Event tree node and relationship types
Every node type and relationship type an event tree recognises, what to write for each, how each is drawn, how splits are ordered, and what may follow what.
Node types
| Node type | Write it as | Drawn under Custom | Rule |
|---|---|---|---|
| Initiating event | Initiating Event, Initiating, Init, Start | Orange ellipse | Starts the tree. Exactly one per tree, and nothing leads to it. |
| Pivotal event | Pivotal Event, Pivotal, Function, Functional Event, Safety Function, Safety System, Mitigating System, Mitigating Factor, Protection Function, System Function, Heading | Blue rectangle | A function that works or fails. Needs at least two branches out. |
| Branch point | Branch Point, Branch, Decision | Purple diamond | A split drawn as a point rather than a box. The same rules as a pivotal event. |
| Success outcome | Success Outcome, Success, Yes | Green rounded rectangle | Ends a sequence. Nothing follows it. |
| Failure outcome | Failure Outcome, Failure, No | Red rounded rectangle | Ends a sequence. Nothing follows it. |
| Consequence | Consequence, End State, Outcome | Pink parallelogram | Ends a sequence. Nothing follows it. |
Under IEC 62502:2010 every node is a plain rectangle, with a heavier border on the initiating event and on consequences, and a branch point is a small filled dot with its label beneath it.
Matching ignores case, spaces, underscores, hyphens and full stops, so Pivotal_Event, pivotal event and PivotalEvent all read as a pivotal event. Wording that matches nothing can be mapped under Node type values on the Data Mapping tab.
Relationship types
| Relationship type | Write it as | Use it for | In a sequence identifier | Connector under Custom |
|---|---|---|---|---|
| Success branch | Success Branch, Success, Yes, True | The branch taken when the function works. | S | Green arrow |
| Failure branch | Failure Branch, Failure, No, False | The branch taken when it fails. | F | Red arrow |
| Conditional branch | Conditional Branch, Conditional, Other | A further branch at a split, such as partial success. | C | Purple dashed arrow |
| Sequence | Sequence, Next | A step that is the only way on from a node. The default for a blank cell. | Nothing | Grey arrow |
Under IEC 62502:2010 every connector is a plain line without an arrowhead, and a conditional branch is dashed. Unrecognised wording can be mapped under Relationship values on the Data Mapping tab.
How the branches of a split are ordered
At every split, success branches come first, then Next, then conditional, then failure. Left to right, that puts success above failure; top to bottom, success is on the left. Order only decides between branches of the same kind, and two branches of the same kind with the same Order are reported.
What may follow what
| Node | Before it | After it |
|---|---|---|
| Initiating event | Nothing. | One node, usually the first pivotal event, with Relationship Type Next. |
| Pivotal event or branch point | One node. | Two or more branches. One branch is a warning; none is an error. |
| Success outcome, failure outcome or consequence | One node. | Nothing. |