Application Note for “Usage of AC500 digital standard I/Os in functional safety applications up to PL c (ISO 13849-1)”, V1.0.0
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3.4.4 Safety function response time
SFRT calculation shall be done based on the data presented in Chapter 5 [1]. It is
straight-forward for all functional safety modules, like SM560-S and DX581-S, but re-
quire additional analysis for standard (non-safety) ones, like PM573 and DC523.
Under assumptions and implementation aspects listed in Chapter 3.4.1, one can add
the following fault error reaction time (e.g., for Stuck-At-1 errors) for DC523 input chan-
nel:
- 400 ms, if supervised as proposed in Chapter 3.4.1.
As for SFRT input part (it means SFRT up to SM560-S Safety CPU only), one shall
add the fault error reaction time for DC523 path (400 ms in the given example, refer to
Chapter 3.4.1) to a LOW phase supervision time (35 ms in the given example, refer to
Chapter 3.4.1) as the overall worst-case response time, which will result in SFRT for
input part:
- 435 ms, if supervised as proposed in Chapter 3.4.1
DANGER
You still have to do a complete SFRT calculation for the safety
loop(s) (not only input part but also logic processing and output
parts) in your application (see Chapter 5 [1]).
3.5 Calculation of the probability of failure and correspondence to
PL (ISO 13849-1)
For calculation of the probability of failure for Safety PLC part, we will use ABB FSDT
software (see www.abb.com). Our goal is to confirm that PFHavg is (refer to Figure 7)
below or equal to 15% of PL c for Category 2 (refer also to PFHavg values listed in
Table K1 of ISO 13849-1 for given Category, DC and MTTFd values).
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