Siemens CPU 313C Hardware And Installation Manual page 216

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In all cases where the occurrence of failures can result in material damage or injury
to persons, special measures must be taken to enhance the safety of the
installation - and therefore also of the situation. System-specific and special
regulations exist for such applications. They must be observed on installing the
control system (e.g. VDE 0116 for burner control systems).
For electronic control equipment with a safety function, the measures that have to
be taken to prevent or rectify faults are based on the risks involved in the
installation. As of a certain degree of hazard the basic measures mentioned above
are no longer sufficient. That is, additional measures (e.g. redundant
configurations, tests, checksums etc.) must be implemented and certified for the
control equipment (DIN VDE 0801). The prototype of the fail-safe PLC S5-95F was
tested by TÜV (German Institute for Technological Surveillance), BIA and G EM III;
several certificates have been granted. Thus, it is suitable in the same way as the
previously tested fail-safe PLC S5-115F to control and monitor safety-relevant
system areas.
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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, WKH FRQWUROOHU LV XVXDOO\
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UHOHYDQW 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.
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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.
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S7-300 Automation System, Hardware and Installation: CPU 31xC and CPU 31x
A5E00105492-03