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Normal AccidentsSome failures are built into the design

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When It Breaks0149

Normal Accidents

Some failures are built into the design

Failures built into a system rather than caused by somebody's error.

III · The Machinery46 / 53

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0149 · The Primer46 / 53

Charles Perrow studied Three Mile Island and concluded the accident was not a fluke. When a system is complex, so that parts interact in ways nobody predicted, and tightly coupled, so that there is no slack to absorb a fault, serious accidents stop being surprises. Not frequent. Expected. The design produces them.

  • Adding a safety device adds complexity, and complexity adds new interactions. Some safety systems have gone on to cause the exact accident they were fitted to prevent.
  • The practical answer is slack and simplicity: buffers, spare capacity, and fewer hidden dependencies. Efficiency and resilience pull against each other.
SourcePerrow, Normal Accidents (1984)
Clearfield and Tilcsik, Meltdown (2018)

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