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A number of reliability indexes aze used in the electric industry to make it easier to compaze <br />performance of electric utilities. These reliability indexes are: <br />1. ASAI (average service availability index) <br />ASAI is a measure of the average availability of the sub-transmission and distribution <br />systems to serve customers. It is the ratio of the total customer minutes that service was <br />available to the total customer minutes available in a time period. This is normally expressed <br />as a percentage. <br />ERMU's 2011 ASAI is 99.994% Availability. Last yeaz was 99.985% Availability. <br />2. CAIDI (customer average interruption duration index) <br />This is defined as the average length of an intemxption weighted by the number of customers <br />affected. It is calculated by summing the customer minutes off during each interruption in <br />the time period and dividing this by the number of customers experiencing one or more <br />sustained interruptions during the time period. The resulting unit is minutes. The index <br />enables utilities to report the average duration of a customer outage for those customers <br />affected. <br />ERMU's 2011 CAIDI is 105 minutes. Last yeaz's CAIDI was 94 minutes. <br />3. SAIDI (system average interruption duration index) <br />This is defined as the average interruption duration for customers served during a specified <br />time period. It is determined by summing the customer minutes off for each interruption <br />during a specified time period and dividing that sum by the average number of customers <br />served during that period. The unit is minutes. This index enables the utility to report how <br />many minutes customers would have been out of service if all customers were out at one <br />time. <br />ERMU's 2011 SAIDI is 31.1 minutes. Last yeaz's SAIDI was 80.0 minutes. <br />4. SAIFI (system average interruption frequency index) <br />This is defined as the average number of timers that a customer is interrupted during a <br />specified time period. It is determined by dividing the total number of customers interrupted <br />in a time period by the average number of customers served. The resulting unit is <br />"interruptions per customer." <br />ERMU's 2011 SAIFI is 0.296. Last yeaz's SAIFI was 0.853. <br />Another statistic staff gets concerned about is the average length of any outage. Elk River <br />Municipal Utilities' 2011 average length of outage is 126.00 minutes. Last year the average <br />length of outage was 108.11 minutes. Electric utilities with a high percentage of underground <br />facilities will exhibit higher length of outage time than electric utilities with more overhead <br />facilities. <br />Staff reviews these statistics in detail to find ways to continually improve our system. These <br />statistics are very good compared to other utilities. Underground faults and branches caused the <br />greatest number of outages. Underground faults resulted in 17.65% of our outage incidents, but <br />only contributed to 0.99% of total outage time. Outages resulting from branches and/or trees <br />make up 25.49% of our outage incidents and contribute to 33.46% of our outage time. This is <br />partially due to the unusually wet spring which resulted in trees being susceptible to blowing <br />down during storms. Ongoing and increased efforts should be taking for tree trimming to <br />improve this outage category. Overall, weather related outages account for 13.73% of the <br />