Important Information - Siemens SIMATIC S7-400 Installation Manual

Splitting the group into safety-relevant areas and areas which are not
savety-relevant
Most plants contain components for handling safety-relevant functions (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 control system is
usually divided into an area that is safety-relevant and an area that is not
safety-relevant. In the non-safety area, no special demands are placed on the
safety of the control equipment, because any failure in the electronic system will
not influence safety in the installation. In the safety-relevant area, however, it is
only allowed to operate controllers or circuits which satisfy the 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 PLC.
• For controllers with balanced areas (e.g. chemical installations, cable cars)
In this case also, the area that is not safety-relevant is controlled with a
standard PLC, whereas a tested fail-safe controller (S7-400F or S7-400FH) PLC
the safety-relevant areas.
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

Even when electronic control equipment has been configured for maximum design
safety -- e.g. with a multi-channel structure -- it is imperative conform with
instructions given in the operating manual. Incorrect handling can render measures
intended to prevent dangerous faults ineffective, or generate additional sources of
danger.
Automation System S7-400 Hardware and Installation
A5E00850741-01
Assembling and Installing Systems
A-33