The document discusses fault tree analysis (FTA), which uses logic diagrams to identify the causes of failures in complex systems. FTA breaks down an undesired event into a logical combination of intermediate and basic events. The document provides an example of constructing a fault tree to calculate the expected frequency of injury to a steam boiler operator based on failure rates of components like water level detectors, pressure switches, and relief valves.

1. Dr. Shailesh Kumar Dewangan
Assistant Professor
Department of Production Engineering,
BIT Mesra Ranchi
Maintenances Engineering

2. • Diagrammatic representation of all possible fault event
• Logical combination and their relationship to the system failure
Fault tree is the logical model of the relationship of the undesired event to more basic
events.
• The top event of the Fault tree is the undesired event.
• The middle events are intermediate events and the
basic events are at the bottom.
• The logic relationship of events are shown by logic symbols or gates.
Fault Tree Analysis

3. Why FTA is carried out
• Identify the cause of a failure.
• Monitor and control safety performance of a complex system.
• To identify the effects of human errors .
• Minimize and optimize resources.

4. Event of fault tree
Primary event symbols
Gate symbols
Basic Event
Conditioning Event
Intermediate Event
Undeveloped Event
External Event
AND GATE
OR Gate
Inhibit

5. Procedure for FTA
Define TOP event
Define Overall
structure
Explore each branch
in successive level of
detail
Solve the fault tree
Perform corrections
if required and make
decision
Procedure for FTA

6. • An operator is required to monitor a steam boiler. Under fault conditions, the operator is at risk
from injury from explosion due either (a) the boiler burner being lit when operating with low
water conditions or (b) overpressure. The expected frequencies of the failures that would lead to
either of these effects are given in the table below:
Given that the operator is at risk for 20 per cent of the time that the boiler is operating, construct and
quantify a simple fault tree to show the expected frequency of the top event (injury to the operator).
Case study (Example Question)
Failure type Frequency (Per Year) Effect
Water level detector 0.1 A
Electrical fault 0.05 A or B
Pressure switch failure 0.1 B
Relief valve failure 0.05 B