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Reference · IEC 61025 Annex A

IEC 61025 fault tree symbols

Every gate and event symbol defined in IEC 61025:2006 Annex A — the international standard for fault tree analysis. Use this page as a quick visual reference; for the underlying mechanics see the glossary entries linked from each symbol.

Logic gates

AND gate

P(out) = ∏ P(in)

Output occurs only if all inputs occur. Flat top, semicircular underside.

OR gate

P(out) = 1 − ∏(1 − P(in))

Output occurs if any input occurs. Curved top, concave underside.

k/n

k-out-of-n (voting) gate

P(out) = Σ over j≥k C(n,j) p^j(1−p)^(n−j)

Output occurs if at least k of n inputs occur. Used for voted-redundancy architectures (2oo3, 3oo3D, etc.).

INHIBIT gate

P(out) = P(in) · P(condition)

Single-input gate enabled by an attached conditioning event. Used to model conditional propagation: input × condition.

XOR

Exclusive-OR gate

P(out) = P(A) + P(B) − 2·P(A)·P(B)

Output occurs if exactly one input occurs (not both). Rarely used in safety FTA, more common in electrical-fault analysis.

PAND

Priority-AND gate

All inputs occur in specified order

Like AND, but inputs must fail in a defined sequence (left-to-right). Models temporal dependence.

NOT gate

P(out) = 1 − P(in)

Negation of input. Use sparingly — fault trees with NOT are non-coherent and lose some intuitive properties.

Event symbols

Basic event

Leaf, λ or P attached

An indivisible failure event with directly assigned probability or failure rate. The leaves of the fault tree.

Undeveloped event

Diamond — not expanded

A failure event that could be expanded but has been left at this level (insufficient information, out of scope, or deliberately black-boxed).

Intermediate event

Rectangle — gate output

An event that results from one or more lower-level events combined through a gate. Most non-leaf nodes in a tree are intermediate events.

House event

P = 0 or 1 (boundary condition)

An event that's deterministically known to be true or false — a system mode, a configuration switch, an environmental condition. Used for what-if analysis.

Conditioning event

Modifier on INHIBIT or PAND

Attached to an INHIBIT or priority-AND gate. Defines the condition under which the gate propagates its input.

→

Transfer in / out

Cross-page link

Triangles linking a sub-tree on one page to its expansion on another. Essential for keeping multi-page fault trees readable.

How FTA Studio implements this All eleven symbols above are drawn to IEC 61025 Annex A geometry exactly. The Boolean semantics match the standard. Round-trippable via IEC JSON export, suitable for sharing with reviewers and submission as part of a safety case.

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