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III. Outage Causes <br />Equipment failure, extreme weather events, wildlife and vegetation are some of the most common causes <br />of electric system outages. However, certain factors, such as regional weather and animal/vegetation <br />patterns, can make a different set of causes more prevalent to a specific group of utilities. The following <br />section of this report include graphs depicting common causes of outages for your individual utility, all <br />utilities in your region, and all utilities using the eReliability Tracker. The charts containing aggregate <br />information are customer-weighted to account for differences in utility size for a better analytical <br />comparison. <br />For example, a particularly large utility may have a large number of outages compared to a small utility; in <br />order to avoid skewing the data towards large utilities, the number of cause occurrences is divided by <br />customer size to account for the differences. In Figures 8-13, the data represent the number of <br />occurrences for each group of 1,000 customers. For instance, a customer-weighted occurrence rate of "1" <br />means 1 outage of that outage cause per 1,000 customers on average in 2019. <br />Note that the sustained outage cause analysis is more comprehensive than the momentary outage cause <br />analysis due to a bigger and more robust sample size for sustained outages. Regardless, tracking both <br />sustained and momentary outages helps utilities understand and reduce outages. To successfully use the <br />outage information tracked by your utility, it is imperative to classify and record outages in detail. The <br />more information provided per outage, the more conclusive and practical your analyses will be. <br />III.1. Sustained Outage Causes <br />In general, sustained outages are the most commonly tracked outage type. In many analyses of sustained <br />outages, utilities tend to exclude scheduled outages, partial power, customer-related problems, and <br />qualifying major events from their reliability indices calculations. While this is a valid method for reporting, <br />these outages should be included for internal review to make utility-level decisions. In this section, we <br />evaluate common causes of sustained outages for your utility, corresponding region, and for all utilities <br />that use the eReliability Tracker. It is important to note that in this report, sustained outages are classified <br />as outages that last longer than five minutes, as defined by IEEE 1366. <br />1.41.27 <br />1.2 <br />1 <br />0.71 <br />0.8 <br />0.6 <br />0.38 <br />0.37 <br />0.29 <br />0.4 <br />Occurrence Rates <br />0.2 <br />0 <br />TreeStormEquipmentSquirrelUnscheduled <br />TreesHumanWeatherSquirrelUnknown <br />Outage Causes Types <br />Figure 8. Top five customer-weighted occurrence rates for common causes of sustained outages <br />Ў <br />for all utilities that use the eReliability Tracker Service <br />5For each utility, the number of occurrences for each cause is divided by that utility's customer size (in 1,000s) to create an occurence rate <br />that can be compared across different utility sizes. <br />12 <br />66 <br />