2-channel or two 1-channel sensors Us = 24 V DC screw terminal.