A Complete List of Fault Tree Analysis Symbols

Fault Tree Analysis is a diagram used to represent different causes of system failure. It helps in analyzing the origin of the problem or issue using specific fault tree analysis symbols. Companies also use this tool to check the system's reliability while investigating a series of events logically.

1. What Are Fault Tree Analysis Symbols

Fault tree analysis is used across many industries, including engineering, high profile industries where faults may disrupt the life of many such as power transmission and also in software engineering. Since it is a widely used tool, we need some standard fault tree analysis symbols identified largely, and everyone on the team or in multiple teams can relate with their meaning. Hence, we use a standard set of fault tree symbols in the diagrams.

There are two basic types of fault tree diagram symbols: they are events and logic gates. Events are the conditions or faults causing or leading to the main failure. Different fault tree analysis symbols show if the event is normal, independent, conditional, or contributing. Events are the blocks or conditions that happen. The gate fault tree analysis symbols, on the other hand, show the logical relationship among different events and other elements in the fault tree analysis.

FTA symbols
Source: EdrawMax Community

2. Fault Tree Analysis Symbols Explained

2.1 Event Symbols

An event block in a fault tree uses several graphical block representations to show different types of events. The primary or basic failure event is represented with a circle. In contrast, an external event that is guaranteed or expected to occur uses a fault tree analysis symbol that looks like a house. Similarly, there are undeveloped events and conditioning events with their specific symbols discussed in the following table.

Symbol Name Description
Fault Tree Analysis Symbol - Basic Event Basic Event A basic initiating failure fault
Fault Tree Analysis Symbol - External Event (House Event) External Event (House Event) An event that is normally expected or guaranteed to occur. Generally, they have a fixed probability of 0 or 1.
Fault Tree Analysis Symbol - Undeveloped Event Undeveloped Event Undeveloped events need no further breakdown. It is an event that is not further developed because the further analysis is not possible because of a lack of information. It is a basic event that does not need resolution.
Fault Tree Analysis Symbol - Conditioning Event Conditioning Event A conditioning event is a specific condition or restriction that can apply to any gate.

The below FTA diagram graphically analyzes the cause and effect of a fire explosion. It shows different events that may lead to an explosion, such as Electrostatic discharge is not grounded or if the sensory part of the equipment fails. The logical gates connect the possible events to show how they can influence the top event.

Fault Tree Analysis Event Symbol Example
Source: EdrawMax Community

2.2 Gate Symbols

Gate symbols describe the logical relationship between input and output events causing the main event. These fault tree analysis symbols are derived from Boolean logic symbols. The AND and OR gates and others are the basic gates that show how the output event occurs when a certain number of the input events occur. Let us see the detailed description of the gates in the following table.

Symbol Name Description
Fault Tree Analysis Symbol - AND Gate AND Gate In an AND gate, the output event is positive if all input events occur. In system reliability terms, we can say that all components must fail for the system to fail.
Fault Tree Analysis Symbol - OR Gate OR Gate In an OR gate, the output event happens even if one of the input events occurs. In system reliability terms, this implies that if any input component fails, then the system will fail.
Fault Tree Analysis Symbol - Exclusive OR gate Exclusive OR gate Any but not all, this is the rule of exclusive OR gate. An output event only occurs if one of the input conditions is met, but not if all conditions are met.
Fault Tree Analysis Symbol - Priority AND gate Priority AND gate Priority AND gate mean the output occurs only after several input events occur in a specific sequence.
Fault Tree Analysis Symbol - Inhibit gate Inhibit gate An event will only occur if all input events occur and an additional, conditional event also takes place. An inhibit gate is an AND gate with an additional event.

This water pump failure analysis shows that valve failure and valve closed events are combined by an AND gate. It means if both events take place, the water pump will fail. In the second branch, if any single or more than one of the events from the faulty indicator, light fail, control command fails, or operator unable to open valve takes place, the water pump will fail.

Gate Symbol Example
Source: EdrawMax Community

2.3 Transfer symbols

Transfer symbols are a convenient means to connect related fault trees, including fault trees of a subsystem, to its system.

Symbol Name Description
Fault Tree Analysis Symbol - Transfer in Transfer in Transfer in fault tree analysis symbol indicates that the tree is developed further at another point within the tree.
Fault Tree Analysis Symbol - Transfer out Transfer out Transfer out shows that this portion of the tree is attached to another location within the same tree.

In the below FTA tree, you can see two transfer controls. These controls show that these points are connected within the same tree. It means that point A and point B are connected.

Transfer symbol Example
Source: EdrawMax Community

3. Tips for Using Fault Tree Analysis Symbols

EdrawMax Online is an excellent drawing software supporting 280+ types of diagrams and drawings. It has separate comprehensive libraries for each type of diagram. So, we also have a fault tree analysis symbol library with all event, logic, or gate and transfer controls. Also, save time and effort by using the fully customized designer-made templates for common FTA from many fields available at EdrawMax Community for a quick start.

Here are some tips for using EdrawMax to use fault tree analysis symbols.

3.1 Use a Professional Fault Tree Analysis Maker

Find fault tree analysis symbols at EdrawMax Online>Symbol Library>Business>Fault Tree Analysis. You will see a complete library with an exhaustive collection of fault tree symbols. Fault Tree Analysis Maker

3.2 Create Symbols by Yourself

EdrawMax Online has a comprehensive symbols library for events, gates, and transfer fault tree analysis symbols. However, if you want to add any unique symbol or graphic element, you can easily do it with a few clicks. You can import images, symbols, or data from files format such as SVG, VSSX, etc. Navigate to the library option on the left-hand sidebar and click on the import icon. In the new pop-up window, you will find options for importing your file in different formats.

You can also learn more from this video. YouTube.

4. Conclusion

While performing a fault tree analysis, you systematically determine the reason for an unwanted event using the exclusive fault tree analysis symbols. It is used in many fields, especially in industries where the repercussions of failures are very serious. So in some situations, the minimum criterion for success is that no single failure should occur that can cause loss of control over the process. While in other industries, there is more tolerance for failure, quick recovery is needed, and FTA is a handy tool for identifying and correcting the fault. Using EdrawMax Online saves a lot of time and effort while drawing the FTA and hence is a great support in critical situations. Find more fault tree analysis examples in the templates community.

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