Page 183 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Fig 2 shows this phasor addition for a situation where the loads are balanced and the currents are equal. The
result is that the current in the neutral line I is zero.
N
Therefore, for a balanced load the neutral conductor carries no current.
Earthing of neutral conductor: Supply of electrical energy to commercial and domestic consumers is an
important application of three-phase electricity. For `low voltage distribution’ - in the simplest case, i.e. supply of
light and power to buildings - there are two requirements.

1 It is desirable to use conductors operating at the highest possible voltage but with low current in order to save
on expensive conductor material.
2 For safety reasons, the voltage between the conductor and earth must not exceed 250V.

A voltage distribution system according to criterion 2, only possible with a low line voltage below 250 V. However,
this is contrary to criterion 1. On the other hand, with a star connection, a line voltage of 415V is available. In this
case, there is only 240V between the supply line and the neutral conductor. Criterion 1 is satisfied and, to comply
with 2, the neutral conductor is earthed.
Indian Electricity Rules: I.E.Rules insist that the neutral conductor must be earthed by two separate and
distinct connections to earth. Rule No.61(1)(a), Rule No.67(1)(a) and Rule No.32 insist on the identification of
neutral at the point of commencement of supply at the consumer’s premises, and also prevent the use of cut outs
or links in the neutral conductor. BIS stipulate the method of earthing the neutral. (Code No.17.4 of IS 3043-1966)
Cross-sectional area of neutral conductor: The neutral conductor in a 3-phase, 4-wire system should have a
smaller cross-section. (half of the cross-section of the supply lines).

Fig 1 Fig 2

Two phase system

Two electrically insulated windings are placed 90 degree electrical apart in a uniform magnetic field.
If the windings are rotated with constant angular velocity, the EMF generated will have a phase difference of 90
degree.

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CITS : Power - Electrician & Wireman - Lesson 26-29