Fault Tree Events & Symbols

This page classifies and describes all types of fault tree events and symbols found

in the Functional and Hardware Fault Tree Analysis.

The Functional Fault Tree in MADE consists of items represented as a rectangular

box. Within each item is either a failure mode response or fault taken directly from

the failure diagram. Under each item box is a single fault tree symbol (except for some

intermediate events). These symbols include:

• Basic Events
• Undeveloped Events
• OR Gates

The Hardware Fault Tree is a hardware representation (no failure modes) of the

system model with items and RBD groups represented as rectangular boxes. Within each

rectangular box is the icon and display name of each individual icon or RBD group.

These symbols include:

• Basic Events
• OR Gates
• AND Gates
• Inhibit Gates (Complex RBD Groups only)

Basic Events

Basic events are failure events that initiate a path leading to 'Top Event' such

as a system failure. Basic event are failures of which probabilities of occurrence

are calculated and displayed in the FTA editor.

There are two failure concepts that are classified as basic concepts: faults or

component failure modes. The symbol for a basic event is a circle underneath the

failure concept container.

[Image: partBasicEvent.png]

[Image: componentEvent.png]

Undeveloped Events

Undeveloped events are failure events that initiate a path leading to a Top

Event but its probability of occurrence are not known.

These events contain failure mode responses from an item in the system. The

symbol used for this is a diamond shape.

[Image: undevelopedevent.png]

Intermediate Events

Intermediate events show the failure mode response of an item in a failure path

between a Basic Event and the Top Event. This event acts as a link in the failure

and although there are no symbols associated with this event, it may contain an AND

or OR gate underneath if there are multiple failure mode responses or faults

upstream.

The figure below shows two intermediate events:

1. An Engine component failure mode response with an OR gate for multiple basic events
2. A Diesel Engine subsystem failure mode response without any symbols

[Image: intermediateevent.png]

Top Events

Top events are the final failure responses the item of interest selected by the

user when generating the functional fault tree e.g. the system level failure

response. Top events do not have a fault tree symbol underneath the rectangular box

representing the top event item.

The user can select multiple top effects (if applicable) from the drop-down menu

to the left of the fault tree diagram (see below).

[Image: TopEvent.png]

Fault Tree Gates

OR Gates

OR gates are shown in the Hardware fault tree when one input failure mode

response leads to an output failure mode response. OR gates are represented by the

symbol shown in the figure below:

[Image: ORgate.png]

AND Gates

AND gates are shown in both Functional and Hardware fault trees when all input

failure mode responses are required to progress to an output failure mode response.

AND gates are represented by the symbol shown in the figure below:

[Image: ANDgate.png]

INHIBIT Gates

INHIBIT gates are shown in a Hardware fault tree when the input occurs under and

enabling condition specified by a conditioning event. The conditioning event in

this case is the 'ON' or 'OFF' state of a key item in a Complex RBD Group. INHIBIT

gates are represented by a hexagon symbol shown below:

[Image: INHIBITgate.png]