Handbook for electrical distribution and transmission fieldworkers

Fieldworkers

It is important electrical workers and fieldworkers to have a good working knowledge of the principles and theory of electricity so that they can work safely on electric lines and associated equipment. This is especially so because electricity cannot be seen and is only evident from the effects that can be experienced, (for example, light, heat and force).

Unfortunately this increases the risk of harm to people working on or using electric lines or associated equipment.

High voltage overhead systems

The two systems most commonly used for transmission and distribution are:

1. Single-phase
2. Three-phase

High voltage single-phase system

This system is generally associated with the distribution of low power levels over relatively short distances. Single-phase systems are generally fed from a three-phase line.

The single-phase line consists of two conductors, neither directly earthed to the general mass of earth. In this system there is no neutral conductor (see Figure 1).

For safety reasons alone, it is important to remember that each phase is alive to earth and that a de nite voltage exists between each phase and the equipment connected to the ground.

High voltage three-phase system

This system is widely used for the transmission of high power levels and is also the standard system used in distribution and reticulation.

It consists of three conductors, each called a “phase”. To standardise the identification of the phases, they are known as A, B and C phases or red, white and blue phases respectively. The voltage in each phase alternates, in a similar manner to the alternating voltage wave form but one follows the other in regular order (see Figure 2).