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Fault tree analysis (FTA)

What is fault tree analysis?

Fault tree analysis (FTA), sometimes known as event tree analysis, is a method of identifying the possible causes of a system failure. A fault tree is used to graphically illustrate the different potential causes of a failure in the form of a diagram. By using FTA, you can determine what factors contributed to an event (known as a failure), and the probability of it occurring. Once the major causes are identified and addressed, FTA can help maintenance teams prioritize corrective actions.

Who uses fault tree analysis?

Fault tree analysis is used by system designers, process designers, project managers, and engineers in manufacturing. These personnel often use FTA alongside the Kaizen methodology and root cause analysis (opens in new tab) to prevent or solve system failures.

How is fault tree analysis performed?

FTA uses a fault tree analysis diagram to show the different events or conditions that could lead to an undesired outcome, like equipment failure. The FTA process consists of three main steps:

  • Create a fault tree diagram
  • Identify failure events, initiating events, and contributing factors from your diagram
  • Evaluate relationships between failures and initiating events (or contributing factors)

The symbols used on a fault tree diagram are called events, conditions, or states. These can occur at any point in time during system operation. Lines connect the symbols together to show how one event may lead to another until we reach the end of our line, an undesired event (known as a fault). The faults represent things that go wrong within your system. Below is an example of how these diagrams can look:

The image shows a fault tree diagram represetend with symbols and lines. The diagram shows the illustration of a pump or motor assembly not having any flow and the possible root causes.

In this fault tree diagram example, we see an illustration of a pump or motor assembly not having any flow. This event is the main failure, and below it, we can see the initiating events: Mechanical failure and electrical failure. To the right of the fault tree diagram, below electrical failure, we can see further events and failures, one being a motor failure and the other being a fuse failure. Below the fuse failure, we see that a circuit overload event is occurring and, below it, two different basic events: Wire shorted and/or a power surge.

What are fault tree analysis symbols?

Every industry uses the same set of symbols and naming conventions for fault trees. The fault tree illustrates the link and flow between different activities and is read from top to bottom. Events and gateways (known as gates) are the two categories under which the activities fall.

Event symbols

Events occur when a system or process fails. The types of events that appear in fault trees have been detailed below.

The image shows the types of events that are used in fault tree diagrams.

Below are definitions of each event type illustrated above:

  • Top event (TE): These types of events are at the top of the fault tree and prompt an investigation into the system failure. It has a single input but no relative outputs because it is the start of the failure.
  • Intermediate events (IE): These events are generally caused by one or more events. It has both an input and output. Another event may cause its failure, and most likely causes further failures down the fault tree.
  • Basic events (BE): These types of events are generally the root cause of the top event. They sit at the bottom of the fault tree.
  • Underdeveloped events (UE): These events don’t have enough information and are placed as a subtree.
  • Transfer events (TE): These types of events happen when a fault tree is too long to fit on paper. Larger parts of the tree are hidden with a symbol and expand in a separate tree. There are two types: Transfer-out and transfer-in events. Transfer-out has a triangle and output to the right, and transfer-in events have input on the top of the triangle.
  • Conditional events (CE): These events happen as conditions for a type of gate called an inhibit gate.
  • House events (HE): These types of events are used to turn an event off and on. If the event is set to 0 means it will not occur, but if it is set to 1 it means that it will occur. House events are used to allow parts of the fault tree to be included or not included.

Gate symbols

Gates represent the various ways that failures can occur in an asset or system. Sometimes a single event can cause a top-level failure (or catastrophic failure). Sometimes a combination of different events can cause a top-level failure event. The types of gates in FTA are detailed below.

The image shows the types of gates that are used in fault tree diagrams.

Below are definitions of each gate type illustrated above:

  • AND gate: This type of gate is connected to output events. The events only occur if the input events to the gate occur.
  • Priority AND gate: This gate occurs if all the input events happen in a specific order.
  • OR gate: This type of gate may have one or more inputs, and an output event will occur if one or more of the input events happen.
  • XOR gate: This gate is slightly less common. An output happens only if one input element occurs.
  • k/N or VOTING gate: This gate is similar to OR gate visually. There will be a number of input events ‘N’ and one output event ‘k.’ The output event occurs when the number of input events occurs. The exact number of inputs needs to be met to trigger this gate.
  • INHIBIT gate: This type of gate will have an output event when all input and conditional events occur.

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What are the advantages of using fault tree analysis?

FTA is a top-down method that can be used to analyze the effects of a single failure on a system. Below are some of the other advantages of using FTA:

  • It narrows down the cause of a failure event, which saves you time and money in finding the root cause.
  • It identifies ways to mitigate the consequences of a failure before it happens. For example, if you are designing an airplane's propeller, you may use fault tree analysis to determine what would happen if the propeller breaks and how it might be repaired. Using this information, you could figure out how to prevent those failures from happening in the first place.
  • It helps you determine which failures are more likely to occur, allowing you to focus your efforts on preventing those failures first.
  • It identifies common failure modes across multiple systems or products (such as similar components), which can help you determine where design changes are most needed.

Fault tree analysis helps you prevent and solve potential failures

Fault tree analysis can be used by teams to analyze failures for complex systems and assets. It helps determine the cause of a failure and the root causes to prevent them in the future. Fault tree analysis can help prioritize issues so that teams can make better decisions on improvements.

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