ABB AC500-S Safety User Manual page 399

NOTICE!
Ä Equation 2, page 398 is taken for SFRT calculation with the following reasoning:
– Device_WD1 and Device_WD2 as worst case delay times for Safety I/Os can be directly calcu-
lated as it is shown in Fig. 126 and Fig. 127.
– It is not always easy to calculate the worst case delay time of "Black channel" components (see
AC500 Non-safety modules in Fig. 126 and Fig. 127). Thus, we propose to use half of
F_WD_Time1 and F_WD_Time2 instead. F_WD_Time1 and F_WD_Time2 can be empirically
obtained for the given AC500 system configuration by tracing the values of tResponseTimeMS for
given Safety I/Os in the safety application. Use PROFIsafe instance for the given Safety I/O
Ä Chapter 4.6.3 "SafetyBase_PROFIsafe_AC500_V22_Ext.lib" on page 231). F_WD_Time1 and
(
F_WD_Time2 shall be set about 30 % higher than the worst case value observed in the tRespon-
seTimeMS for the given Safety I/O.
– We propose to take F_Host_WD time instead of the worst case delay time of SM560-S Safety
CPU. F_Host_WD can be calculated as three times the value set using SF_WDOG_TIME_SET
POUs. The correct value for SF_WDOG_TIME_SET can be empirically obtained using tracing
MAX_TIME output of the same POU in a test run. SF_WDOG_TIME_SET value shall be set about
30 % higher than the worst case value (MAX_TIME) observed in the given safety application to
avoid potential availability problems due to triggering of SM560-S Safety CPU watchdog.
– F_WD_Time1 and F_WD_Time2 are the only potential candidates for Longest ∆T_WD, because
F_Host_WD, Device_WD1 and Device_WD2 are already equal to their worst case delay times.
Thus,
Longest ∆T_WD = Max (0.5 * F_WD_Time1; 0.5 * F_WD_Time2)
NOTICE!
One could achieve even better SFRT values than those obtained using
more detailed technical analysis. Contact ABB technical support for further details.
NOTICE!
You have to set F_WD_Time1 and F_WD_Time2 at least 2 times bigger than the value set using
SF_WDOG_TIME_SET time to avoid unintended system stop due to PROFIsafe watchdog expiration.
30.03.2017 AC500-S
Safety times
Safety function response time
Ä Equation 2, page 398 with a
399