The Failure Mode

Form EIA-861 collects a year of distribution outage data from every reporting electric utility in the United States. The 2024 row of the table built from it carries five SAIDI values for that single year. Two of them are 662.6 and 131.6. Both are minutes per customer. Both are 2024. Neither corrects the other. Divide by sixty and the familiar headlines appear. 662.6 minutes is 11.04 hours, and the U.S. Energy Information Administration's write-up of the year says customers "experienced an average of 11 hours of electricity interruptions in 2024." 131.6 minutes is 2.19 hours, and the same write-up says interruptions "that aren't triggered by major events routinely average about two hours per year." The 11-hour year and the 2-hour year are the same year, on the same form, printed on the same line. What separates them is one test applied a day at a time, and it is not a fixed severity. It is a number each utility computes from its own recent history. ...

Alternating-current frequency is one shared number across an interconnected grid. Every generator, motor and protective relay inside the same interconnection sees the same value at nearly the same instant, and it moves whenever generation and load stop matching. For the Texas Interconnection, NERC's frequency-response analysis starts its arithmetic from a figure it labels the starting frequency, 59.970 Hz, and works down to a low-frequency limit of 59.300 Hz. That limit is not an arbitrary floor. The report states that “the low-frequency limits to be used for the IFRO calculations (Table 2.2) should be the highest step in the Interconnection for regionally approved UFLS systems.” That is written as an instruction for choosing the number, not as a finding about any particular program, and the report does not say which program each figure comes from. The same four values are then carried into the calculation table beside the starting frequencies, where the row is called the ...

On a grid running short of supply, some customers lose power and others do not, and the boundary between them is not drawn where most people assume. It is not drawn by neighborhood, by how much anyone pays, or by who called the utility first. It is drawn by which distribution circuit an address happens to sit on, and by which of two entirely separate mechanisms is doing the shedding at that moment. Those two mechanisms are worth separating carefully, because coverage of any given shortage tends to blur them into one event. One of them is a chain of declarations made by people, ending in a utility opening breakers it selected in advance. The other is a set of relays that watch system frequency and act without asking anyone. They produce a similar outcome — load disconnected — through completely different logic, on completely different timescales, and with completely different rules about who is spared. The directed path runs on definitions The escalation is not improvis...