Barriers are the safety measures or controls in a bowtie diagram. Barriers interrupt the scenario so that the threats do not result in the top event, when control is lost over the hazard. Barriers can also ensure that the top event does not escalate into an actual impact (the consequences). Barriers in the bowtie appear on both sides of the top event.
Type of barriers
Barriers can be catagorised using any classifaction system desired. A common classification system is if the functioning of the barrier is dependend on human behaviour or technology. Categorizing barriers very often creates a greater understanding about how risks are managed. You can build on this basic barrier structure further to deepen you understanding of where the weaknesses are. Barrier categorizing your barries as a certain type, it is also advised to include information on barrier effectiveness.
Barrier effectiveness lets you assess how well a barrier performs. If you have barriers in place that are not performing well, the risk exposure is only slightly reduced. To keep barriers performing adequatly, the barriers need to be implemented and maintained. In bowtie diagramms barrier effectiveness is often indicated by a stoplight system, where green barrier perform well, and red barriers are under performing.
After that you can look at the activities you have to implement and maintain you Barriers. This essentially means mapping you Safety Management System on the Barriers. Also determining who is responsible for a Barrier and assessing the criticality of a Barrier are things you can do to increase your understanding of the Barriers.