Public Fault Tree Analyser (PFTA)

Free and open-source fault tree analysis.

• For rudimentary documentation, see DOCS.md.
• For an overview of the mathematics, see MATHS.md.
• For example fault trees, see examples/.

Features

• Enforces declaration of at least one value of time t.
• Supports the general fault tree analysis framework where objects can have both a failure probability q(t) and a failure intensity ω(t) (with failure rate λ(t) = ω(t) / (1 − q(t))).
• Supports Monte Carlo (sampling of distributions) for failure parameters out of the box.

Limitations

• Does not support negative logic (NOT gates). Only supports coherent fault trees (i.e. those whose logic is reducible to AND gates and OR gates).
• Does not support common cause failure groups.

Textual input

PFTA reads a textual representation of a fault tree. For example:

- times: 1, 2, 3, 5, 10, 100, 1000
- time_unit: h
- seed: Candley McCandleface
- sample_size: 1000

Gate: CAN
- label: Candlelight fails
- type: OR
- inputs: IGN, EXP, EXT

Gate: IGN
- label: Candle fails to ignite
- type: AND
- inputs: MAT, LTR

Event: MAT
- label: Single match fails to ignite candle
- model_type: Fixed
- probability: triangular(lower=0.1, upper=0.3, mode=0.2)
- intensity: 0

Event: LTR
- label: Lighter fails to ignite candle
- model_type: Fixed
- probability: loguniform(lower=0.001, upper=0.01)
- intensity: 0

Event: EXP
- label: Candle explodes on ignition
- appearance: Undeveloped
- model_type: Fixed
- probability: 0
- intensity: 0

Event: EXT
- label: Candle extinguishes
- model_type: ConstantRate
- mean_failure_time: 3
- mean_repair_time: inf

This allows for text-based version control of a fault tree.

Output

Output files consist of:

• a table (TSV) of events,
• a table (TSV) of gates,
• a table (TSV) of cut sets under each gate, and
• vector graphics (SVG) for each top gate and paged gate at each value of time.

Installation

$ pip3 install pfta

• If simply using as a command line tool, do pipx instead of pip3 to avoid having to set up a virtual environment.
• If using Windows, do pip instead of pip3.

Usage (command line)

$ pfta [-h] [-v] ft.txt

Perform a fault tree analysis.

positional arguments:
  ft.txt         fault tree text file; output is written to `{ft.txt}.out/`

options:
  -h, --help     show this help message and exit
  -v, --version  show program's version number and exit

Usage (scripting example)

from pfta.core import FaultTree

fault_tree = FaultTree('''
- times: nan

Event: A
- model_type: Fixed
- probability: 0
- intensity: 0.9

Event: B
- model_type: Fixed
- probability: 0.7
- intensity: 0

Event: C
- model_type: Fixed
- probability: 0
- intensity: 1e-4

Gate: AB
- type: AND
- inputs: A, B

Gate: AB_OR_C
- type: OR
- inputs: AB, C
''')

fault_tree.gates[0]
# Gate(id_='AB', is_paged=False, type_=<GateType.AND: 2>, vote_threshold=None, input_ids=['A', 'B'], is_top_gate=False, flattened_indexer=FlattenedIndexer(time_count=1, sample_size=1, flattened_size=1), computed_expression=<...>, computed_probabilities=<...>, computed_intensities=<...>, computed_rates=<...>)

fault_tree.gates[0].computed_rates
# [0.63]

fault_tree.gates[1]
# Gate(id_='AB_OR_C', is_paged=False, type_=<GateType.OR: 1>, vote_threshold=None, input_ids=['AB', 'C'], is_top_gate=True, flattened_indexer=FlattenedIndexer(time_count=1, sample_size=1, flattened_size=1), computed_expression=<...>, computed_probabilities=<...>, computed_intensities=<...>, computed_rates=<...>)

fault_tree.gates[1].computed_rates
# [0.6301]

Licence

Copyright 2025 Conway.
Licensed under the GNU General Public License v3.0 (GPL-3.0-only).
This is free software with NO WARRANTY etc. etc., see LICENSE.