Siemens SIMATIC S7-300 Hardware And Installation Manual page 233

Splitting the group into safety-relevant areas and areas which are not safety
relevant
Most plants contain equipment performing safety-relevant operations (e.g.
EMERGENCY-OFF switch, protective gates, two-hand controls). To avoid the need
to examine the entire controller from the aspect of safety, the controller is usually
divided into an area that is safety-relevant and an area that is not safety-
relevant In the non-safety-related area, no special demands are placed on the
safety of the control equipment because any failure in the electronics will have no
effect on the safety of the installation. In the safety-relevant area, however, it is
only allowed to operate controllers or circuits compliant with corresponding
regulations.
The following divisions are common in practical situations:
• For control equipment with few safety-related functions (e.g. machine controls)
The conventional PLC is responsible for machine control, whereas safety-
related functions are implemented with a fail-safe mini PLC (e.g. S5-95F).
• For controllers with balanced areas (e.g. chemical installations, cable cars)
The non-secure area is implemented with a conventional PLC, while the secure
area requires a tested, fail-safe controller (S7-300F, S7-400F, S7-400FH, S5-
115F or several S5-95Fs).
The entire installation is implemented with a fail-safe control system.
• For control equipment with mainly safety-relevant functions (e.g. burner control
systems)
The entire control system is implemented with fail-safe technology.
Important Information
The instructions in the operating manual MUST be followed, even if the electronic
control equipment has been configured for maximum design safety - e.g. with a
multi-channel structure. Incorrect handling can render measures intended to
prevent dangerous faults ineffective, or generate additional sources of danger.
S7-300 Programmable Controller Hardware and Installation
A5E00105492-02
Appendix
12-31