FMEA v FTA

Have you ever found it difficult to decide which technique you should be using to analyse a system – Failure Modes and Effects Analysis or Fault Tree Analysis? If so, then here are a few tips to help you decide.

1. FMEA is a “Bottom-Up” technique which examines the failure modes of the components within a system (i.e. the failure symptoms), and traces forward the potential effects of each component failure mode on system performance. As such it is a CAUSE => EFFECT model.
2. FTA is the reverse of FMEA in that it is concerned with the identification and analysis of conditions (including component failures) that lead to the occurrence of a defined effect. In contrast with FMEA it is therefore a “Top-Down” technique, and so is an EFFECT => CAUSE model.
3. FMEA will be more appropriate than FTA when you suspect that a large number of distinct system conditions exist with a range of unacceptable consequences.
4. Consider using FTA rather than FMEA when you are particularly concerned about one or just a few system conditions that pose unacceptable consequences . These conditions will usually have been identified as the system level “effect” using FMEA or some other hazard identification technique.
5. FTA is very good at showing how robust a system will be to one or more initiating faults. Thus for systems with high levels of redundancy and/or diversity, or for those with majority voting logic, FTA will be more appropriate.
6. FMEA is more suited to analysing systems that contain little or no redundancy and does not examine the effects of multiple failures at system level (apart from common cause failures).
7. Consider using FMEA when the system contains novel technology and the effects of failure of the components contained within the system need to be explored with insightful judgement. For any system FMEA is generally good for exhaustively identifying and recording the local effects within it that arise from component failures and then inferring the effects of those failures at system level.
8. Consider using FMEA when there is a need (a) to establish appropriate levels of redundancy within the design of a system, (b) to ensure “fail safe” outputs, (c) to increase the derating of components, or (d) to otherwise enhance the design generally.
9. FTA enables the fault/failure logic within a system of a particular effect of interest to be represented in diagrammatic form, whereas FMEA records the system effects of each failure cause in a tabular format.
10. First order cut-sets which are identified using FTA (i.e. conditions or factors which on their own would lead to a defined adverse effect) should also be identified as “single points of failure” using the FMEA technique. Whilst FTA focuses on a defined adverse effect, FMEA implicitly considers all adverse effects that may occur as a result of any single failure.

In summary

• FTA will identify combinations of conditions and component failures which will lead to a single defined adverse effect.
• FMEA on the other hand considers all single component failures in turn and identifies the range of their effects on the system.