Give me five

When the concept of reliability formally became an integrated engineering approach in the 1950s, it was associated with failure rate. Today, the term reliability is used as an umbrella definition covering a variety of subjects including availability, durability, quality, and sometimes the function of the product. Currently, in the power industry, when the subject is reliability, it is very common to see statements about five 9s. The implication is that this term somehow defines a specific level of reliability. It is only one component of reliability and must be used in its intended manor. This article will present the industry-accepted definitions along with examples to show what five 9s really mean in specific instances and with specific electrical distribution systems. The main issue is that reliability is a term with a specific definition, and the five 9s refer to availability; it means an availability of 0.99999. As we will see later, it requires more than a single term to adequately define the expectations for reliable operation of critical facilities.