Major Incidents
Power cuts due to either problems on the actual infrastructure of the supergrid (defined in the Grid Code, as the transmission system operated by National Grid, which in England and Wales, comprises lines energised at 275,000 volts and 400,000 volts), or due to lack of generation to supply it with sufficient energy at each point in time, are exceedingly rare. The nominal standard of security of supply is for power cuts due to lack of generation to occur in nine winters in a hundred.
The overall performance measure for electricity transmission is published on NGET’s website: - this site includes a simple high level figure on transmission availability and reliability of supply – for 2008-9 this was 99.99979%. It is issues affecting the low voltage distribution systems, for which National Grid is not responsible, that cause almost all the 60 minutes or so per year, on average, of actual domestic power cuts. Most of these low voltage distribution interruptions are in turn, the fault of third parties such as workmen drilling through the street mains (or subterranean higher voltage) cables; this does not happen for transmission lines, which are mostly overhead on pylons. For comparison with supergrid availability, Ofgem, the electricity regulator, has published figures on the performance of 14 electricity distributors.
Since 1990, there have only been two power cuts of high national promimence, that were linked to National Grid (although one was actually due to generation issues) :
Read more about this topic: National Grid (Great Britain)
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