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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Ó
2015 ABB Ltd.
NOTICE
MTTFd values for AC500 standard (non-safety) modules were ob-
tained based on MTBF values (contact AC500 technical support at
www.abb.com/PLC
to obtain MTBF values for selected AC500
modules). The following relation, which complies with ISO 13849-1,
was used:
MTTFd = 2 * MTBF
This relationship is based on the following assumptions:
- It is assumed that statistically only every second failure is a
potentially dangerous failure
- The permissible ambient conditions are met
- Mean Time to Repair (MTTR) is significantly less than the
MTBF.
The following MTTFd values were used in the calculation for AC500 standard modules
(all AC500 modules, which are involved in the internal communication of dynamic test
pulses up to SM560-S Safety CPU):
- PM573-ETH
à
2 * 170 years = 340 years
- DC523
à
2 * 88 years = 176 years
- DX581-S (1 out of 3 electronic boards is involved in the internal communication)
à
2 * (3 * 73 years) = 438 years
- TB511-ETH
à
2 * 292 years = 584 years
- TU582-S
à
2 * 2757 years = 5514 years
- TU516
à
2 * 2942 years = 5884 years
Using formula D.1 from Annex D, ISO 13849-1, MTTFd value for the input part, which
is composed of all components contributing to the safety function and listed above,
was calculated:
- MTTFd for input part in Figure 5
à
77 years
DC = 90% was used for standard modules in the input path (see Figure 5). All safety
values for safety modules SM560-S and DX581-S are available from the TUV certifi-
cation process and are a part of the FSDT library.
Figures 8 shows a screen shot from FSDT safety calculation.
