ITPEC IP October 2021 Question 59

Topic: System ComponentAlso asked in October 2018

Which of the following is an appropriate description concerning reliability?

a)The availability of the device increases if the MTBF and MTTR are lengthened.b)The availability of the device increases if the MTBF is lengthened and the MTTR is shortened.c)If the MTBF is lengthened, the MTTR is shortened.d)If the MTTR is shortened, the MTBF is lengthened.

Explanation

Longer MTBF and shorter MTTR raise availability — availability is the share of time the device is working.

MTBF (mean time between failures) is the average operating time from one repair to the next failure; MTTR (mean time to repair) is the average time each repair takes. Availability = MTBF ÷ (MTBF + MTTR).

For example, with an MTBF of 90 hours and an MTTR of 10 hours availability is 90 ÷ 100 = 0.9. Lengthening the MTBF to 190 hours gives 190 ÷ 200 = 0.95; shortening the MTTR to 5 hours gives 90 ÷ 95 ≈ 0.947. Both changes raise it.

Answer (b)

Why not others:

•(a) — a longer MTTR means longer downtime per failure, which lowers availability
•(c) and (d) — MTBF and MTTR are independent: MTBF depends on how rarely the device fails (reliability), MTTR on how quickly it is repaired (maintainability); improving one does not change the other

Key rule: Raise availability from two sides: fail less often (longer MTBF) and recover faster (shorter MTTR).

System Component · AI-written, may contain errors

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