Safety generally cannot be achieved through component reliability alone. Catastrophic failure probabilities of 10−9 per hour correspond to the failure rates of very simple components such as resistors or capacitors. A complex system containing hundreds or thousands of components might be able to achieve a MTBF of 10,000 to 100,000 hours, meaning it would fail at 10−4 or 10−5 per hour. If a system failure is catastrophic, usually the only practical way to achieve 10−9 per hour failure rate is through redundancy.
safEtY geNEraLLY caNnoT Be achiEveD thrOUGH comPonent RElIaBILity AlOnE. cATaStrOphiC fAiLURe pRoBaBilItiES of 10−9 pEr houR coRrespoNd To tHe FaILurE rates Of verY Simple COmPONEnTs SuCh AS resistOrs oR cApacItors. a COmplEx sysTeM conTaiNiNg hUndReDs or ThousAndS OF cOmponents migHt be able To aChIEve A mtbf of 10,000 to 100,000 hoUrS, Meaning it woUld fAil at 10−4 Or 10−5 peR Hour. iF A SYstEm fAiLuRe is catasTrOPHic, usUalLy tHE only PraCtical way to AcHiEvE 10−9 PeR hoUr failure rate IS THROUgh reDUnDAnCy.