Emergency stop shutdown to SIL 2 or PL c with a contactor with F-PLC-IN and 3SK2 safety relay
The 3SK2 safety relay monitors the emergency stop device.
When the emergency stop device is actuated, the safety relay opens the enabling circuits and switches the power contactor off by means of its fail-safe input (F-PLC-IN) in a safety-related way.
If the emergency stop device is unlatched and the feedback circuit is closed, the Start button can be used to switch on again.
To achieve PL c, a single-channel design of the emergency stop device is sufficient. For consistency of representation, a two-channel design has been selected here. This is necessary for an SIL 2 application.
Please note that the hardware fault tolerance (HFT) must also be observed in addition to the PFHd values for the SIL when using the contactor with an F-PLC-IN acc. to IEC 62061. In this example, SIL 2 is achieved, but due to the HFT of 0, welding of the contactor contacts can lead to a loss of the safety function.
The mirror contact of the contactor is used for fault diagnostics and must be monitored in the higher-level application (evaluation unit) in order to be able to initiate appropriate response in the event of a fault.
For example, a circuit-breaker can be used here as the second shutdown path. More information is available in the following FAQ: Which Safety Integrity Level can be achieved using a contactor and a circuit breaker? 40349715
When transferring this example as a subsystem to a complete application acc. to ISO 13849, the architecture of the subsystem and common cause failures (CCF) must also be taken into account in order to obtain the corresponding PL. This means that it is not possible to directly transfer a SIL to a PL.