Biomedical Engineering Reference
In-Depth Information
CASE STUDY 7.5
If the probabilities of failure in one branch are as described earlier, determine the chances of the
system failing due to a host computer Internet failure and hence estimate a value for O (FMEA).
Using Equation (7.6) the probability of host Internet connection failure is
P /1
000
1/5
00
1/5
000 000
.
323 /312 5
.
If this were our only occurrence factor then we can see (from Table 7.11(d)) that this gives a
value for O of between 2 and 3. It is, arguably, closer to 2 so we would use this in our FMEA.
More importantly the fault tree helps illustrates all the ways a fault can occur, and it does not
let us forget the stupidity of the end-user!
If you wish to know more about fault tree analysis and reliability, I refer you to O'Connor (2002)
and Carter (1997) , but you need to have a good background in statistics and applied mathematics
to follow them. You should also obtain a copy of BS EN 61025:2007 or its equivalent.
CASE STUDY 7.6
A circuit in an OR anesthesia machine was found to fail 1/100 uses. An engineer suggests that
using three in parallel would improve reliability. Show that this is true.
1/100
Case 1
Case 2
1/100
1/100
1/100
In the single case (case 1) the probability of failure is
P () 1/10 0
In the second case, when three circuits are in parallel, failure can only occur when all three have
failed. Hence this is a joint probability. The probability of two independent things happening at
the same time is given by
() ().()
PABPAPB
(7.8)
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