We own and operate the electricity transmission network in England and Wales, the gas transmission network in Great Britain, and electricity transmission networks in the northeastern US. We are also responsible for the operation of the electricity transmission networks in Scotland.

Our Transmission business operates in both the UK and the US. As a consequence of the differences in economic and regulatory environments, we report the results of Transmission as two segments: Transmission UK and Transmission US.

Transmission UK

Our transmission operations in the UK encompass both electricity and gas transmission, comprising the following principal activities:

Electricity transmission owner: We own the electricity transmission system in England and Wales. Our electricity assets comprise approximately 7,200 kilometres of overhead line, about 690 kilometres of underground cable and 337 substations at 241 sites.

Gas transmission owner: We own the gas national transmission system in Great Britain. This comprises approximately 7,600 kilometres of high pressure pipe and 26 compressor stations, connecting to 8 distribution networks and to third party independent systems for onward transportation of gas to end consumers.

Electricity system operator:  We are the Great Britain System Operator, responsible for managing the operations of both the England and Wales transmission system that we own and also the two high voltage electricity transmission networks in Scotland. Day-to-day operation of the Great Britain electricity transmission system involves the continuous real-time matching of demand and generation output, ensuring the stability and security of the power system and the maintenance of satisfactory voltage and frequency. We are also designated as system operator for the new offshore electricity transmission regime.

Gas system operator: We operate the gas national transmission system. Day-to-day operation includes balancing supply and demand, maintaining satisfactory system pressures and ensuring gas quality standards are met. 

French interconnector: We own and operate the UK assets, and a portion of the subsea cables, that comprise the electricity interconnector between England and France as part of a joint arrangement with the French transmission operator.

LNG storage: We own and operate four liquefied natural gas (LNG) storage
facilities in Great Britain. With the scheduled closure of Dynevor Arms during 2009 the number of facilities will decrease to three.

Transmission US

In the US, we are involved in electricity transmission and our principal activities are as follows:

Electricity transmission owner:  We own and operate an electricity transmission network of approximately 13,800 kilometres spanning upstate New York, Massachusetts, Rhode Island, New Hampshire and Vermont. Our US electricity transmission facilities operate at voltages ranging from 69 kV to 345 kV, utilising nearly 13,700 kilometres of overhead line, nearly 140 kilometres of underground cable and 524 substations. We are the largest electricity transmission service provider in New England and New York by reference to the length of these high voltage transmission lines.
 
Canadian interconnector: We own and operate a 224 kilometre direct current transmission line rated at 450 kV that is a key section of an interconnector between New England and Canada.

As one of several transmission owners, we work with two distinct independent system operators in New England and New York. These non profit system operator entities for New England and New York are responsible for operating organised wholesale markets for energy, for operating reserves and capacity, for maintaining the operating reliability of the New England and New York networks, for coordinating the activities of the transmission owners, and for managing transparent transmission expansion planning processes.

The transmission owners are responsible for certain aspects of the operation of the facilities they each own, such as maintenance, equipment restoration and switching operations.

National Grid also works closely with the independent system operators in New England and New York to support efficient market and network operations and transmission investment.