Safety and SIS · Glossary

What is Fault Tree Analysis?

Symbol: FTAtop down logic modelSafety and SIS

Fault tree analysis is a top down method that models how combinations of basic failures lead to an undesired top event using AND and OR gates.

Fault Tree Analysis Explained

The top event, such as SIF fails on demand, is broken down into causes. AND gates need all inputs to fail; OR gates need any one.

It handles complex architectures and common causes better than simple formulas, and shows which components dominate.

AND: P = P₁ × P₂ OR: P ≈ P₁ + P₂ (small P)

P	probability of the event

Example: Two sensors in 1oo2 with PFD 0.01 each: AND gate gives 10⁻⁴, before common cause.

What is Fault Tree Analysis?

Fault tree analysis is a top down method that models how combinations of basic failures lead to an undesired top event using AND and OR gates.

Fault tree or event tree?

Fault trees work back from one event to its causes; event trees work forward from an initiating event to outcomes.

Why is the OR formula approximate?

Exactly, P = 1 − (1 − P₁)(1 − P₂); for small probabilities the product term is negligible.

What is SIL (safety integrity level)

A practical guide on instrumentationblog.in with more detail on fault tree analysis and how it is applied on real plants.

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