The Weekly Reflektion 26/2023

Multiple barriers are often essential to ensure safety and security. This is particularly important where the systems are complex and the value of the people, environment and assets that are being protected are high. Sometimes the number of barriers in place creates an illusion that the probability of all barriers being breached is very low. That is the holes in the barriers are few and small. Common mode failures however can both compromise the barriers and lead to the holes in the barriers lining up. Your system may be more vulnerable than you think.

Do you have common mode failures that could compromise your safety and security systems?

Our Reflektion in week 24/2023 concerned the Independent Air Flight 1851, that struck the mountain Pico Alto while on approach to Santa Maria Airport in the Azores. 144 people died in the crash. The subject of the Reflektion was failures in multiple barriers that were in place to prevent such a crash. Thank you to Stuart Girling for his reflektions on multiple barriers and the danger of common mode failures and for bringing an article by David Vose to our attention.

Vose’s article was on a security breach at the White House in August 2014. Omar Gonzalez, a US war veteran who had been wounded in Iraq and psychologically affected by his experience, had managed to enter the White House with a knife, evading all security.

In the article Vose describes eight barriers that failed and highlights what he considers a common mode failure that compromised all eight barriers. The discipline of the Secret Service personnel employed to protect the president and his family. He highlights the challenge in maintaining disciplinein an environment where ‘nothing much happens’. At the same time, he emphasizes the importance of management to retain the focus on objectives and the maintaining the integrity of barriers in place.

In the article Vose uses an equation for the calculation of probability to show how we can be deceived by multiple barrier systems. This is particularly relevant for engineers who often find solace in the black and white world of mathematics.

If the probability that any individual barrier functions as required is P(i) and the number of barriers in the system is k, the probability of the failure of the system can be calculated as:

P(fail) = ((1- P(1)) * (1 – P(2)) * …* (1-P(k)) 

There are of course uncertainties related to the determination of these probabilities that need to be understood and there are qualitative and quantitative factors that need to be accounted for. The important point is that the calculation assumes that the barriers are independent and not subjected to any common mode failure. In the worst case the probability of failure of the system may be equal to the probability of the failure of the barrier with the highest integrity. 

P(fail) = (1 – (P(highest))

Given that there are inherent accuracies in the estimates of the calculation of the individual probability, the probability of the failure of the system lies between these two values. For a system with 4 barriers and a probability that any barrier will function of 90% then the probability of failure lies between 10% and 0.01%. A significant difference.

Common mode failures may be technical, individual, or organisational and understanding these and how they affect the integrity of your safety and security systems is vital if you want to achieve your objectives.