6.2. ERMUSR 01-15-2008 - Laserfiche WebLink

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This year we experienced minimal outages due in part to no severe weather and no transmission or substation outages. A number of reliability indexes are used in the electric industry to make it easier to compare performance of electrical utilities. These reliability indexes are: 1. ASAI (average service availability index) ASAI is a measure of the average availability of the sub-transmission and distribution systems to serve customers. It is the ratio of the total customer minutes that service was available to the total customer minutes demanded in a time period. It is normally expressed as a percentage. ERMU's 2007 ASAI is 99.99%. This is a very good percentage for any utility. 2. CAIDI (customer average interruption duration index) This is defined as the average length of an interruption, weighted by the number of customers affected, for customers interrupted during a specific time period. It is calculated by summing the customer minutes off during each interruption in the time period and dividing this sum by the number of customers experiencing one or more sustained interruptions during the time period. The resulting unit is minutes. The index enables utilities to report the average duration of a customer outage for those customers affected. ERMU's 2007 CAIDI is 63 minutes. 3. SAIDI (system average interruption duration index) This is defined as the average interruption duration for customers served during a specified time period. It is determined by summing the customer-minutes off for each interruption during a specified time period and dividing the sum by the average number of customers served during that period. The unit is minutes. This index enables the utility to report how many minutes' customers would have been out of service if all customers were out at one time. ERMU's 2007 SAIDI is 7.41 minutes. 4. SAIFI (system average interruption frequency index) This is defined as the average number of times that a customer is interrupted during a specified time period. It is determined by dividing the total number of customers interrupted in a time period by the average number of customers served. The resulting unit is "interruptions per customer". Separate SAIFI indexes are sometimes calculated for long and for short interruptions. ERMU's 2007 SAIFI is .118. Another statistic staff gets concerned about is the average length of any outage. ERMU's 2007 average length of outage is 68.75 minutes. Electric utilities with a high percent of underground facilities will exhibit higher length of outage times than electric utilities with high percent of overhead facilities.