From Transmission and Distribution Electrical Engineering, Third Edition


Overhead lines are, in essence, air-insulated cables suspended from insulated supports with a power transfer capacity approximately proportional to the square of the line voltage. Overhead lines are cheaper in initial capital cost and are generally more economic than cable feeders. For the transmission of equivalent power at 11 kV a cable feeder would cost some 5 times the cost of a transmission line, at 132 kV 8 times and at 400 kV 23 times. Such comparisons must, however, be treated in more depth since they must take into account rights of way, amenity, clearance problems, planning permissions associated with the unsightly nature of erecting bare conductors in rural and urban areas, and ongoing maintenance requirements.


In order to match both the mechanical and electrical characteristics of the overhead line conductor to the environmental conditions climatic details must first be collected and analysed. The parameters required are as described in Chapter 17, Section 17.2.1.


The maximum, minimum and average ambient temperature influences conductor current rating and sag. For temperate conditions typically 20 C with 55 C temperature rise. For tropical conditions 35 C or 40 C with 40 C or 35 C temperature rise. Maximum conductor operating temperature should not exceed 75 C for bare conductors to prevent annealing of aluminium. Conductor temperatures up to 210 C are possible with GAP conductor (see Section 18.3.2).

Wind velocity

Required for structure and conductor design. Electrical conductor ratings may be based on cross wind speeds of 0.5 m/s...

Products & Services
Coaxial Cables and Triaxial Cables
Coaxial cables have an inner conductor insulated by a dielectric material and then surrounded by an outer conductor that is shielded with braid or foil. Triaxial cables add an extra layer of insulation and a second conductive sheath. They are commonly known as coax and triax, and are chosen because of their protection against external electromagnetic interference.
Mobile Electrification Systems
Mobile electrification systems provide power to moving vehicles and equipment such as AGVs, mobile hoists and cranes in automated factory, material handling and other specialized applications.
Hookup Wires
Hookup wires are used in low current, low voltage (under 1000 V) applications within enclosed electronic equipment. They are also used in control panels, meters, computers, business machines, and appliances.
Pulling and Support Grips
Pulling grips are mechanical devices used for the holding of a wire or cable so that it can be pulled without damage to the wire or cable jacket.

Topics of Interest

19.1 INTRODUCTION This chapter describes the testing and commissioning necessary before power transmission and distribution equipment is ready for service. Consideration is given under the following...

9.1 Introduction This chapter looks briefly at the various types of conductor on the market with particular reference to covered and insulated conductors. Aspects such as current carrying capacity,...

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...

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...

7.1 Scope The next two chapters look at conductors the workhorse of the overhead line. Chapter 8 looks at the electrical choice for conductors and how to calculate what is required. This chapter,...