Welcome to European Tribune. It's gone a bit quiet around here these days, but it's still going.
Perhaps the largest single cause of unpredicted failures in complex systems is that multiple components, supposedly independent and redundant, can all fail at the same time for unforeseeable reasons. These can be "commonmode" failures--multiple failures of identical, redundant components in the same manner--or "common-cause" failures--multiple failures, caused by a single initiating event, of components that are different from each other but are supposed to do the same task. For example, identical valves can fail at the same time if they are all exposed to conditions for which they were not designed, or if they were designed or built wrongly: a common-mode failure. Different energy systems that are supposed to back each other up independently-- for example, programs for mining coal, making oil from shale, and generating electricity from coal and uranium--could all fail to be built because Wall Street will not pay for them or because Westerners do not want them: a common-cause failure. Common-mode and common-cause failures cannot be identified simply by cataloguing individual failure modes and their probabilities.

by rootless2 on Sun Jun 12th, 2011 at 10:50:58 AM EST

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