At 16:52:33 on Friday 9 August 2019, lightning struck a 400 kV mainland transmission line between Eaton Socon and Wymondley north of London, causing small generators, Little Barford Power Station (641 MW) and Hornsea (737 MW) to unexpectedly disconnect a combined 1,878 MW (1,691 MW maximum single-time) supply within minutes, greater than the 1,000 MW limit. Grid frequency declined, initially below the 'exceptional circumstances' limit at 49.5 Hz, triggering additional generation and then more importantly below 48.8 Hz at which point the first stage of ‘Low Frequency Demand Disconnection’ (LFDD) a form of automatic load shedding commences to stabilise the grid. As part of this 1.1 million customers were disconnected; rail services were disrupted with 371 trains cancelled, 220 part cancelled, and 873 trains delayed. Power was restored at 17:37. Hornsea disconnected due to a software flaw, which was fixed the next day.
