Catalogs > Safety Products Catalog > Principles, Standards and Implementation > System Design According to ISO/EN 13849-1:2006
System Design According to ISO/EN 13849-1:2006
| System Design According to ISO/EN 13849-1:2006 |  |
Safety System Architectures (Structures) | |
Mission Time | |
Mean-Time-to-Dangerous Failure (MTTFd) | |
Diagnostic Coverage (DC) | |
Common-Cause Failure (CCF) | |
| Systematic Failure | |
Performance Level (PL) | |
Subsystem Design and Combinations | |
Validation | |
Machine Commissioning | |
Fault Exclusion | |
Subsystems that conform to a PL can be combined into a system using Table 20. The rationale behind this table is clear. First, the system can only be as good as its weakest 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 20: PL calculation for series combined subsystems | ||
In the system shown in Figure 159 the lowest Performance Levels are at Subsystems 1 and 2. Both are PLb. Therefore, using Table 20, 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.
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| Figure 159: Combination of series subsystems as a PLb system |