System Average Interruption Duration Index (SAIDI)
This index is defined as the system average interruption duration for customers served per year.
| SAIDI = | Total Customer-Hours of Interruptions |
| Total Customers Served* |
*Total customers served represents the number of end customers the utility is delivering electricity to.
System Average Interruption Frequency Index (SAIFI)
This index is defined as the average number of interruptions per customer served per year.
| SAIFI = | Total Customer-Interruptions |
| Total Customers Served* |
*Total customers served represents the number of end customers the utility is delivering electricity to.
Understanding SAIDI and SAIFI
When an interruption or an outage occurs, that’s one event.
For example, if a tree fell and took out a powerline that impacts an entire block of 52 customers, that is now 52 customer interruptions. The utility repairs the line and restores power in 45 minutes. The total duration is calculated in customer hours of interruption (CHI) or Customer minutes of interruption (CMI). With 52 customers each at 45 minutes that is now 2,340 CMI or 39 CHI.
Let’s now assume another interruption occurs due to a severe ice storm - another interruption impacting 550 customers for 2 hours or a total of 1100 CHI.
Assuming the total customers served was 950 and these events were the only interruptions for the utility that year. That would provide our example utility with a SAIDI of 1.2 hours per year, and a SAIFI of 0.63 occurrences per year. In other words, each customer on their network could experience a total of 1.2 customer hours in the year, over 0.63 times in the year.
The CEA Bulk Electricity System (BES) program uses SAIDI and SAIFI to measure transmission system performance. Since transmission is fundamentally different than distribution and not delivered to many end customers, the BES program uses system delivery points within the formula instead of total customers served.
Momentary and Sustained Outages
The IEEE Standard 1366-2012 identifies momentary outages at 5 minutes. Momentary outages are not measured against utility performance. In Canada and many parts of the world, 5 minutes is considered too long for a momentary outage. In several jurisdictions in Europe a momentary outage is identified at 3 minutes or even 1 minute. Canada the CEA reliability committees follow the 1-minute rule for identifying momentary outages.
Major Event Day / Force Majeure
Major Event Days, Force Majeure or Most Prominent Events are considered outside the control of the utility and are not caused from a general malaise of the system or equipment. Major Events are determined by applying the IEEE BETA 2.5 methodology to the utility’s daily performance measures. This methodology can also be found in the IEEE standard. Examples of major events may be outages caused by heavy snow and ice, high winds and more.
Major Events have been identified as extreme weather events and are often linked to climate change. The following chart shows only the data for major events with a 5-year rolling average for both CHI and CI. There is a subtle increase in CI, but a significant change on the CHI highlighting the increased impact of these events on critical infrastructure.