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Fault tree analysis template
Fault tree analysis (FTA) is a top-down method that starts from one undesired event and works down through the combinations of events that could cause it, joined by AND gates (all inputs needed) and OR gates (any one input is enough), to basic events that are not broken down further. With a probability for each basic event over a stated period, the gates give the probability of the top event, and the minimal cut sets show which combinations of basic events cause it. Page 1 of the PDF has a squared drawing area with a key to the standard symbols, an event table with ID, type, parent and probability, the minimal cut sets and the top event probability. Page 2 has how to build and work out a tree, how to find the minimal cut sets and a worked example tree drawn with its numbers. The Excel version takes up to 30 events and works the gates out bottom-up for up to 6 levels. It gives the top event probability, shows how far the top event falls when each basic event is set to 0 and ranks the events, checks the tree for a missing parent, a gate with no inputs or a duplicate ID, and works out the probability of each cut set you type in.
Free to use: print it, copy it and edit it for your team. Enter your name and work email once to download.
When to use it
When to use fault tree analysis
- A failure with several possible paths, where the 5 Whys would follow only one: an unplanned line stop, a machine breakdown, a safety device that did not act.
- When you need to know which single failures can cause the event on their own, and where a second, independent safeguard would close a path.
- When you have failure and stop records and want to compare the paths with numbers, not opinions.
- In a reliability or safety review of equipment, alongside an FMEA that works from the bottom up.
How to fill it in
- 1
Define the top event and the period
One undesired event, stated exactly: what, where, how big. Every probability will refer to the same period, for example one month.
- 2
Break it down
Under each event, ask what immediate events could cause it. Use an OR gate if any one alone is enough and an AND gate if all must happen together.
- 3
Stop at basic events
A circle for an event you will not break down and can give a probability, a diamond for an event you choose not to develop, with an estimate.
- 4
Give each a probability
The chance it happens in the period, from maintenance and stop records or an estimate. Write where each number came from.
- 5
Work up the tree
AND = product of the input probabilities, OR = 1 − product of (1 − p), from the bottom gates up to the top event. The formulas assume the events are independent.
- 6
Find the minimal cut sets and act
List the smallest sets of basic events that cause the top event. Work first on single-event cut sets and on the events whose removal lowers the top event most.
A filled-in example
Illustrative, not a benchmarkAn example: conveyor C3 stops for more than 10 minutes, with probabilities over one month from the maintenance and stop records (illustrative, as on page 2 of the PDF and the Excel worked example).
- The top event is an OR of the drive stopping (G1) and the belt stopping (G2). G1 is an OR of a motor overload trip (0.05), power to the drive lost (0.02) and the gearbox seizing (G3).
- G3 is an AND: the gearbox seizes only when a lubrication PM is missed (0.10) and the temperature alarm fails to warn (0.20), so G3 = 0.10 × 0.20 = 0.02.
- G1 = 1 − 0.95 × 0.98 × 0.98 = 0.0876. G2 is an OR of the belt running off track (0.04), a jam at the transfer (0.08) and a belt splice failure (0.01, undeveloped): 1 − 0.96 × 0.92 × 0.99 = 0.1256.
- Top event = 1 − (1 − 0.0876) × (1 − 0.1256) = 0.2022, about one month in five. The six minimal cut sets add up to 0.22, the rare-event estimate, a little high.
- With the jam set to 0 the top event falls to 0.1329, the largest drop (0.0694). The motor overload trip is next (0.0420), then the belt running off track (0.0332).
The team worked on the jam at the transfer point first. The missed lubrication and the temperature alarm each lower the top event by the same 0.0163, because preventing either input breaks the AND gate.
Common mistakes
A vague top event
'Conveyor problems' gives a tree with no edges. Name one event with a place and a size, such as a stop of more than 10 minutes.
Mixing periods
A probability per shift and another per year cannot be combined. Put every basic event on the same period.
Treating linked events as independent
Two pumps on one power supply do not fail independently. Make the shared cause its own basic event, or the AND gate will look far safer than it is.
Stopping at the drawing
The tree is only useful when it leads to action: close single-event cut sets, add a second safeguard where a path hangs on one event, and update the numbers when the records change.
Download the template
Free to use: print it, copy it and edit it for your team. Enter your name and work email once to download.
Run this template in LeanSuite
In LeanSuite the RCA Agent in every Kaizen and EWO reads the problem, your attached manuals and SOPs and your plant's past problems, suggests a 5 Whys, 4M1D and fishbone you can edit, and sends the countermeasures you keep to the action plan with an owner and a due date.
In the lean glossary
Guides
- Fault Tree Analysis: Event-Based Root Cause Analysis for Manufacturing
- Root Cause Analysis for Equipment Failures: Methods and Framework
- Top Root Cause Analysis Tools for Manufacturing Problem Solving
- FMEA in Manufacturing: Failure Mode and Effects Analysis Complete Guide
- Reliability-Centered Maintenance: A Practical Guide
FAQ
Fault tree analysis template: common questions
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