Interestingly hunterston - a nuclear power station on the west coast of Scotland - had a failure of its backup diesel generators. Wikipedia describes it as:

> In December 1998, an INES Level 2 incident occurred after severe winds and sea spray disabled all four power lines to the site during the Boxing Day Storm of 1998. After multiple grid failures in a short period of time, emergency diesel generators failed to start. Normally, in the absence of power for the reactor cooling pumps, the reactor would be passively cooled. However, the emergency control system which would have initiated passive cooling failed to act, as it had not been reset. Reactor cooling was reinstated after four hours.

It seems the safety margin was twenty hours of no reactor cooling.

Wikipedia: https://en.wikipedia.org/wiki/Hunterston_B_nuclear_power_sta...