Circuit Diagrams

# Circuit Diagrams

A circuit diagram is an alternate way of viewing a fault tree. It consists of all basic events in the fault tree connected by branches. AND gates and OR gates do not show up as nodes in a circuit diagram. Instead, they define its structure. A circuit diagram can be useful to identify which events in the fault tree result in a failure.

Viewing a Circuit Diagram

While viewing a fault tree, toggle back and forth between fault tree view and circuit diagram view by selecting FAULT TREE | Display | Circuit Diagram or pressing Tab.

Using a Circuit Diagram

A circuit diagram in DPL Fault Tree is analagous to an electric circuit. A light bulb representing the top node is attached to the left end of the circuit diagram. Depending on the structure of the fault tree, initially the bulb will either be yellow (on) indicating that the circuit is closed or gray (off) indicating the circuit is open. When the circuit diagram is created, all nodes for basic events are either green (indicating they are in a false state) or red (indicating they are in a true state) depending on the structure of the fault tree. To toggle a node's state, click on it.

If a basic event is in its false state, it is green. When in its false state, the fault does not occur and the branch associated with it allows current to flow. When a basic event is in its true state, it will be red. When in its true state, the associated branch prohibits current. For events with a tilde (~) in front of their name, the opposite is the case: false (green) prohibits current and true (red) allows current to flow. Use the circuit diagram to determin whether the circuit is closed given the states of events in the diagram. I.e., which assignments of true and false to the basic events break the circuit. Probability values associated with basic events are ignored here. A closed circuit (yellow bulb on) indicates the current true/false settings of the basic events do not result in the top node being true (a fault does not occur); an open circuit (gray bulb off) indicates a fault has occurred (top node is true).

If you set enough basic events to their true state, current will not flow to the light bulb. This indicates that the top event in the fault tree is in its true state (i.e., that the system will fail when the events are in the states as selected in the circuit diagram). A set of events set to true (or false if a tilde appears before the name) that result in the top event being true is called a cut set.

For more on cut sets, see Minimal Cut Sets.

Reference Nodes

If event reference nodes appear in a fault tree, then they will appear multiple times in a circuit diagram as duplicate nodes. Duplicate nodes in a circuit diagram are always in the same state. If you set the state for once occurrence, it will be set everywhere else that node appears.

Time Series

A circuit diagram represents a single time period of the fault tree. If your fault tree uses time series, DPL Fault Tree will prompt you to choose a time period before you can view the circuit diagram.

Versions: DPL Fault Tree