How Electric Reliability Is Measured

Graph of reliability in industry quintiles, rising from 4th quintile in 2005 to 1st quintile in 2013..The reliability of a system is measured by the perceptions of those who use the system—our members—but the PenLight staff also baselines our performance by calculating indices based on industry standards (Institute of Electrical and Electronic Engineers standard IEEE-1366). The standards provide equations for averaging how long each member was out of power and the number of outages each member experienced.

The indices are used in two ways. First, we use them to compare our performance to national statistics, which use a quartile system, after adjusting for major events. A utility with a performance better than 75% of the other utilities in the country is in the first quartile. A utility with performance better than 50% of the other utilities, but not as good as the first 25% is in the second quartile, and so on. Figure 1 demonstrates PenLight’s performance relative to other utilities using IEEE-1366. Note the consistent improvement.

Taken alone, this can be misleading since systems are different and each utility manages its reliability differently. As such, we also use them to compare our performance each year to our performance in previous years. This tells us how we are trending and gives us an indication of the effectiveness of our reliability program. We take this one step further by looking at how many members are affected by each outage. As you can see from Figure 2, this has been improving for quite some time.