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11.3.3
Determining the safety figures
In order to determine the safety figures for the entire system, the figures of the individual subsystems
must be investigated, calculated and evaluated. As the subsystems in both examples (STO and SS1)
are nearly identical, the following chapter applies to STO as well as SS1.
11.3.3.1
Subsystem SB1
Subsystem SB1 contains both position switches B1 and B2. The mechanical switches of type PSEN
me4 from Pilz GmbH und Co. KG serve as a specific example in this case.
Average Diagnostic Coverage – DC
}
DC
avg
}
B1 and B2 are monitored for plausibility, cross circuits and short circuits using the safety relay K1.
Source: DIN EN ISO 13849-1, Appendix E, Table E.1
Failure resulting from common cause – CCF
The following measures are taken in order to fulfill the requirements for the prevention of errors from a
common cause. For each measure category, a specific number of points are allotted. The maximum
value for CCF is 100 points. At 65 points, all CCF requirements are considered to be fulfilled.
}
Separation of the wiring: 15 points in category "Isolation/Separation"
}
Use of NC and NO contacts: 20 points in category "Diversity"
}
Protection from overvoltage and use of tried-and-tested components: 20 points in category
"Design/Application/Experience"
}
FMEA of the wiring example: 5 points in category "Assessment/Analysis"
}
Position switches are set in accordance with the manufacturer specification: 10 points in category
"Setting"
}
-> CCF of subsystem SB1: 70 total points
Source: DIN EN ISO 13849-1, appendix F, table F.1
Nominal life span – B
For the position switch with positive opening B1, fault exclusion for the electrical contact is possible.
For the electrical NO contact of B2, the B
the mechanical part of B1 and B2.
}
B1
(NC)
Elimination of dangerous component errors possible for the electrical contact
B
10D
}
B2
(NO)
B
10D
}
B2
(NO)
B
10D
Source: Pilz GmbH und Co. KG
Manual PMC SR6 safety module
1005344-en-US-01
Intern
of subsystem SB1: 99%
10D
(mechanical): 2,000,000 cycles
(mechanical): 2,000,000 cycles
(electrical): 2,000,000 cycles
avg
value is taken from 2000000 cycles. This is also true for
10D
Pilz | 37
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