Siemens Industry Online Support
Siemens AG
Entry type: Application example Entry ID: 38472027, Entry date: 09/24/2013

Emergency stop shutdown to SIL 2 or PL d with a SIRIUS 3SK1 safety relay

  • Entry
  • Associated product(s)
To be able to safely shut down a machine in an emergency, an emergency stop command device is installed and monitored by a SIRIUS 3SK1 safety relay. Safe shutdown takes place via a contactor. In case of a failure of the contactor the circuit-breaker is tripped.

Operating principle

The SIRIUS 3SK1 safety relay monitors the emergency stop pushbutton with two-channels. When the emergency stop pushbutton is actuated, the safety relay opens its relay output and safely switches the contactor off.

The circuit-breaker acts as a fault reaction that occurs only in the event of a fault. For that matter an undervoltage release is connected to one of the time-delayed outputs of the safety relay. Parallel connected the undervoltage release is provided with voltage through a normally-closed auxiliary contact of the contactor. After the set delay time on the safety relay has elapsed, the normally-closed contact has to be closed (contactor switched off) in order to continue to provide the undervoltage release with voltage. If the contactor is welded and thus the auxiliary contact remains open, the undervoltage release is inevitably tripped after the set delay time.

The architecture is thus a single-channel architecture (contactor) with a specified fault reaction (circuit-breaker)

To avoid an undetected accumulation of faults, the circuit-breaker must be tested after 6 to 12 months at the latest. This test set-up has to be documented in the description of the safety function and in the operating instructions of the machine. The tests performed must also be verifiably documented by the user during the phase of use.

The set time delay influences the maximum reaction time. It must be ensured that, based on the risk assessment, this reaction time is sufficiently short in the event of a fault. After setting the final time value, measures against unwanted, subsequent changes have to be provided. Therefore, it is mandatory that the sealable cover (as an accessory with article number 3ZY1321) is used. The set time value must be verifiably documented.

While this application example focuses on the technical realization of the solution, the below listed FAQ describes the safety-related evaluation.



Content of downloads


 Registrierung notwendig  Safety_Evaluation_E-Stop_SIL2_circuit_breaker_3SK1_en.zip (4,6 KB) 



FAQ „What safety level can be reached when combining a contactor and a circuit-breaker for safe switching?“


  Registrierung notwendig  Wiring_E-Stop_SIL2_circuit_breaker_3SK1.zip (54,5 KB)  

Security information
In order to protect technical infrastructures, systems, machines and networks against cyber threats, it is necessary to implement – and continuously maintain – a holistic, state-of-the-art IT security concept. Siemens’ products and solutions constitute one element of such a concept. For more information about cyber security, please visit
Rate entry
no rating
Requests and feedback
What do you want to do?
Note: The feedback always relates to the current entry / product. Your message will be forwarded to our technical editors working in the Online Support. In a few days, you will receive a response if your feedback requires one. If we have no further questions, you will not hear from us.