What is the meaning of 'defense-in-depth' in the context of plant safety?

• A. Multiple layers of safety and independent protections to prevent failure.
• B. A single-layer safety approach with one protection.
• C. Relying solely on operator action.
• D. No safety concept involved.

Answer: (C)

Defense-in-depth means safety is achieved through several layers of protection, each independent from the others, so a failure in one layer doesn’t lead to an unsafe condition. These layers—from robust design features that prevent events, to automatic protections that detect and stop abnormal conditions, to administrative controls, procedures, operator actions, and emergency responses—work together, with diversity and independence, to cover different failure modes. The core idea is redundancy in a way that reduces the chance that a single fault or a single human error can cause harm. It’s not about relying on one action or on no safety concept at all, but about multiple, independent safeguards that collectively prevent failure or mitigate its consequences.