Fault Tree Analysis(FTA) is one of the several deductive logic model techniques, and one of the most common hazard identification tool.
The deduction begins with a stated top level hazardous/undesired event.
It uses logic diagrams and Boolean Algebra to identify single events and combinations of events that could cause the top event.
Probability of occurrence values are assigned to the lowest events in the tree in order to obtain the probability of occurrence of the top event.
Procedure for Fault Tree Analysis
Process Flow Diagram
Basic Event
A lower most event that can not be further developed.E.g. Relay failure, Switch failure etc.,
An Event/Fault
This can be a intermediate event (or) a top event. They are a result logical combination of lower level events. E.g. Both transmitters fail, Run away reaction
OR Gate
Either one of the bottom event results in occurrence of the top event. E.g. Either one of the root valve is closed, process signal to transmitter fails.
AND Gate
For the top event to occur all the bottom events should occur. E.g. Fuel, Oxygen and Ignition source has to be present for fire.
Incomplete Event
An event which has scope for further development but not done usually because of insufficient data. E.g. Software malfunction, Human Error etc.,
External Event
An event external to the system which can cause failure. E.g. Fire.
Inhibit Gate
The top event occurs only if the bottom event occurs and the inhibit condition is true. E.g. False trip of unit on “maintenance override” not ON.
