Canales en UMTS

There have been several attempts to extend fault trees with additional gates and symbols in order to represent further types of information within the fault tree (e.g. the DFT approach in Vesely et al. (2002) or the TFT approach in Palshikar (2001)) and there are many minor variations in the appearance and layout of fault trees. However, the core set of symbols is common to all fault trees and these symbols, as defined in the Fault Tree Handbook, can be divided into three categories: event, gate and transfer symbols, as shown below. There are also NOT gates but these are not shown in the Handbook.

Figure 5 – Fault Tree event symbols

Basic Event

A basic event is a basic fault that requires no further development or expansion. Basic events form the leaf nodes of the tree and combine to cause intermediate events and (ultimately) the top event. In qualitative analysis, cut sets are composed of basic events, and in quantitative analysis, basic events will usually be assigned failure rates and repair rates so that the top event unavailability can be calculated.

Intermediate Event

An intermediate event is a fault that occurs because of combinations of other events occurring further down the tree; for this reason an intermediate event is almost always a type of logical gate. The top event is a special event of this type at the top of the tree.

Conditioning Event

A conditioning event is an event that serves as a special condition or constraint for certain types of gates (e.g. Priority-AND gates and INHIBIT gates). For example, the INHIBIT gate is only true if both all of its inputs are true and if its conditioning event is true. A conditioning event does not necessarily have to be a fault; it could be that the system is in a certain state.

Undeveloped Event

An undeveloped event is an intermediate event whose contributing events are not considered in the analysis. This may be because of insufficient information about this event or it may be because the event is considered inconsequential. It may, for example, have such a low probability of occurrence that to analyse it in further detail would be unnecessary.

External Event

An external event is an event that is not a fault, i.e. an event that could be expected to occur during the normal operation of a system. It is used to represent events that ordinarily would not cause any problems, but in combination with other events, may lead to an undesired event.

Figure 6 – Fault Tree gate symbols

OR Gate

The OR gate is true if any of its input events are true. The OR gate does not necessarily represent a causal relationship between its inputs and outputs; each of the inputs are often restatements of the output. For example, the output 'valve is failed open' could be further described as 'valve left open during maintenance' or 'valve fails open due to mechanical fault', but both descriptions refer to the same result: the valve is open, and the inputs did not cause the output. OR is represented in this thesis by a '+' symbol in text or in expressions.

AND Gate

The AND gate is true if all of its input events are true. Unlike an OR gate, an AND gate typically represents a causal relationship between its inputs and its outputs; that is to say, the combination of input faults causes the output fault. For example, 'No power to system' could be caused by both 'battery failure' and 'generator failure', but not by just one. AND is represented in this thesis by a '.' (full stop) symbol in text or expressions.

Priority AND (PAND) Gate

The PAND gate is only true if all of its input events are true and they occur in a certain order. The order can be specified by a separate conditioning event, but it is often omitted. The PAND gate will be covered in greater depth later in this chapter (see section 2.4.1).

Exclusive OR (XOR) Gate

The XOR gate is true if one and only one of its input events is true.5

INHIBIT Gate

The INHIBIT gate is a special case of the AND gate in which the output of the gate is only true when the input event is true whilst a conditioning event is also true. For example, an explosive reaction may only take place if above a certain temperature or a catalyst is present, even if the

5

Note that the Fault Tree Handbook also shows a strange diagram with a standard OR gate with a conditioning event attached; this will be mentioned again later in the context of the Priority-OR. Also note that the XOR gate symbol in the Handbook differs from XOR gate symbols found in other fields, e.g. electronics.

constituent ingredients are present. It is also known as an IF gate but in logical terms it functions as a normal AND gate.

Figure 7 – Transfer gates

Transfer In

This indicates that this particular branch of the tree is displayed at the corresponding Transfer Out symbol. It is used to save space or to indicate a shared branch.

Transfer Out

This signifies that this branch connects to the rest of the tree at the corresponding Transfer In. Transfer Out symbols can be used to represent shared branches (i.e. multiple Transfer In gates linking to a single Transfer Out).

Figure 8 – NOT gate & Complement Events

Although NOT gates are not present in the Fault Tree Handbook, when used, they usually have the symbol shown in Figure 8, though other symbols are also used. They flip the value of an event, i.e. if we have an event X, then NOT(X) is true only if X did not occur, and false if it did occur. NOT gates can also be represented implicitly as the complement of a basic event. NOT is represented in this thesis by the '¬' symbol.