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Part Four
Improvement
Preventing failure occurring
Once a thorough understanding of the causes and effects of failure has been established, the
next responsibility of operations managers is to try to prevent the failures occurring in the
first place. The obvious way to do this is to systematically examine any processes involved
and ‘design out’ any failure points. Many of the approaches used in Chapters 4 and 5 on
process and product/service design and Chapter 17 on quality management can be used to
do this. In this section we will look at three further approaches to reducing risk by trying
to prevent failure: building redundancy into a process, ‘fail-safeing’ some of the activities in
the process, and maintaining the physical facilities in the process.
Redundancy
Redundancy
Building in redundancy to an operation means having back-up systems or components in
case of failure. It can be expensive and is generally used when the breakdown could have
a critical impact. It means doubling or even tripling some parts of a process or system in case
one component fails. Nuclear power stations, spacecraft and hospitals all have auxiliary
systems in case of an emergency. Some organizations also have ‘back-up’ staff held in reserve
in case someone does not turn up for work. Rear-brake lighting sets in buses and trucks
contain two bulbs to reduce the likelihood of not showing a red light. Human bodies contain
two of some organs – kidneys and eyes, for example – both of which are used in ‘normal
operation’ but the body can cope with a failure in one of them. The reliability of a component
together with its back-up is given by the sum of the reliability of the original component and
the likelihood that the back-up component will both be needed and be working.
where
R a+b = R a + (R b × P (failure))
R a+b = reliability of component a with its back-up component b
R a = reliability of a alone
R b = reliability of back-up component b
P (failure) = the probability that component a will fail and therefore component b
will be needed.
Worked example
The food manufacturer in the earlier worked example has decided that the cheese depositor
in the pizza-making machine is so unreliable that it needs a second cheese depositor
to be fitted to the machine which will come into action if the first cheese depositor fails.
The two cheese depositors (each with reliability = 0.9) working together will have a
reliability of:
0.9 + [0.9 × (1 − 0.9)] = 0.99
The reliability of the whole machine is now:
0.95 × 0.99 × 0.97 × 0.99 × 0.98 = 0.885
Redundancy is often used for servers, where system availability is particularly important.
In this context, the industry used three main types of redundancy.