FMECA or FTA?

September 1, 2009 | 1 Comment

There are two ways to approach fault analysis — Failure Modes, Effects and Criticality Analysis (FMECA), which is currently the most common approach, and Fault Tree Analysis (FTA). Both look at the effects of component failures on a system, but come at it from different ways. FMECA starts from the lowest levels of the system and considers the effect at the top level of the system of a failure at the lower level. FTA starts with a possible failure mode of the system, and then works down to see what could cause it. The Reliability Toolkit¹, gives selection criteria for when to use either of these two approaches.

The primary concern is safety of public or operating and maintenance personnel
A small number of clearly differentiated “top events” can be identified
Completion of a functional profile is of critical importance
There is a high potential for failure from “human error”
There is a high potential for failure from “software error”
The primary concern is a quantified “risk evaluation”
Product functionality is highly complex and/or it contains highly-interconnected functional paths

On the other hand, the FMECA is the preferred approach when:

“Top events” cannot be explicitly defined or limited to a small number
The primary concern is the identification of “all possible” failure modes
The product has little human or software intervention

It is clear that, from this analysis, FTA is the more appropriate approach in many cases.

In addition, FTA is an easier and faster mode of analysis for (at least) two reasons. Firstly, it focuses on a select subset of the possible modes, those that have a “catastrophic” consequence. Secondly, it is my experience that it seems more relevant to the engineering mind as the consequence provides a focus. Also, it is generally done graphically so is easier to visualize than the tables of a FMECA.

Even more so than a FMECA, the FTA is an invaluable tool in the initial design stages as it can be applied even at the block diagram stage to determine critical areas. Unlike a FMECA, operator, software and external input effects can be easily included. An added benefit is that the FTA provides a basis for maintenance troubleshooting procedures.

References:

1. Reliability Toolkit: Commercial Practices Edition, Rome Laboratory & Reliability Analysis Center.

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