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Normal Accidents: Living with High-Risk Technologies

Perrow · Safety & Error, Complexity & Systems · Basic Books · 1984 · Open access

The founding text of Normal Accident Theory, built to explain why certain systems generate catastrophic failures that no amount of operator training or safety procedure eliminates. Opens with a deliberately mundane illustration before the theory arrives: a morning where the coffee pot cracks, the car keys are locked inside, the neighbour's car has a dead generator, and a bus strike leaves no way to an interview. Each failure is trivial alone and each had a backup, but the failures interact, and Perrow's point is that the honest answer to 'what caused this' is none of the individual failures: the cause is the system's structure. He formalises this into system accidents, defined against component failure accidents by one criterion: whether the interaction between failures was anticipated by the people who designed the system. Two properties determine how prone a system is to this kind of accident: interactive complexity (unfamiliar, unplanned, or invisible sequences of interaction between parts) and tight coupling (no slack, no buffer, so a change in one part propagates directly into another before anyone can intervene). Systems high on both are the ones where normal accidents become, in the technical sense Perrow insists on, normal: not frequent, but an inevitable expression of the system's own characteristics rather than a statement about how often they happen. Crucially, the book refuses an easy fix: complex, tightly coupled systems are also more efficient, so the losses in slack, redundancy, and generalist understanding that would make them safer are the same losses that make them worth building in the first place. Three Mile Island runs through the book as the central case; petrochemical plants, aviation, marine transport, dams, and the space programme extend the argument.

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