Most Bowtie diagrams have a problem at the centre. The top event — the most critical element in the entire framework — is wrong.
When the bowtie top event is incorrectly defined, everything built around it becomes unreliable. Barriers are positioned in the wrong place. Escalation factors are identified for the wrong conditions. The entire diagram, carefully constructed, is built on a faulty foundation.
Understanding how to correctly identify a top event is not a technical detail. It is the difference between a Bowtie that functions as a risk management tool and one that functions as a document.
What Is a Bowtie Top Event?
The top event sits at the centre of the Bowtie diagram. On the left side, threats push toward the top event — these are the causes that could trigger loss of control. On the right side, consequences flow away from it. Prevention barriers sit between threats and the top event. Recovery barriers sit between the top event and consequences.
The top event itself has a precise definition: it is the moment of loss of control of the hazard. Not the hazard source. Not the consequence. The exact moment the hazard is released and control is lost.
A correctly defined bowtie top event is specific, physical, and definable. It describes a state change — from the hazard being contained to the hazard being released.
Mistake 1 — The Hazard Source Is Used as the Top Event
The most common error in Bowtie construction is placing the hazard source at the centre of the diagram instead of the moment of release.
“Electricity” is not a top event. “Flammable gas” is not a top event. “Working at height” is not a top event. These are hazard sources — they describe what exists in the system, not the moment control is lost.
The top event that corresponds to these hazards would be: “Contact with live conductor above safe voltage,” “Loss of containment of flammable gas at the vessel,” or “Person falls from work platform.”
The distinction is critical because your prevention barriers need to be positioned to stop the moment of release. If your top event is “electricity,” what does it mean to prevent “electricity”? The question is unanswerable. But if your top event is “contact with live conductor,” prevention barriers become specific and testable: physical isolation, lockout/tagout, interlocks.
Mistake 2 — A Consequence Is Used as the Top Event
The opposite error is equally common: placing a consequence at the centre of the Bowtie instead of the triggering event.
“Fatality” is not a top event. “Explosion” is not a top event in most cases. “Injury to workers” is not a top event. These are outcomes that occur after the top event — they belong on the right side of the Bowtie as consequences, not at the centre.
When a consequence is placed as the top event, the prevention barriers on the left become incoherent. What barrier prevents a fatality directly? None — because a fatality is the outcome you recover from after the bowtie top event has occurred, not the event itself.
The top event that precedes an explosion from a gas release would be: “Loss of containment of flammable gas above lower explosive limit.” The explosion is the consequence of that top event, not the event itself.
Mistake 3 — The Top Event Is Too Vague to Be Actionable
A third error is using a top event that is technically correct in form but too vague to support the construction of specific barriers.
“Incident occurs” is a top event written this way. So is “Loss of control” with no further specificity, or “Hazardous situation develops.”
These descriptions cannot support a Bowtie because they cannot be tested. A barrier is only valid if its function can be verified — it either prevents the top event or it does not. But if the top event is “hazardous situation develops,” what does it mean for a barrier to hold?
A well-defined bowtie top event passes one test: can you state the exact physical or operational condition that represents loss of control? Can a worker on site observe whether the top event has occurred? If yes, the top event is well-defined. For a pressure vessel scenario, the top event might be: “Overpressure of vessel beyond rated maximum allowable working pressure.” This is observable, specific, and testable.
How to Check Your Bowtie Top Event
Review each Bowtie diagram in your system. For each top event, ask three questions:
Is it the hazard source? If yes, move it to the threat side and replace it with the moment of release.
Is it a consequence? If yes, move it to the consequence side and replace it with the event that triggers the consequence.
Can you describe the exact condition it represents? If the answer is vague, tighten the definition until a person on site can observe whether it has occurred.
A correctly defined bowtie top event is the foundation of effective barrier management. Get it right, and your barriers become specific, testable, and defensible. Get it wrong, and the most carefully built Bowtie becomes a document rather than a risk tool.
Building Bowtie diagrams for major hazard risk in your organisation? Cikgu Barrier’s Barrier Management: Bow-Tie Analysis program covers top event identification, barrier definition, and escalation factor analysis — with hands-on diagram construction for your actual hazard scenarios. Contact us to discuss in-house delivery.