November 21, 2007 | Ken Reed

Fault with Fault Trees

Many of us have had experience working with fault trrees.  We have worked throught the painstaking process of determining how a particular material failure will get us to a specific observed failure.  It is an excellent tool to structure your thinking when you need to see exactly what could possibley have caused the failure we are investigating.

Too often, we seem to focus only on the material failures.  The fault tree will have items such as “Relief Valve Fails”, Vessel Wall Fatigue Failure”, “Overpressure”, “Power Supply Fails.”  This is very comfortable, because it is only a piece of “stuff” that failed, and I can easily lay the blame for the failure on an inanimate object.  The fault tree will use the basic event (or equivalent) symbol, signifying that we need look no deeper than this event.

The more realistic investigator may add in a set of events that include “Human Error” into the mix.  More often than not, however, these events are catagorized as Incomplete Events, or those that we just don’t have enough information to further develop.  It is easier to just leave these events on our fault trees as incomplete, and we just accept that the human error is just going to occur.  After all, I can’t replace the human like I can a part.  And parts don’t get their feelings hurt or complain about the result!

Yet, deep down we know (and studies have proven over and over again) that these “human errors” are at the root of over 90% of equipment failures.  Therefore, although our fault trees may eventually get you to the point of discovering what part or condition contributed to the final failure, we still never seem to get to the bottom of 90% of our failures.

It is important that, when conducting a fault tree analysis, we include and run down the human errors that lead to the failure conditions.  The failure conditions that we discover should be considered Causal Factors (“Relief Valve did not lift”), and then a proper root cause analysis can be conducted on the reasons why the relief valve did not lift.  Use Equifactor® to help populate your fault trees.  Then take the results of your fault tree, plug them into your SnapCharT®, and finish your investigation.  Using the rest of the TapRooT® system to assist in your equipment troubleshooting process, you will get much further beyond where the fault tree drops you.

Root Cause Analysis
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