Fundamentals, Terminology, and Core Concepts
Build a precise vocabulary first
Start with the chain from hazard to harm: a hazard is a potential source of harm; harm is injury or damage to people, property, or environment; risk combines likelihood and severity of that harm. Do not treat hazard and risk as synonyms in questions. Related concepts you must recognize: exposure, tolerable risk, residual risk, ALARP, defense in depth, safety function, safety instrumented system, reliability, availability, maintainability, independence, redundancy, diversity, and common-cause failure. Know the safety lifecycle: concept, scope definition, hazard and risk analysis, safety requirements, design, implementation, operation, modification, and decommissioning.
Traps that appear in questions
- Calling a reliable system "safe"; reliability and safety are not the same.
- Averaging ordinal risk-matrix categories (e.g., severity 4 and likelihood 2 becoming "risk 3").
- Confusing a safety function with a single device; the function includes sensor, logic, and final element.
- Assuming redundancy eliminates common-cause failure; identical devices with shared utilities can fail together.
Worked rule of thumb
Use a risk matrix, not arithmetic, for categorized risk. If severity is 4 and likelihood is 2 on a 5×5 matrix, read the cell (often high) and record the basis; never average to 3. Document assumptions so another reviewer can reproduce the rating.