5.1 Introduction

The purpose of this chapter is to provide an overview of the design, manufacture, construction, testing and maintenance of the various components which go to make up overhead transmission (visit http://www.cigre.sc22.org) and distribution lines. The transmission of electrical energy is carried out over long distances at voltages of 66 kV and above, whereas distribution is carried out over short distances at voltages of between 11 and 66 kV using the same technology.

In the early days of electricity transmission, the methods already established by telegraph authorities, using telephone wires fixed to wood pole supports, were adopted by the electricity supply authorities. This involved the fixing of bare, uninsulated conductors to small porcelain insulators (Figure 5.1 a) screwed on to metal spindles which were mounted on to metallic or wooden crossarms supported by wooden poles; see Figure 5.1 b.

Figure 5.1: (a) Section drawing of toughened glass (left) and porcelain pin-type insulators (b) Overhead line construction (11 kV)

As transmission/distribution distances increased, it became necessary to increase the system voltages. Consequently, the pin-type insulators were increased in size to provide longer creepage paths to earth; conductor sizes were also increased to allow higher current carrying capacities (ampacities). This resulted in increased vertical loads due to the conductor mass and increased transverse wind loads due to the increased conductor projected area. Insulator spindle diameters were of necessity increased to cater for the greater mechanical loads, and similarly single wood poles were replaced by 'A' and 'H'...