Skip to content

Free template

Updated

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.

Page 1 of the fault tree analysis templateFields for the top event, period, line, team and date, a squared drawing area with a top event box and a key to the event, OR gate, AND gate, basic event and undeveloped event symbols, and an event table with ID, type, parent ID, description and probability, minimal cut sets and the top event probability.
Drawing area on squared paperFTA symbols keyEvent table with parent IDsAND and OR gate formulasMinimal cut setsTop event probabilityEach event set to 0Worked example tree
Filled-in Fault tree analysis template: conveyor C3 stopping for over 10 minutes
The same sheet filled in with the worked example (page 2 of the PDF, illustrative numbers). Open full size (opens the picture in a new tab)

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. 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. 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. 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. 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. 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. 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 benchmark

An 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 example fault tree with its monthly probabilities

  • ORConveyor C3 stops for more than 10 minutes0.2022
    • ORG1: the drive stops0.0876
      • Motor overload trip0.05
      • Power to the drive lost0.02
      • ANDG3: the gearbox seizes0.02
        • Lubrication PM missed0.10
        • Temperature alarm fails to warn0.20
    • ORG2: the belt stops0.1256
      • Belt runs off track0.04
      • Jam at the transfer0.08largest single effect
      • Belt splice failure (undeveloped)0.01

Top event 0.2022, an OR of the drive stopping, 0.0876, and the belt stopping, 0.1256. The gearbox seizing is an AND of a missed lubrication PM, 0.10, and a failed temperature alarm, 0.20, so 0.02.

OR gate: 1 − product of (1 − p). AND gate: product of p, so G3 = 0.10 × 0.20 = 0.02. Removing the jam lowers the top event the most, to 0.1329.
  • 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.

See it in a 45-minute demo

FAQ

Fault tree analysis template: common questions

  1. 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 and OR gates, to basic events. With a probability for each basic event it gives the probability of the top event and shows which combinations matter most.

  2. For independent events, an AND gate is the product of the input probabilities and an OR gate is 1 minus the product of (1 − p) for each input. For two inputs of 0.1 and 0.2, the AND gate gives 0.02 and the OR gate gives 0.28. All probabilities must refer to the same period.

  3. The smallest set of basic events that together cause the top event: take away any one of them and that combination no longer causes it. A cut set with a single event is a single point of failure.

  4. A rectangle is the top event or an intermediate event. The OR gate, with a curved base, means any one input causes the event above it; the AND gate, with a flat base and a round top, means all inputs must happen together. A circle is a basic event and a diamond is an undeveloped event, one that is not broken down further.

  5. An FMEA works bottom-up: it takes each component or process step, asks how it could fail and what the effect would be. Fault tree analysis works top-down from one undesired event to the combinations of causes. They are often used together: the FMEA finds the failure modes and the fault tree shows how they combine.

More free templates

All templates
Browse all 199 templates

Can't find the template you need?

Tell us what you're looking for. We'll make it and send it to your inbox, free.

A shift handover sheet, a red tag log, a changeover checklist for one machine: if your team uses it on the floor, ask.

By sending this form you agree to our Terms of Service and Privacy Policy.

Pass it on

We want LeanSuite to be the best place on the internet for lean help. If this was useful, send it to someone on your team or in your network who needs it.