Fail-Safe Systems
A5E00085588-03
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1 Product Overview
1.1 Overview
SIMATIC S7 F/FH Systems
The S7 F/FH Programmable Controllers (F-Systems) are used in systems with
increased safety requirements. The aim of the S7 F/FH System is to control
processes that can immediately be returned to a safe state. In other words, when
these processes are suddenly shut down, it represents no danger to either man or
the environment.
Safety Requirements
The S7 F/FH System fulfills the following safety requirements:
• Requirement classes AK1 to AK6 in accordance with DIN V 19250/DIN V VDE
0801
• SIL1 to SIL3 (Safety Integrity Level)
in accordance with IEC 61508
• Categories 1 to 4 in accordance with EN 954-1
Principle Behind the Safety Functions
Fail-safe behavior is achieved by means of safety functions primarily in the
software. Safety functions are executed by the S7 F/FH programmable controller in
order to return the system to a safe state, or keep it in a safe state when a
hazardous event occurs.
The safety function for the process can be executed by means of a user safety
function or a fault reaction function. If the F-System can no longer execute its
actual user safety function in the event of a fault, it executes the fault reaction
function. For example, the associated outputs are switched off and the Safety
Program or parts of the Safety Program are disabled, if necessary.
For example: The F-System has to open a valve when there is excess pressure
(user safety function). In the event of a dangerous fault occurring in the CPU, all
the outputs are switched off (fault reaction function), thus opening the valve and
returning the other actuators to a safe state
. If the F-System were intact, only the
valve would be opened.
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