ITPEC FE Subject A October 2024 Question 44
A device that operates 24 hours a day, 360 days a year has an MTBF value of 1,440 hours. Which of the following is the average number of failures for this device for 360 days? Here, the result is rounded to the closest whole number, and the MTTR of the device is ignored.
Explanation
6 failures — divide the total operating time by the mean time between failures.
- •Operating time in 360 days: 24 × 360 = 8640 hours
- •Average number of failures: 8640 ÷ 1440 = 6
Checked another way: 1,440 hours is 1440 ÷ 24 = 60 days of continuous operation, and 360 days hold 360 ÷ 60 = 6 such intervals. MTTR is ignored, so all 8,640 hours count as operating time.
Answer (b)
Why not others:
- •(a) 3 — would mean one failure every 2,880 hours, twice the stated MTBF
- •(c) 9 — would mean one failure every 960 hours, shorter than the stated 1,440
- •(d) 12 — would mean one failure every 720 hours, half the stated MTBF
Key rule: MTBF is the average operating time between failures, so expected failures = operating time ÷ MTBF, with both in the same unit.
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