Overhead-line practice

Overhead lines have been in use in the UK since the beginning of electricity supply, those erected prior to 1882 being based on American practice. The 1882 Act established rules regarding the consent for the erection of lines, but made no reference to technical considerations. Further Electricity Acts followed, but it was not until 1928, when the UK Electricity Commissioners issued their first general code, ELC 53, that there was any great change towards modern-day practice. The British electricity boards´ specifications are developed from the BS1320 design for 11 kV and BEBS L1 for m.v. The latest work on line design is in consequence of the revised Overhead Line Regulation (statutory instrument 1355, 1970), and the industry´s adoption of the metric system. The modern standards will differ from previous ones as all components will have their own standards. The supply industry produces the standards, which, where applicable, are accepted by Department of Trade and Industry as conforming to the Electricity Supply Regulations. Conductors represent some 45% of the total cost of overhead lines and for the sake of economy aluminium has replaced copper as the material used. There are problems in the use of aluminium, but these have been overcome by the development of differing designs of components and the development of fittings and connectors. The development of the bimetallic-compression joint has removed the corrosion problem in the connection of copper to aluminium. The m.v. system has changed to discrete systems around transformers with no interconnection between them, and the modern standard will cater for this, together with new methods of servicing. The advent of the BS1320 specification permitting unearthed systems, and the development of the highspeed autoreclosing circuit breaker have revolutionalised the performance of 11 kV overhead systems. Insulators have varied little over the years, but the manufacture of glass insulators has now ceased in the UK and t- - he production of porcelain ones is limited. The introduction of resin-bonded glassfibre in the construction of insulators opens up a new field in design. While earlier developments improved the operational characteristics of 11 kV lines, the addition of hot-stick maintenance techniques from America and the development of live-line connection methods by area boards in the UK is changing the maintenance philosophies of the boards and the training of the staff. It is obvious that the preservation of timber has been a problem that has exercised the minds of engineers over the years. The prime preservative has been creosote, although water-based preservatives are available and have been tested. Rot occurs in poles due to fungi, and several methods available for combating it are discussed. The rural electrification programme in the UK, as required by the 1947 Act, has been completed in the last five years, and the present construction requirement is only for short extensions and reinforcements. The emphasis in the future will be on the maintenance of lines with the consequential changes in the design of tools and equipment and the training of labour. The length of overhead lines in operation is shown in Table 1.