In the electricity sector in the United Kingdom, the National Grid is the high-voltage electric power transmission network serving Great Britain, connecting power stations and major substations and ensuring that electricity generated anywhere on it can be used to satisfy demand elsewhere. The network covers the great majority of Great Britain and several of the surrounding islands. It does not cover Northern Ireland, which is part of a single electricity market with the Republic of Ireland. The GB grid is connected as a wide area synchronous grid nominally running at 50 hertz. There are also undersea interconnections to other grids in the Isle of Man, Northern Ireland, the Republic of Ireland, France, Belgium, the Netherlands and Norway. On the breakup of the Central Electricity Generating Board in 1990, the ownership and operation of the National Grid in England and Wales passed to National Grid Company plc, later to become National Grid Transco, and now National Grid plc. In Scotland the grid was already split into two separate entities, one for southern and central Scotland and the other for northern Scotland, with interconnectors. The first is owned and maintained by SP Energy Networks, a subsidiary of Scottish Power, and the other by SSE. However, National Grid plc continues to be the transmission system operator for the whole GB grid. (en)