A safety relay evaluates emergency-stop buttons, guard-door switches and light curtains, and removes power through redundant, monitored contacts. The SIRIUS 3SK range covers compact single-function units through to expandable modules, and 3RK safety system components extend the same concept over AS-Interface.
Confirm the required performance level (PL/SIL) for the circuit before selecting — it determines single- or dual-channel and whether feedback monitoring is mandatory.
Check the number and type of enabling circuits (instantaneous vs delayed) you need.
Match the supply voltage (24 V DC is most common) and whether the unit needs a separate expansion module.
Manual reset vs automatic start is a part-number difference, not a setting, on several models.
Frequently asked questions
Can I replace a safety relay with a standard relay?
No. A safety relay uses redundant, force-guided contacts with internal fault detection so a single component failure cannot defeat the stop function. A standard relay provides none of that and would invalidate the machine's safety assessment.
Which 3SK do I need for a simple E-stop circuit?
For a single dual-channel E-stop with two enabling contacts, a compact 3SK1 unit is usually enough.