Top event
The top event is the moment control of a hazard is lost — the central point of a bowtie diagram, after prevention has failed but before the consequences have played out. Example: loss of containment of flammable hydrocarbon.
Every bowtie diagram is built around exactly one top event. It is not the hazard itself (hydrocarbon under pressure is a hazard all day, every day) and it is not the consequence (the fire, the injury, the outage). It is the pivot between them: the release, the loss of separation, the unauthorised access — the instant the situation stops being controlled.
Choosing the top event well is the most important modelling decision in a bowtie. Set it too early ("corrosion occurs") and the diagram fills with trivia; set it too late ("explosion") and the recovery side has nothing left to say. The test that works: the top event is undesirable but not yet damaging, and both prevention and recovery still make sense around it.
Good top events are written as observable states, not judgements: loss of containment of flammable hydrocarbon, critical pump failure causing unplanned shutdown, unintended drug overdose from infusion pump. Each of those is the centre of a worked template you can open and read.
Bowtie Risk Engine writes the top event as the risk at the centre of the diagram — the label differs, the concept is identical. If your organisation's procedures say "top event", nothing changes but the word on the box.
Last reviewed 2026-08-09 · Back to the glossary
See the method in working software.
Six worked bowtie examples ship with the product — open one read-only, no sign-in, and read every control in full.