ITPEC IP April 2019 Question 15
Topic: System Component
When MTBF is 600 hours and MTTR is 12 hours, what is the approximate availability?
Explanation
0.98 — availability is MTBF ÷ (MTBF + MTTR), the share of time the system is running.
Over one cycle it runs for the MTBF, 600 hours, then is under repair for the MTTR, 12 hours:
600 ÷ (600 + 12) = 600 ÷ 612 ≈ 0.98
Answer (d)
Why not others:
- •(a) 0.02 — the unavailability, 12 ÷ 612: the fraction of time spent under repair, not running
- •(b) 0.20 — would mean the system is down 80% of the time, an MTTR of 2,400 hours rather than 12
- •(c) 0.88 — would mean 12% downtime, an MTTR of about 82 hours; 12 hours in 612 is only 2%
Key rule: Availability = MTBF ÷ (MTBF + MTTR); when the repair time is small next to the time between failures, the answer must be close to 1.
System Component · AI-written, may contain errors
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