RAM System Reliability Analysis 3 — Questions and Answers
Question 1: A system has an MTBF of 500 hours and an MTTR of 10 hours. What is its steady-state availability?
- 0.980 (Correct answer)
- 0.950
- 0.990
- 0.975
Correct answer: 0.980
Steady-state availability = MTBF / (MTBF + MTTR) = 500 / (500 + 10) = 500 / 510 ≈ 0.980.
Question 2: What is the 'bathtub curve' in reliability engineering used to describe?
- The relationship between system cost and reliability
- The variation in failure rate over a product's lifetime (Correct answer)
- The distribution of repair times for complex systems
- The probability of failure during stress testing
Correct answer: The variation in failure rate over a product's lifetime
The bathtub curve illustrates how failure rate changes over a product's life: decreasing during infant mortality, constant during useful life, and increasing during wear-out.
Question 3: Which redundancy strategy activates the backup component only when the primary component fails?
- Active (hot) redundancy
- Standby (cold) redundancy (Correct answer)
- Warm standby redundancy
- K-out-of-N redundancy
Correct answer: Standby (cold) redundancy
Cold (standby) redundancy keeps the backup in a non-powered, non-operating state until the primary fails, at which point a switch activates the backup.
Question 4: In reliability-centered maintenance (RCM), what is the primary criterion for selecting a maintenance task?
- The task must be cost-effective and technically feasible for addressing the failure mode (Correct answer)
- The task must be performed on a calendar-based schedule
- The task must eliminate all possible failure modes
- The task must be approved by the original equipment manufacturer
Correct answer: The task must be cost-effective and technically feasible for addressing the failure mode
RCM selects maintenance tasks based on two criteria: the task must be technically feasible (capable of preventing the failure) and worth doing (cost-effective).
Question 5: What does the Risk Priority Number (RPN) in FMEA represent?
- The product of Severity, Occurrence, and Detection ratings (Correct answer)
- The ratio of failure frequency to repair cost
- The sum of all failure mode probabilities
- The time interval between critical failures
Correct answer: The product of Severity, Occurrence, and Detection ratings
RPN = Severity × Occurrence × Detection, and is used to prioritize which failure modes require corrective action in FMEA.
Question 6: A component follows an exponential failure distribution with a failure rate λ = 0.002 failures/hour. What is its reliability at t = 100 hours?
- 0.819 (Correct answer)
- 0.980
- 0.200
- 0.905
Correct answer: 0.819
For exponential distribution, R(t) = e^(-λt) = e^(-0.002 × 100) = e^(-0.2) ≈ 0.819.
Question 7: Which type of common cause failure (CCF) occurs when a single external event simultaneously affects multiple redundant components?
- Dependent failure
- Common mode failure (Correct answer)
- Cascading failure
- Systematic failure
Correct answer: Common mode failure
Common mode failure occurs when a single root cause — such as a power surge or environmental event — simultaneously causes multiple components to fail in the same manner.
RAM System Reliability Analysis 3
A system has an MTBF of 500 hours and an MTTR of 10 hours.
What is its steady-state availability?