About Functions

To prevent failures within a system, the functions of the system need to be defined. In an RCM Analysis, a function defines what the owner of the system wants it to do. A system can have one or more functions, each of which is defined in a separate record. To define all the possible functions of a system, it can be helpful to think through the functions of each individual piece of equipment and location belonging to a given system. For example, a function for a cooling tower may be defined with the following statement:

Provide 500 gallons per minute of water at a maximum of 90 degrees Fahrenheit under all ambient conditions.

The analysis team should also define whether each function is evident or hidden. Evident functions are those in which, under normal circumstances, an operating crew would notice a failure. Hidden functions are those in which a failure may not be noticed.

The SAE Standard JA1012, "Evaluation Criteria for Reliability-Centered Maintenance (RCM) Processes," specifies the following requirements for functions:

The operating context of the asset shall be defined.
All the functions of the asset/system shall be identified (all primary and secondary functions, including the functions of all protective devices).
All function statements shall contain a verb, an object, and a performance standard (quantified in every case where this can be done).
Performance standards incorporated in function statements shall be the level of performance desired by the owner or user of the asset/system in its operating context.