Definition
The knot of the bowtie is the top event: the moment control over the hazard is lost. Everything before it belongs to prevention, everything after it to mitigation. Its position therefore sets the geometry of the whole diagram.
Three tests
- Observable — when it happens, somebody on site could notice it.
- Neutral — it contains neither the cause nor the consequence.
- A single sentence — two events joined by "and" are two separate bowties.
What is not a top event
| Written as | Actually |
|---|---|
| A worker is injured | Consequence — belongs on the right |
| The valve fails | Threat — belongs on the left |
| Maintenance is skipped | Escalation factor — it weakens a barrier |
| Flammable liquid is stored | The hazard itself — the heading of the bowtie |
Too late and too early
Set too late ("a fire started"), there are no preventive barriers left to draw and the diagram grows on one side only. Set too early ("the pump was started"), the left side fills with normal operating steps and the bowtie becomes a process flow drawing. The right place is between the two: no harm yet, but no normal operation either.
Not the same as the hazard
A hazard is inherent to the work and usually cannot be removed — pressure, height, energy, substance. The top event is the moment that hazard breaks loose. One hazard can have several top events, and each of them deserves its own bowtie.