I’m assuming that the type of conversion that you are talking about is to convert an NPRD failure rate based on operating hours to one based on calendar hours. The hours and failures that RIAC collects to generate the NPRD failure rates are generally expressed in terms of operating hours in order to be compatible with those prediction methodologies that are based solely on operating hours and do not explicitly consider the failure rates associated with extended periods of nonoperation and/or various frequencies of power off/on cycling. The RIAC 217Plus software includes a supporting database of NPRD failure rate data that has been converted to an equivalent “calendar hour” life unit using a duty cycle of 25%. This represents an average duty cycle based on the RIAC assessment of all of the data sources comprising the NPRD data set, so it should be the only duty cycle factor applied to NPRD data. Simply multiply your current NPRD failure rate by 0.25 to get the corresponding failure rate in terms of failures per million calendar hours.
2011-02-11T16:55:14-05:00February 11th, 2011