The main constructional features of the 66-kV and 33-kV sections of the British grid system are briefly described under headings dealing with overhead lines, cables, switching and transforming stations, and protective equipment.Illustrative examples are given of both normal construction and special constructions of particular interest, together with the most important basic data relating to the general design of the equipment.Methods of earthing are described, with particular reference to their relation to the functioning of discriminative protective equipment.The types of protective equipment used on different classes of lines and apparatus are indicated.