R t
ð Þ ¼ exp Àλt
ð
Þ 1 þ λt þ
1
2
λt
ð Þ
2
h
i
ð9:12Þ
The term
1
2 λt
ð Þ
2 exp Àλt
ð
Þgives the increase in reliability arising from a second
additional standby unit.
If the less than perfect reliability of the switching system is considered, then these
improvements are, of course, reduced.
9.7 Measures of Reliability
Reliability is measured in different ways for different infrastructure services, examples being:
(1) for electricity, by the number, length, and time of power outages, and by changes
in voltage and frequency levels;
(2) for freight transport, by the meeting of delivery schedules and by loss or damage
to goods being transported;
(3) for passenger transport, by delays in travel time;
(4) for telecommunications, by delays in dial tone, uncompleted telephone connections, and interferences with the sound; and
INPUT
Switching System
Components or units
OUTPUT
1
2
3
n
Fig. 9.7 Standby system
9.7 Measures of Reliability
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