FTA vs RBD

Abstract

Chris and Fred discuss the differences, pros and cons of FTA (fault tree analysis) and RBD (reliability block diagrams). Need to learn more about how to model system reliability? Do you struggle to spell FTA or RBDs? Listen to this podcast!

Key Points

Join Chris and Fred as they discuss the differences between these things we call fault trees and reliability block diagrams. If you need to model system reliability, you really need to know what these are … and which one might work best for you.

Topics include:

System reliability modeling. This involves creating something that allows you to convert component reliabilities into system reliability. So if your system has seven components, and their reliabilities at 2 years are 75%, 89%, 45%, 92% and 56% … what is the reliability of your system? A system reliability model can use these figures to give you that number.
What is a fault tree (FT)? It is a diagram that uses logic gates and circles (called basic events – representing component failure) that are connected with lines up to the ‘top event’ (usually representing system failure). The logic gates are often ‘AND’ or ‘OR’ gates, and they allow you to work out what combinations of components need to fail for your system to fail.
What is a reliability block diagram (RBD)? It is a diagram where a ‘block’ represents each component, and lines are going from left to right through these blocks. If you can follow one of these lines from left to right passing only through working ‘blocks’ or components, then your system works. RBDs can often mimic the physical layout of your system where FTs do not.
Why use one over the other? RBDs and FTs are usually just as useful at modeling system reliability as each other. While FTs can’t mimic the physical layout of a system, they are often much easier for us to analyze because they are laid out logically. FTs can also incorporate other things besides components failing (such as human error, environmental changes and so on). RBDs can’t do this. FTs are often useful for root cause analysis, so if you already have a FT for your system from RCA, perhaps it is easier to keep using them.
So which one should you use? It’s up to you! Know the pros and cons of each one, and work out which one might be better for your problem. Also … perhaps your team is more fluent with FTs instead of RBDs, or vice versa. Take this into consideration. Work out what problem you are trying to solve, which decision you are trying to inform, and then make an informed decision!

Enjoy an episode of Speaking of Reliability. Where you can join friends as they discuss reliability topics. Join us as we discuss topics ranging from design for reliability techniques to field data analysis approaches.