Availability
Topic appears in the following WE1 Exams:
- 2021 07 26 Q2 (5 points)
- 2021 02 05 Q3 (3 points)
Therefore it appears in 33% of the WE1 exams and it is worth 4 points on average.
Summary
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Mean Time to Repair (
MTTR)$MTTR$): Average time between the occurrence of a fault and service recovery, also known as the downtime -
Mean Time To Failures (
MTTF)$MTTF$): Mean time between the recovery from one incident and the occurrence of the next incident, also known as uptime -
Mean Time Between failure (
MTBF)$MTBF$): Mean time between the occurrences of two consecutive incidents
We can then define:
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Availability: The probability that a component is working properly at time
t$t$: $$A = MTTF / (MTTF+MTTR)$$ A given component could have failed in the interval $(0,t)$, but it could have been repaired before t -
Reliability: The probability that a component has always been working properly during a time interval $(0,t)$, it requires that the component never fails in the interval. $$R =
e-e^{-λtt} \space where \space λ = 1/MTTFMTTF$$
Availability is typically specified in nines notation. For example 3-nines availability corresponds to 99.9%, 5-nines availability corresponds to 99.999% availability.
Calculating availability
Availability in series
If failure of a part leads to the combination becoming inoperable.
The combined availability is the product of the availability of the component parts