I'm not sure you are looking in the right place - the regs tells you to plan your design more or less, its not really a fault find aid.
How  big an over-voltage are you looking at and how long is its duration?

However predictable (sort of regualr) short duration over-voltages are usually related to the switching of inductive loads like motors, where all the stored energy suddenly gets 'dumped' . Less predictable but often more devastating, are currents induced by nearby (or direct !) lightning strikes. Longer duration over voltages usually relate to network faults or generator faults, where a substation transformer tap changer jams, or the neutral -earth bond fails (and up to 400V appears where 230 was expected), or on gensets problems with the regulator/ speed stabiliser, usually coupled with frequency going walk about as well....

Mike

I cannot find in the 18th edition Regs causes of severe temporary overvoltages

Try the section titles in chapter 44 - e.g. 442 - due to earth faults in the HV system and faults in the LV system, 443 - atmospheric origin or switching

   - Andy.

There's a section the the book on surge protection.

The clue in a question like that is "severe temporary overvoltage". Just the examiner looking up a regulation and setting a question to it. Two minutes to find it! Best go straight to the index, look up overvoltage or temporary. The question number should alert you to the Part of the regs that you should be drawn to. So for example, questions 13-27 are on Part 4 so you can ignore any references that dont start with a 4.

Thanks everyone, great advice.