Page 127 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Voltage operated ELCB
This device is used for making and breaking a circuit. It automatically trips or breaks the circuit when the potential
difference between the protected metal work of the nstallation and the general mass of earth exceeds 24V. This
voltage signal will cause the relay to operate (Fig 1).
Voltage operated ELCBs are meant to be used where it is not practicable to meet the requirements of IEE wiring
regulation by direct earthing or where additional protection is desirable.

Current operated ELCB: This device is used for making and breaking a circuit and for breaking a circuit
automatically when the vector sum of current in all conductors differs from zero by a predetermined amount.
Current operated ELCBs are much more reliable in operation, easier to install and maintain.
Construction of current operated ELCB: It consists of a Torroid ring made of high permeability magnetic
material. It has two primary windings each carrying the current flowing through phase and neutral of the
installation. The secondary winding is connected to a highly sensitive electro - magnetic trip relay which operates
the trip mechanism.

Working principle
The residual current device (RCD) is a circuit breaker which continuously compares the current in the phase with
that in the neutral. The difference between the two is called as the residual current which is flowing to earth.
The purpose of the residual current device is to monitor the residual current and to switch off the circuit if it rises
from a preset level (Fig 2&3).

Fig 1 Fig 2

Fig 3

The main contacts are closed against the pressure of a spring which, provides the energy to open them when the
device trips. Phase and neutral current pass through identical coils wound in opposing direction on a magnetic
circuit, so that each coil will provide equal but opposing numbers of ampere turns when there is no residual
current. The opposing ampere turns will cancel and no magnetic flux will be set up in the magnetic circuit.
In a healthy circuit the sum of the current in phases is equal to the current in the neutral and vector sum of all
the current is equal to zero. If there is any insulation fault in the circuit then leakage current flows to earth. This
residual current passes to the circuit through the phase coil but returns through the earth path and avoids the
neutral coil, which will therefore carry less current.

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CITS : Power - Electrician & Wireman - Lesson 20-25