Fault Tree Analysis (FTA)
An undesired event sits at the top, and beneath it the conditions that must combine for it to happen. The tree is built from logic gates; enter probabilities on the basic events and the probability of the top event is calculated for you.
Register nowWhen to use it?
- Seeing which combinations of conditions can produce a failure or accident.
- Talking about risk in numbers: how much each branch contributes to the total.
- Showing which branch a given safety measure cuts.
Step by step
- 1The menu at the top left switches between trees in the same project, and lets you add, rename or delete one.
- 2Create the top-event box on the empty screen, or load the ready-made example.
- 3Right-click a box and use Edit to set its name, description and — on basic events — its probability.
- 4From the same menu add an event beneath it: event, basic event, undeveloped event or conditioning event.
- 5From that menu you also add logic gates: AND, priority AND, OR, exclusive OR or inhibit.
- 6Enter probabilities as percentages on the basic events; the gates above and the top event are computed from them.
- 7Drag boxes to arrange them and use the mini map at the bottom right to navigate a large tree.
Keyboard shortcuts (desktop)
Tips
- An AND gate multiplies the probabilities below it — everything must happen, so the result shrinks. An OR gate needs only one, so the result grows.
- Branches without a probability are not counted; the number at the top only covers the data you entered.
- Basic events are circles, undeveloped events are diamonds: marking the branches you did not dig into keeps the tree honest.
Worked example
Example: cold store temperature rose above the limit
In a food warehouse the temperature stayed above the limit for two hours and the stock had to be destroyed. The top event is written down and the tree works downwards through logic gates, showing which failures had to happen together.
Top event
- Cold store above the temperature limit for two hours
OR gate — any one is enough
- Cooling stopped
- Heat got in
- The alarm did not fire and nobody noticed
Under "cooling stopped" (OR)
- Compressor failure
- Power cut
- Thermostat set wrongly
Under "alarm did not fire" (AND)
- Sensor faulty
- Backup sensor never installed
- Remote notifications switched off
The tree shows that cooling failing is not enough on its own — the alarm has to fail as well. So the cheapest countermeasure is not a new compressor, it is fitting the backup sensor. This is where fault tree analysis sends money to the right place.
Frequently asked questions
What is fault tree analysis (FTA)?
A method that puts an undesired event at the top and works downwards through logic gates to show which combinations of failures would produce it. It came from aerospace and nuclear engineering and is now used in safety and process analysis generally.
What is the difference between an AND gate and an OR gate?
Under an OR gate, any one of the events below is enough to cause the event above. Under an AND gate, all of them must happen together. That distinction is the heart of the method: AND gates show you where the system is protecting itself.
Fault tree analysis or 5 Whys?
5 Whys traces a single chain backwards from something that already happened. A fault tree maps every route to an event that has not happened yet. One looks at the past, the other at the future.
How far down should the tree go?
Down to events you can no longer split and can act on directly. "Sensor faulty" is low enough, because you can write a countermeasure against it. "The system does not work" is not.
Is it free?
Yes. Klarsti is free and ad-free right now, and you do not need an account to build a fault tree.