Ieee std 1366-2012 (Revision of ieee std 1366-2003) ieee guide for Electric Power Distribution Reliability Indices

5. Information about the factors that affect the calculation of reliability 
indices 
5.1 Rationale behind selecting the indices provided in this guide 
One view of distribution system performance can be garnered through the use of reliability indices. To 
adequately measure performance, both duration and frequency of customer interruptions must be examined 
at various system levels. The most commonly used indices are SAIFI, SAIDI, CAIDI, and ASAI, which all 
provide information about average system performance. Many utilities also calculate indices on a feeder 
basis to provide more detailed information for decision making. Averages give general performance trends 
for the utility; however, using averages will lead to loss of detail that could be critical to decision making. 
For example, using system averages alone will not provide information about the interruption duration 
experienced by any specific customer. It is difficult for most utilities to provide information on a customer 
basis. This group believes the tracking of specific details surrounding interruptions, rather than averages, 
may be accomplished by improving tracking capabilities. To this end, the working group has included not 
only the most commonly used indices, but also indices that examine performance at the customer level 
(e.g., CEMI
n 
and the CELIDs).
5.2 Factors that cause variation in reported indices 
Many factors can cause variation in the indices reported by different utilities. Some examples are 
differences in: 
⎯
Level of automated data collection 
⎯
Geography 
⎯
System design 
⎯
Data classification (e.g., Are major events in the data set? Planned interruptions?) 
To ensure accurate and equitable assessment and comparison of absolute performance and performance 
trends over time, it is important to classify performance for each day in the data set to be analyzed as either 
day-to-day or MED. Not performing this critical step can lead to false decision making because MED 
performance often overshadows and disguises daily performance. Interruptions that occur as a result of 
outages on customer-owned facilities, or loss of supply from another utility, should not be included in the 
index calculation.