Reliability indices

In order to deal meaningfully with reliability as a design criterion for distribution, it is necessary to be able to measure it and set goals. A bewildering range of reliability indices, or measures are in use within the power industry. Some measure only frequency of interruption, others only duration. A few try to combine both frequency and duration into a single value, which proves to be a nearly impossible task. Some measures are system-oriented, looking at reliability averaged over the entire customer base. Others are customer or equipment oriented, meaning that they measure reliability only with regard to specific sets of customers or equipment. To complicate matters further, most utilities use more than one reliability index to evaluate their reliability. The large number of indices in use is compelling evidence that no one index is really superior to any other. In fact, the author is convinced that no one index alone is particularly useful