Engineering Design, Systems, and Safety
Design and systems questions test whether you can turn requirements into a safe, reliable, maintainable system, and whether you know where safety effort belongs in the hierarchy of controls.
Core rules
- Requirements must be measurable and traceable, and verified by analysis, inspection, demonstration, or test.
- Codes and standards define minimums; meeting a code does not by itself make a design safe for every use.
- Series reliability R_s = ΠR_i; parallel redundancy R_p = 1 − Π(1 − R_i). Availability = MTBF/(MTBF + MTTR).
- FMEA scores severity, occurrence, and detection, with RPN = S × O × D; high-RPN items get priority for corrective action.
- Risk = probability × consequence. Hierarchy of controls, most to least effective: elimination, substitution, engineering controls, administrative controls, then PPE.
- Human factors: design for expected use, not ideal use; poka-yoke prevents error at the source.
Worked example
Three series components have reliabilities 0.98, 0.95, and 0.99, so R_s = 0.98 × 0.95 × 0.99 = 0.9217. Making the 0.95 component redundant raises that block to 1 − 0.05² = 0.9975, and the system to 0.98 × 0.9975 × 0.99 = 0.9678. Redundancy applied to the weakest link yields the largest gain.
Traps
- Adding reliabilities rather than multiplying them for a series system.
- Treating availability as if it were reliability; they differ by repair time.
- Listing PPE first as a control measure rather than last.
- Choosing a safety factor without weighing the consequence of failure and load uncertainty.