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Fault tree analysis 61052 is a top-down, deductive failure analysis in which an undesired state of a system is analyzed using Boolean logic to combine a series of lower-level events.
This analysis method is mainly used in the fields of safety engineering and reliability engineering to understand how systems can fail, to identify the best ways to reduce risk or to determine or get a feeling for event rates of a safety accident or a particular system level functional failure.
Fault tree analysis
FTA is used in the aerospace nuclear powerchemical and process   pharmaceutical petrochemical and other high-hazard industries; but is also used in fields as diverse as risk factor identification relating to social service system failure. These conditions are classified by the severity of their effects. The most severe conditions require the most extensive fault tree analysis. These system failure conditions and their classification are often previously determined in the functional hazard analysis.
Watson, under a U. Subsequently, within the U. Inthe U.
This change adopted failure probability criteria for aircraft systems and equipment and led to widespread use of FTA in civil aviation. Inthe FAA published Order Early in the Apollo project the question was asked jec the probability of successfully sending astronauts to the moon and returning them safely to Earth. A risk, or reliability, calculation of some sort was performed and the result was a mission success probability irc was unacceptably low. This result discouraged NASA from further quantitative risk or reliability analysis until after the Challenger accident in After the Challenger accident, the importance of probabilistic risk assessment PRA and FTA in systems risk and reliability analysis was realized and its use at NASA has begun to grow and now FTA is considered as one of the most important system reliability and safety analysis techniques.
Within the nuclear power industry, the U. Today FTA is widely used in system safety and reliability engineering uec, and in all major fields of engineering. Any sufficiently complex system is subject to failure as a result of one or more subsystems ice. The likelihood of failure, however, can often be reduced through improved system design.
Fault tree analysis maps the relationship between faults, subsystems, and redundant safety design elements by creating a logic diagram of the overall system. The undesired outcome is taken as the root ‘top event’ of a tree of logic.
For instance the undesired outcome of a metal stamping press operation is a human appendage being stamped. Working backward from this top event we might determine there are two ways this could happen: This condition is a logical OR. Considering the branch of occurring during normal operation perhaps we determine there are two ways this could happen: This is another logical OR.
We can make a design improvement by requiring the operator to press two buttons to cycle the machine—this is a safety feature in the form of a logical AND. The button may have an intrinsic failure rate—this becomes a fault stimulus we can analyze. When fault trees are labeled with actual numbers for failure probabilities, computer programs can calculate failure probabilities from fault trees.
When a specific event is found to have more than one effect event, i. Graphically speaking, it means this event will appear at several locations in the tree. Common causes introduce dependency relations between events. The probability computations of a tree which contains some common causes are much more complicated than regular trees where all events are considered as independent. Not all software tools available on ifc market provide such capability.
The tree is usually written out using conventional logic gate symbols. A cut set is a combination of events, typically component failures, causing the top event.
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If no event can be removed from a cut set without causing the top event, then it is called a minimal cut set. Some industries use both fault trees and event trees see Probabilistic Risk Assessment. An event tree starts from an undesired initiator loss of critical supply, component failure etc. As each new event is considered, a new node on the tree is added with a split of probabilities of taking either branch. The probabilities of a range of ‘top events’ arising from the initial event can then be seen.
Government to evaluate the safety and reliability of nuclear reactorsthe Space Shuttleand the International Space Station. Outside the US, the software RiskSpectrum is a popular tool for fault tree and event tree analysis, and is licensed ic use at almost half of the world’s nuclear power plants for probabilistic safety assessment. The basic symbols used ie FTA are grouped as events, gates, and transfer symbols.
Minor variations may be used in FTA software. Event symbols are used for primary events and intermediate events.
Primary events are not further developed on the fault tree. Intermediate events are found at the output of a gate.
The event symbols are shown below:. An intermediate event gate can be used immediately above a primary event to provide more room to type 16025 event description. Gate symbols describe the relationship between input and output events. The symbols are derived from Boolean logic symbols:.
Transfer symbols are used to connect the inputs and outputs of related fault iiec, such as the fault tree of a subsystem to its system. Events in a fault tree are associated with statistical probabilities.
A fault tree is often normalized to a given time interval, such as a flight hour or an average mission time. Event probabilities depend on the relationship of the event hazard function to this interval.
Unlike conventional logic gate diagrams in which inputs and outputs hold the binary values of TRUE 1 or FALSE 0the gates in a fault tree output probabilities related to the set operations of Boolean logic.
The probability of a gate’s output event depends on the input event probabilities.
An AND gate represents a combination of independent events. That is, the probability of any input event to an AND gate is unaffected by any other input event to the same gate.
In set theoretic terms, this is equivalent to the intersection of the input event sets, and the probability of the AND gate output is given by:. Since failure probabilities on fault trees tend to be small less than. An exclusive OR gate with two inputs represents the probability that one or the other input, but not both, occurs:. Many different approaches can be used to model a FTA, but the most common and popular way can be summarized in a few steps. A single fault tree is used to analyze one and only one undesired event or top event, which may be subsequently fed into another fault tree as a basic event.
Though the nature of the undesired event may vary dramatically, a FTA follows the same procedure for any undesired event; be it a delay of 0. Due to labor cost, FTA is normally only performed for more serious undesired events.
FTA is a deductivetop-down method aimed at analyzing the effects of initiating faults and events on a complex system. This contrasts with failure mode and effects analysis FMEAwhich is an inductivebottom-up analysis method aimed at analyzing the effects of single component or function failures on equipment or subsystems.
FTA is very good at showing how resistant a system is to single or multiple initiating faults. It is not good at finding all possible initiating faults. FMEA is good at exhaustively cataloging initiating faults, and identifying their local effects. It 61205 not good at examining multiple failures or their effects at a system level. A dependence diagram is equivalent to a success tree analysis STAthe logical inverse of an FTA, and depicts the system using paths instead ice gates.
DD and STA produce probability of success i. From Wikipedia, the free encyclopedia. This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources.
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