The most widely used reliability indexes are SAIFI, CAIDI and SAIDI (pronounced SAY-fee, KAY-dee, and SAY-dee)
SAIFI (System Average Interruption Frequency Index) indicates the average number of outages for the system over a specified time period.
CAIDI (Customer Average Interruption Duration Index) indicates the average duration of a single outage.
SAIDI (System Average Interruption Duration Index) indicates the average outage duration for the system over a specified time period.
The indexes are mathematically related as follows:
SAIDI = (SAIFI) x (CAIDI)
CenterPoint Energy reports reliability index results to the Texas Public Utility Commission (PUCT).
CenterPoint Energy maintains statistics of outage frequencies and durations for each distribution circuit and for the electric system as a whole. While there is some variation from year to year, the average figures customers may experience are as follows:
Duration of Sustained Outages = 3 hours per year
Number of Sustained Outages = 2 per year
Average Power Quality
We also maintain statistics of momentary outage frequencies for each distribution circuit and the whole system. While there is some year-to-year variation, the average figure that customers may experience is as follows:
Number of Momentary Outages = 10 per year
Besides momentary outages, instantaneous voltage sags are the most common utility-related power quality event that adversely affects our customers. While we don't keep statistics on voltage sags, we did participate in a national power quality survey that accurately reflects what typical customers may experience:
Number of Sags Below 90% of Nominal Voltage = 70 per year
Of these 70 sags, 23 will dip below 70% of nominal voltage, which is generally considered the threshold for causing motors and other sensitive equipment to drop off-line.