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29.15 Reliability engineering 973

Engineeringapplication29.9

Availabilityofanelectricalsupplytoalargefactory

Theelectricalsupplytoalargefactoryhasameantimebetweenfailuresof350hours.

When the supply fails it takes 3 hours to repair the failure and restore the supply.

Calculate the average availability of the electricalsupply tothe factory.

Solution

Failure rate of the supply is k where k = 1/350 failures per hour. Using Equation

(29.3) withT r

= 3 hours gives

A =

1

= 0.992 that is,99.2%

1 +3/350

The supply isup and running for99.2% of the time.

So far we have only examined systems in which failure was caused by one or more

componentseachwiththesamefailurerateorMTBF.Amorecommonsituationisonein

whichthedifferentcomponentsofasystemhavedifferentdegreesofreliability.Itisstill

useful to be able to calculate the overall reliability of the system although the analysis

ismorecomplicated.Inordertodosoitisnecessarytodefinethetermreliability.From

Equation(29.1)weknowthatP(t)definestheprobabilityofoneormorefailuresduring

atimeperiod,t.Thereforetheprobabilityofnofailuresisgivenby1−P(t).Thequantity

1−P(t)iscalledthereliabilityofthesystemduringatimeperiodt,andisdenotedR(t),

thatis

R(t) =1−P(t) =e −kt (29.4)

R(t) can be interpreted as the probability a component works properly during a period

t. We now examine the reliabilityoftwo simple systemconfigurations.

29.15.1 Seriessystem

A series system is one in which all the components of a system must operate satisfactorily

if the system is to function correctly. Consider a system consisting of three

components, shown in Figure 29.23. The reliability of the system is the product of the

reliabilitiesofthe individual components,that is

R =R A

R B

R C

(29.5)

This formula is a direct consequence of the fact that the failure of any one of the components

is an independent event. So the probability of the system not failing, that is its

reliability,istheproductoftheprobabilitiesofeachofthecomponentsnotfailing.(See

Section28.7 forindependentevents.)

Input A B C Output Figure29.23

Seriessystem.

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