ITPEC FE Morning October 2022 Question 57
A system is planned to be operational 22 hours per day. However, there are two (2) failures with downtimes of 0.5 hours and 1.5 hours in 100 days. Which of the following is the approximate MTBF of this system during the period in hours?
Explanation
About 1,100 hours — MTBF is the operating time divided by the number of failures.
- •Planned operation over the period: 22 × 100 = 2200 hours
- •Total downtime: 0.5 + 1.5 = 2 hours
- •Actual operating time: 2200 - 2 = 2198 hours
- •MTBF: 2198 ÷ 2 = 1099, about 1,100 hours
The downtime is so small against 2,200 hours that 2200 ÷ 2 = 1100 gives the same approximate answer.
Answer (d)
Why not others:
- •(a) 0.00091 — is 1 ÷ 1099, the failure rate per hour, the reciprocal of MTBF
- •(b) 1.0 — is 2 ÷ 2, the downtime per failure, which is MTTR
- •(c) 2.0 — is the total downtime of the two failures, not a time between failures
Key rule: MTBF = operating time ÷ number of failures, MTTR = total downtime ÷ number of failures, failure rate = 1 ÷ MTBF, and availability = MTBF ÷ (MTBF + MTTR).
System Component · AI-written, may contain errors
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