PT Notes
Prevention versus Mitigation Barriers in Bow Tie Analysis
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Prevention and mitigation barriers typically differ in several ways.
Focus
Prevention barriers are shown on the left side and mitigation barriers are shown on the right side of bow tie diagrams as they follow the timeline of hazard scenarios from left to right. Left-side barriers focus on controlling operations to prevent top events from occurring, with the goal of no incidents, whereas right- side barriers focus on ensuring that an unexpected event does not result in undesirable consequences, with the goal of no harm. The balance between the two needs to be optimal without reliance being placed on just one side.
Timing of failure
Left-side barriers often fail over days, weeks, or years before the top event occurs whereas right-side barriers often fail over minutes or hours after the top event occurs. Thus, opportunities for identifying and intervening for right-side barrier failures are fewer. This difference leads to higher reliance on right-side barriers working correctly the first time.
Company culture
Some companies may favor prevention barriers by placing a high degree of confidence and trust in their systems and practices and rely on left-side barriers to block top events through a pre-emptive policy. Other companies may doubt the robustness of left-side barriers but value the ability of employees to solve problems to get out of trouble and emphasize right-side barriers to prevent or reduce consequences through a reactive policy. Neither policy is advisable as they each result in reliance on one type of barrier.
Availability of information on barrier failures
Failures of left-hand barriers usually are treated as unsafe conditions or situations whereas failures of right-hand barriers usually are treated as near misses. Near misses frequently are underreported compared to unsafe conditions, likely because the perception of blame attaches to near misses but not unsafe conditions. This can create the false impression that operations are conducted under greater control with left side barriers than is actually the case. However, it should also be noted that good performance of recovery barriers masks the need to improve the strength of prevention barriers.
Ownership
Left-hand barriers often are owned by management and engineering, for example, safeguard designs, whereas right-side barriers often are owned by operations, for example, control of ignition sources. This difference in ownership can produce different perceptions of the importance of prevention and mitigation barriers and influence the investments made in them. For example, management may need to pay more attention to right-side barriers.
The selection and management of left-side and right-side barriers is influenced by their differences in focus, timing of failure, company culture, availability of failure information, and ownership. The role of these differences should be considered when selecting and managing barriers.
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