Catalogs > Safety Products Catalog > Principles, Standards and Implementation > System Design According to ISO/EN 13849 and SISTEMA
System Design According to ISO/EN 13849 and SISTEMA
| SISTEMA Software PL Calculation Tool |  |
System Structure | |
Reliability Data | |
Methods of Data Determination | |
Diagnostic Coverage | |
Common-Cause Failure | |
Mission Time | |
| Systematic Faults | |
Fault Exclusion | |
Performance Level (PL) | |
Subsystem Design and Combinations | |
Validation | |
Machine Commissioning | |
If the PLs of all the subsystem are known, they can be combined simply into a system using Table 13. The rational behind this table is clear. First, that the system can only be as good as its weakest link (subsystem). Second, the more subsystems there are, the greater the possibility for failure.
| PLlow | Nlow | PL |
| a | >3 | Not allowed |
| =<3 | a | |
| b | >2 | a |
| =<2 | b | |
| c | >2 | b |
| =<2 | c | |
| d | >3 | c |
| =<3 | d | |
| e | >3 | d |
| .3 | e | |
| Table 13: PL calculation for series combined subsystems | ||
In the system shown in Figure 135, the lowest Performance Levels are at Subsystems 1 and 2. Both are PLb. Therefore, using Table 13, we can read across b (in the PLlow column), through 2 (in the Nlow column) and find the achieved system PL as b (in the PL column). If all three subsystems were PLb the achieved PL would be PLa.
Note: The application of this table is not mandatory. The use of Annex K of the standard (or SISTEMA) is the preferred method. This table is only intended to provide a very simple approach for small systems.
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| Figure 134: Combination of series subsystems as a PLb system |