System Safety Engineering and Analysis Flashcards

A Preliminary Hazard Analysis (PHA) is most appropriately conducted:

Answer: Early in the conceptual or design phase, before detailed designs are finalized

PHA is specifically designed for use in the early design or conceptual phase when detailed information is limited. Its purpose is to identify major hazards and define safety requirements before costly design commitments are made. Conducting it after the design is finalized limits the ability to make fundamental changes.

In a Fault Tree Analysis (FTA), a 'basic event' symbol (circle) represents:

Answer: A primary failure that requires no further development and has an assigned probability

In FTA, the circle (basic event) is a leaf node representing an elementary failure for which a probability can be directly assigned from historical data or engineering judgment. It requires no further decomposition. The diamond represents an undeveloped event, rectangles represent developed events (gates), and the top event is the undesired outcome being studied.

Which system safety analysis technique uses a matrix or table to systematically evaluate what happens when each component fails in different modes (e.g., fails open, fails closed, fails to operate)?

Answer: Failure Mode and Effects Analysis (FMEA)

FMEA systematically examines each component, identifies all potential failure modes (how it could fail), and evaluates the effects of each failure on the overall system. FTA works top-down from an undesired event. HAZOP uses guide words applied to process parameters. Change analysis compares a changed system to a baseline.

A HAZOP study uses 'guide words' applied to process parameters. Which guide word indicates that a parameter (such as flow) is entirely absent?

Answer: NO / NONE

In HAZOP, the guide word NO/NONE means the intended parameter (e.g., flow, pressure, temperature) is completely absent—no flow where flow was intended. MORE means a quantitative increase, LESS means a quantitative decrease, and REVERSE means the parameter acts in the opposite direction (e.g., backflow). HAZOP is especially used in process industries.

The minimum cut set in a Fault Tree Analysis represents:

Answer: The smallest combination of basic event failures that is sufficient to cause the top event

A minimum cut set is the smallest group of basic events whose simultaneous occurrence guarantees the top (undesired) event occurs. Identifying minimum cut sets reveals critical failure combinations and single points of failure (cut sets of size 1). Eliminating or protecting against minimum cut sets is the most efficient way to reduce top-event probability.

In system safety, the concept of 'defense in depth' is best described as:

Answer: Implementing multiple independent layers of protection so that no single failure leads to a catastrophic outcome

Defense in depth (also called the 'Swiss cheese model' concept) relies on multiple independent barriers so that even if one layer fails (a 'hole in the cheese'), other layers prevent the hazard from reaching the target. It is a foundational system safety principle used in nuclear, aviation, chemical process, and general industry applications.