Probability of Failure

The first step in determining the criticality score of an asset is to determine the likelihood or probability of failure. Entirely eliminating the possibility of failure is unrealistic; however, utilities can work to minimize failure, especially for critical assets that have high consequences of failure. There are four modes by which an asset can fail: mortality failure, capacity failure, level of service failure, and financial inefficiency.  

A variety of factors contribute to each of these four failure modes. These factors can range from O&M history to customer expectations to condition of the asset. There is not one factor that is the most common cause of failure, and these factors may impact green and gray assets differently. When considering how each asset might fail, all four modes need to be explored. Also, consider which mode is most likely to affect each asset. 

To calculate the criticality of an asset, quantify the probability of failure of that asset and assign a probability of failure (PoF) rating. These ratings should be based on a PoF rating structure created by utility staff. Each failure rating in the structure should have a general description of likelihood of occurrence. It is important to create these rating systems so all staff members can rate the probability of failure of a given asset similarly. The ratings should not be compared to other systems; this is meant as an internal tool only. The goal is for a utility to determine which of their assets are more likely to fail than other assets in their system.