Had an interesting mains incoming fault the other week.
Of the infeeding 11KV 3 phases to the local substation two had shorted and gone bang.
That left one phase live - mine.
The result was "interesting"
The RMS value of the incoming supply to the house shot up from 240 volts to 281Volts before 'settling down' to 275Volts.
...and there it sat - while I got on the phone and after about an hour they wound down the tap changing transformer somewhere further back to reduce it to around 250volts while they pondered what to do next. (install a generator on a lorry outside the local substation was the answer while the cable was mended over the next few days)
I noticed it as I was in a room with an incandescent lamp which suddenly got much brighter and my computer's UPS intervened to safeguard things as the volts were way too high.
I was rushing round the house switching things off!