Also known in Great Britain as the Grid, the National Grid or the transmission system. The system consisting (wholly or mainly) of high voltage electric lines owned or operated by transmission licensees within Great Britain, in the territorial sea adjacent to Great Britain and in any renewable energy zone, and used for the transmission of electricity from one generating station to a substation or to another generating station or between substations or to or from any interconnector. This includes any electrical plant or meters owned or operated by any transmission licensee within those areas in connection with the transmission of electricity. For more information, see Practice note, Electricity industry: overview.

In Great Britain it is formed mainly of three separately owned but connected transmission systems and a number of offshore transmission lines, with a common system operator (SO), National Grid Electricity System Operator (NGESO). Northern Ireland has a separate transmission system. For more information, see Practice note, Electricity industry: overview: Transmission network. There are plans for significant extensions to the onshore transmission network to be independently owned in the future under the Competitively Appointed Transmission Owner (CATO) regime (see Practice note, Competitively appointed transmission owner (CATO) projects: overview).