HD
That "once in a life time" happened to me on three occasions. I must be very old! Or unlucky.
Most memorable time was at W Drayton one evening when it all went dark in the ops room (just the emergency lighting working). Then the lights went on, then back out again then on. I was on a break at the time, and was conscious of the eery quieteness (no strip printers running)....although every controller in the ops room was on his feet, reaching for the emergency RT handsets. I went back to my sector to lend a hand. I forget the cause now, it was something to do with the power supply modules deciding they were no longer un interruptable.....ho hum.