For a given failure, the Risk Priority Number (RPN) Occurrence factor is derived from the failure rate of the object associated with that failure and the percentage of that item’s overall failures that are attributable to that failure.
The calculation also takes into consideration the expected useful life of the item—either the repair item (object) or the item under test (the full system). The standard calculation involves the following steps:
Retrieving the failure rate of the object associated with the given failure
Multiplying that value by the failure’s failure ratio (percentage of the object’s failures) to derive the failure’s failure rate.
Converting the failure’s failure rate to failures per hour
Calculating the likelihood of failure (Q) using the following equation:
Q = 1 – e-λt
where λ is the failure’s failure rate (in failures per hour) and t is the Occurrence Time Interval (also in hours).
Mapping the value Q (which will fall between 0 and 1) to a 10-point RPN occurrence rating.
Calculating the RPN Values using Special Occurence Attributes
RPN Occurence calculations can overide the normal calculated values by creating an attributes for failure effects called “OccurenceRating”, “OccurenceTimeInterval” or “OccurenceMap”. When one or more of these attributes are included as columns in an eXpress FMECA chart, the calculated RPN will use these values (rather that the automatic calculations) as the corresponding factor.
