MeanTimeBetweenFailures(MTBF)
ReliabilityisquantifiedasMTBF(MeanTimeBetweenFailures)forrepairableproduct
andMTTF(MeanTimeToFailure)fornon-repairableproduct.Acorrectunderstanding
ofMTBFisimportant.ApowersupplywithanMTBFof40,000hoursdoesnotmean
thatthepowersupplyshouldlastforanaverageof40,000hours.Accordingtothetheory
behindthestatisticsofconfidenceintervals,thestatisticalaveragebecomesthetrue
averageasthenumberofsamplesincrease.AnMTBFof40,000hours,or1yearfor1
module,becomes40,000/2fortwomodulesand40,000/4forfourmodules.Sometimes
failureratesaremeasuredinpercentfailedpermillionhoursofoperationinsteadof
MTBF.TheFITisequivalenttoonefailureperbilliondevicehours,whichisequivalent
toaMTBFof1,000,000,000hours.TheformulaforcalculatingtheMTBFis
Sita=T/R.
Sita=MTBF
T=totaltime
R=numberoffailures |