Functional Failure

Identifying Functional Failures is an integral part of a thorough reliability centered maintenance analysis.

The definitions of Failure and Functional Failure

To truly understand the definition of the term "Functional Failure", it is important that you first understand "Failure".
Most people look at Failure as a black or white term – a component either failed or was working – a component was running or shut down. In reality, the definition of Failure is very broad and can often be subjective. Consider the following definitions of the word Failure:

Merriam-Webster's Collegiate® Dictionary defines Failure as “a state of inability to perform a normal function”.

Nowlan and Heap defined Failure as “an unsatisfactory condition”.

Read these definitions and try to relate them to your equipment or the process you work with. Is it clear to everyone what your “normal functions” are? Does everyone know what a “satisfactory or unsatisfactory condition” is? If we asked everyone who worked with or operated this equipment, would they all have the same definition of Failure, normal functions, and unsatisfactory condition? Of course not, and this is what drives us to become more specific in defining Failure. As the definition becomes more specific, our ability to clearly understand and pinpoint Failure increases.

To understand Failure, we must first understand the criteria that define Failure. These criteria should be set when defining the performance standards for the function of your system (Main Function) and the components that make up the system (Support Functions). In defining these performance standards, you will in turn help clearly define what Failure is for your process or equipment.

In the world of RCM, we use the term Functional Failure to help clarify the understanding of what Failure is.

Nowlan and Heap defined Functional Failure as “the inability of an item (or the equipment containing it) to meet a specified performance standard".

Now with the understanding that it is the Functional Failure of an item (component or system) that dictates how we now define Failure, it should be clear to all people working with or operating the equipment when the equipment has failed.

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