From High Voltage Engineering and Testing, 2nd Edition

F.J. Liptrot

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 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'...

Products & Services
Electrical Insulators
Electrical insulators, high voltage bushings and porcelain insulator assemblies provide electrical isolation of power line conductors in transmission and distribution (T & D), substation, transformer, capacitor and power protection applications.
Circuit Breakers
Circuit breakers are mechanical switching devices capable of breaking currents under specified abnormal circuit conditions.
Power Transformers
Power transformers convert power-level voltages from one level or phase configuration to another.  They can include features for electrical isolation, power distribution, and control and instrumentation applications.
AC Power Cords
AC power cords carry alternating current from one area to another. There are many different standard plugs and voltages, depending upon the country.
Electric and Gas Utilities
Electric utilities and gas utilities supply electric power and/or natural gas or connect lines to industrial, commercial, and residential customers.

Topics of Interest

5.2 Towers and Supports 5.2.1 General The tower or supporting structure is required to carry the overhead line conductors and earth conductors, each of which will be subjected to a variety of forces.

5.6 Insulator Design 5.6.1 General As indicated in Section 5.1, it is usual to employ 'pin-type' insulators for distribution voltages up to 30 or 40 kV. Above this voltage and up to about 132 kV...

6.1 INTRODUCTION This chapter describes the different types of overhead line and substation insulators, their design characteristics and their application. Conductors are attached to their support by...

Environmental considerations have increased the use of aluminum, concrete, fiberglass, and steel to support electric transmission and distribution circuits. Wood-pole structures have generally proved...

Overhead conductors are supported on the distribution pole using insulators, crossarms, or conductor supports. The National Electrical Safety Code (NESC) establishes the criteria to be followed in the...