Major Event Days: Why Storm Days Are Excluded

8 min · difficulty 4/10

An 11-hour derecho knocks out power to a huge share of a utility’s customers in a single day. This year’s SAIDI is now hundreds of minutes higher than last year’s. Before you conclude the grid got worse, ask one thing: should that one storm day count in the comparison at all?

For routine reporting, the answer is no, and the reason is arithmetic. A single extreme day can produce more interruption minutes than every blue-sky day of the year combined. If a utility logs 110 minutes of SAIDI across all its normal days and one storm adds several hundred more, a raw year-over-year number stops measuring how the system performed and starts measuring how the weather behaved. To keep the trend meaningful, utilities pull those days out and report them separately.

The trick is deciding which days count as storm days without picking a number out of the air. IEEE 1366 handles this with the statistical 2.5-beta method. It takes several years of the utility’s own daily SAIDI, fits a log-normal curve to those values, and sets a threshold called TMED at the log-mean plus 2.5 standard deviations. Any day whose SAIDI clears TMED is designated a Major Event Day and reported on its own, outside the blue-sky indices. There is no universal minute cutoff to memorize: TMED is derived from that utility’s own trailing data and recomputed every year, so it differs from utility to utility and shifts as the history rolls forward.

Daily-SAIDI control chart with the computed Major Event Day threshold A gold formula banner reads T sub MED equals e to the power alpha plus 2.5 beta equals e squared point five, approximately 12.18 minutes. Below it, twenty small green blue-sky daily SAIDI points cluster near the bottom of the chart, all well under a dashed gold TMED threshold line at 12.18 minutes. Far to the right and far above that line sits a single red point labeled derecho day, Major Event Day, excluded from blue-sky SAIDI, dwarfing every normal day. TMED = e^(α + 2.5β) = e^2.5 ≈ 12.18 min Sample of blue-sky days vs. one derecho, same threshold TMED ≈ 12.18 min Blue-sky days, below TMED Derecho day Major Event Day excluded from blue-sky SAIDI 170 0 Daily SAIDI (min) Day of year (sample window)
The IEEE 1366 threshold is computed from a utility's own history. A derecho clears it by a wide margin and gets excluded from routine SAIDI.

Run the derecho through that filter and it lands far above the utility’s computed line, so it comes out of the annual SAIDI and gets its own line item. Whether you are a buyer sizing reliability into a purchase, an operator reading the annual scorecard, or an engineer defending your numbers to a regulator, the lesson is the same: a raw year-over-year SAIDI comparison is misleading unless Major Event Days are called out separately. When someone hands you a reliability trend, the first question is always whether the storm days were pulled.

SAIDI and SAIFI targets drive capital planning, and capital planning drives what gets ordered and when. The Feeder tracks the reliability investment and regulatory signals behind those orders, free, every week.

Question 1 of 3

A utility runs the IEEE 1366 method on five years of daily SAIDI. The natural-log values have mean alpha = 0.5 and standard deviation beta = 0.8. The Major Event Day threshold is TMED = e^(alpha + 2.5*beta). Compute TMED, in minutes.

Educational material only. This is not engineering, safety, or procurement advice. Confirm any value against manufacturer documentation and a licensed professional before specifying equipment.