7 Fault consideration, fault exclusion

7.1 General

I n accordance with the category selected, safety-related parts shall be designed to achieve the required
performa n ce level (PLrJ. The ability to resist faults shall be assessed.

7.2 Fault consideration

I SO 13849-2 lists the important fau lts and failures for the var ious technologies. The lists of faults are not exhaustive and, if necessary, additional faul ts shall be considered and listed. In su ch cases, the method of evaluation should also be clearly el aborated. For new components not mentioned in  ISO 13849-2, a failure m ode a n d effects analysis (FMEA, see IEC 60812) shall be carried out to establish the faults that are to be considered for those components.

I n general, the following fault criteria shall be taken into account:
- if, as a consequen ce of a fault, fur ther components fail’the first fault together with all following fau lts sh all be considered as a single fault﹔

- two or more separate faults having a common cause shall be considered as a sin gle fault (known as
a CCF）﹔

- the simultaneous occurrence of two or m ore faul ts having separate causes is considered highly
unlikely and therefore  need not be considered.

7.3 Fault exclusion

I t is not always possibl e to evaluate SRP/CS without assum ing that certai n faults can be excluded. For
detailed information on fault exclusions, sec ISO 13849-2.

Fault excluson is a compromi se between technica l safety requirements and the theoretical poss ibili ty of occurrence of a fau lt.

Fault exclusion ca n be based on

- the technical improbability of occurrence of som e fau lts,
- generally accepted technical experience, independent of the considered application, and
- technical requirements related to the application and the specific hazard.

If faults are excluded, a detailed ju stification shall be given in the technica I documentat ion.