(the size of the largest single expected loss) that the operator was maintaining at the time. The grid frequency fell to 48.8 Hz before automatic load-shedding disconnected 5% of the local distribution networks (1.1 million customers) for 15 to 20 minutes; this action stabilised the remaining 95% of the system and prevented a wider blackout. Although power was maintained at all times to the railway network (but not to the signalling system), the reduction in frequency caused 60 Thameslink Class 700 and 717 trains to fail. Half were restarted by the drivers but the others required a technician to come out to the train to restart it. This led to substantial travel disruption for several hours on the East Coast Main Line and Thameslink services. The supply to Newcastle Airport was also disrupted, and a weakness was exposed in backup power arrangements at Ipswich Hospital. An investigation by Ofgem concluded in January 2020. It found that Little Barford and Hornsea One had failed to remain connected to the grid following the lightning strike, and their operators – RWE and Ørsted respectively – agreed to each pay £4.5 million to Ofgem's redress fund. Additionally, Ofgem fined distribution network operator UK Power Networks £1.5M for beginning to reconnect customers before being cleared to do so, although this breach of procedure did not affect the recovery of the system. Minor incidents November 2015 On 4 November 2015 National Grid issued an emergency notice asking for voluntary power cuts because of "multiple plant breakdowns". No power cuts occurred but wholesale electricity prices rose dramatically, with the grid paying up to £2,500 per megawatt-hour. See also Demand response Cost of electricity by source Economics of nuclear power plants – for cost comparisons Energy security and renewable technology Intermittent energy source TV pickup 2007 switching station flood at Walham, Gloucestershire List of energy storage projects List of major power outages Spark spread – calculating the cost of back-up Load management Three-phase electric power List of HVDC projects List of high voltage underground and submarine cables National Grid Reserve Service Energy in the United Kingdom High-voltage substations in the United Kingdom References Further reading External links The Transmission System, National Grid's Seven Year Statement (2008) The Statement of the Use of System Charging Methodology , National Grid Useful Information, National Grid National Grid live data, ELEXON UK National Grid Status UK Electricity Networks: The nature of UK