ITPEC FE Morning October 2019 Question 16
ITPEC FE Morning October 2019 — Question 16 of 80
System Availability (Series) — calculate availability of two identical devices connected in series.
Given: MTBF = 45 hours, MTTR = 5 hours, two devices in series.
Step 1 — Single device availability:
- A = MTBF / (MTBF + MTTR) = 45 / (45 + 5) = 0.90
Step 2 — Series connection (multiply):
- A_system = 0.90 × 0.90 = 0.81
Why not others:
- (b) 0.90 — availability of a single device, not the whole system
- (c) 0.95 — no valid combination produces this
- (d) 0.99 — this would be parallel: 1 − (1 − 0.9)² = 0.99
Key rules:
- Availability = MTBF / (MTBF + MTTR)
- Series: A₁ × A₂ (reliability decreases)
- Parallel: 1 − (1 − A₁)(1 − A₂) (reliability increases)
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