In my view, the key thing is not knowing just the correct time to break the rules but to make such a decision for the right reasons when armed with the right information. Additionally, any such rule breaking must be made by the person with the right level of responsibility and this should then be documented accordingly so that the poor sod with his hands at the controls doesn't find him, or herself, at the wrong end of an about-face by the decision maker. Current safety thinking is not about arse covering and risk aversion but about informed decision making and the protection of the people who used to get the blame when the excrement hits the rotating air blower.