8. An electricity line consists of either an overhead line or an underground cable, or both. A typical National Grid overhead line route uses three main types of lattice steel tower (or pylon). These are:
- suspension towers which support the conductors on straight stretches of line;
- deviation towers at points where the route changes direction;
- and terminal towers where lines terminate at substations or are connected to underground cables.
Appendix II illustrates these features.
9. National Grid’s substations are necessary for the efficient operation of the transmission system, for the specific role of switching circuits or transforming voltage. They are normally sited between power stations and the transmission network, and between the transmission network and the local distribution companies’ networks. They can be sizeable developments, and including connecting terminal pylons, can occupy up to 20 hectares. However, advances in technology means that the equipment located at substations is now more compact than that of the 50s and 60s when many of the existing substations were built. Hence new substations are considerably smaller in size, both in height and area covered, and in certain circumstances, can be sited inside a building which resembles an industrial unit. Substations are usually contained within steel palisade fencing to ensure public safety, and the structures, excluding pylons, are not usually more than 15m in height. Road access is necessary for staff, and for the transport of equipment during construction, maintenance or repair. Very occasionally, transformers or other very large items of plant may need to be moved into or out of sites as abnormal indivisible loads.