One of the lesser used functionalities in BowTieXP are Barrier types. These allow you to categorise barriers and assess the mix of different types of barriers. Generally, having different types of barriers is good for two reasons.

Different barrier types decreases the chance of common mode failure. Different barrier types can compensate for each other’s weaknesses. There are different categorisations possible, but they generally describe either barrier functions or barrier systems.

## Barrier function

´A barrier function is a function planned to prevent, control, or mitigate undesired events or accidents´

This definition by Sklet [1] gives a good summary of barrier function. Instead of describing what is there in an organisation, the barrier function should describe why it’s there. One of the first systems to describe the differences in barrier function was developed in the early 70’s. Haddon described 10 strategies for countering energy damage of any form in a classic article [2]. This was one of the first categorizations of barrier functions and it would have a lot of influence on later categorizations.

The ten strategies (taken from the original article) are:

1. The first strategy is to prevent the marshalling of the form of energy in the first place
2. The second strategy is to reduce the amount of energy marshalled
3. The third strategy is to prevent the release of the energy
4. The fourth strategy is to modify the rate of spatial distribution of release of the energy from its source
5. The fifth strategy is to separate, in space or time, the energy being released from the susceptible structure, whether living or inanimate
6. The very important sixth strategy uses not separation in time and space but separation by interposition of a material ‘barrier’
7. The seventh strategy, into which the sixth blends, is also very important - to modify appropriately the contact surface, subsurface, or basic structure, as in eliminating, rounding, and softening corners, edges, and points with which people can, and therefore sooner or later do, come in contact.
8. The eighth strategy in reducing losses in people and property is to strengthen the structure, living or nonliving, that might otherwise be damaged by the entry transfer.
9. The ninth strategy in loss reduction applies to the damage not prevented by measures under the eight preceding - to move rapidly in detection and evaluation of damage that has occurred or is occurring, and to counter its continuation and extension.
10. The tenth strategy encompasses all the measures between the emergency period following the damaging energy exchange and the final stabilization of the process after appropriate intermediate and long-term reparative and rehabilitative measures.

Later systems tried to describe barrier functions on a higher level. Prevent, control and mitigate/protect are the three high level barrier functions that in some form or another, ended up in most classification systems. It is important to understand that all of these functions are relative to the progression of an incident. A barrier (such as a wall) might have a preventive function in one incident, while it has a mitigation function in another incident. Barrier functions are not an absolute property of any barrier, but describe a relationship within an incident sequence.

## Barrier system

´A barrier system is a system that has been designed and implemented to perform one or more barrier functions´

This definition by Sklet [1], defines barrier systems as the means to implement a barrier function. For example. A fire extinguisher is a barrier system that is there to implement a mitigation function. The type of barrier system is inherent to the barrier itself, unlike the barrier function, which is a relative property. There are different ways to categorise barrier systems as well. ARAMIS, a European project to implement the Seveso II Directive, has 11 different types [3]. In this article we’ll talk about a simpler categorisation with 5 types.

A barrier system can first be divided into active or passive. A passive barrier is for instance a fence or dyke. Active hardware can be further divided using the Detect-Decide-Act (DDA) principle. A complete active barrier system includes all three. Depending on whether these three elements are performed by people or technology determines what kind of active hardware it is.

If the DDA system is completely represented by people, we call it a Behavioural Barrier. If the whole cycle is hardware based, we call it Active hardware. If the DDA cycle is a mix between people and hardware, we call it a Socio-Technical (or Man-Machine) barrier system. The last type is one where there is no detection, but a continuous action (like for instance a ventilation system). This is called Continuous hardware.

## Conclusion

Barriers can be categorised in different ways. The lists and models presented here are not definitive by any means. Looking at barrier functions and barrier systems both has its benefits, but you have to decide for yourself how you want to use barrier types to aid decision making.

1. Sklet, S. (2006). Safety barriers: Definition, classification, and performance. Journal of Loss Prevention in the Process Industries, 19(5), 494-506
2. Haddon, W. (1995). Energy damage and the 10 countermeasure strategies. 1973. Injury prevention : journal of the International Society for Child and Adolescent Injury Prevention, 1(1), 40-4.
3. Guldenmund, F., Hale, A., Goossens, L., Betten, J., & Duijm, N. J. (2006). The development of an audit technique to assess the quality of safety barrier management. Journal of hazardous materials, 130(3), 234-41.