Response Times Of Safety Functions - Siemens SIMATIC PCS 7 Operating Manual

Process control system compendium part b process safety v8.2
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Configuring the safety program
7.11 Monitoring times and system response times
7.11.6

Response times of safety functions

Definition of response time
The response time is the time between the detection of an input signal and the changing of a
linked output signal.
The actual response time is always between a minimum and a maximum response time.
When configuring your plant, you must always assume the maximum response time.
The maximum response time of a safety function must be shorter than the process safety
time.
Definition of process safety time
The process safety time is the time within which the process can be left to its own devices
without creating a dangerous situation.
Within the process safety time, the S7 F/FH system controlling the process is not under
control; in other words, it might malfunction or fail completely. The process safety time
depends on the type of process and must be specified individually.
How to calculate the response time
In S7ftimea.xlsm, the route of a signal from the sensor to the actuator via the system can be
broken down into five possible sections:
1. Input:
Runtime from sensor to user program
2. Processing in the 1st F-CPU:
Cycle time and run time of F-program
3. F CPU-to-F CPU communication:
Sends the signal to a second F-CPU (optional)
4. Processing in the 2nd F-CPU:
Cycle time and run time of F-program in the second F-CPU (optional)
5. Output:
Runtime from user program to actuator
Different variants may be used in each section. To calculate the maximum reaction time,
select the relevant variant from the header line of each section and enter the corresponding
values in the variants.
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Compendium Part B - Process Safety (V8.2)
Operating Manual, 07/2016, A5E35031794-AB

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