Техническое чтение для энергетиков. Бухарова Г.П. - 35 стр.

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to earth the system at one point, this being done conveniently at the generator

common junction as shown in Fig. 8.

Fig. 9. Connections for Three-Phase Four-wire System

Three-Phase Four-Wire System.– This is a system of connections which permits

the employment of a three-phase load and three single-phase loads simultaneously, as

shown in Fig. 9. It re-introduces the fourth conductor to act as a common return for

the three singlephase loads, this conductor being called the neutral. The other three

conductors are called the line conductors. The system is earthed as before, by

connecting the neutral conductor to an earthed plate or other earthing connection.

Colour Scheme.– For purposes of standardization it is now the general practice to

mark each particular phase by a distinctive colour. The three phases are coloured red,

white (or yellow), and blue, respectively, while the neutral conductor is coloured

green.

ELECTRICAL MEASUREMENTS

The measurement of any physical quantity applies a determination of its magnitude in

terms of some appropriate unit. It follows, therefore, that before we can measure we

must decide upon a system of units which will be convenient for the purpose. In the

case of simple fundamental quantities such as length, mass, or time, the units them-

selves are simple.

Electrical and magnetic quantities are, however, much less simple than length, mass,

or time, and cannot be measured directly by comparison with a material standard. The

units in which we express these quantities have to be defined in terms of their

observable, effects obtained in experimental work, e. g. the weight of silver deposited

in one second by a current when it is passed through a solution of silver nitrate is a

measure of the magnitude of this current.

Electrical measurements can be classified broadly as either absolute measurements,

or secondary measurements, but we need concern ourselves very little with the first