remote control equipment - Indian Railways Institute of Electrical ...

e = Er / R were introduced in the voltage produced on r without the circuit II. This means that
reaction produced on I by II is negligible.
Fig. 8.1
Conductive coupling occurs often between two circuits, using to some extent, the earth
as conductor. Very weak couplings of this kind exists, obviously between all circuits because
of imperfect insulation. In practice, conductive coupling exists when electrified railways use
rails as a return conductor. In telephony, coupling of this kind may arise in all circuits in which
earth is used as an auxiliary or third conductor. Especially all circuits using common batteries.
In all such cases disturbances may occur, if the earth connection of a telecommunication
circuit is near enough to an earthing point on the power system.
In practice interference by conductive coupling between lines can be neglected.
Conductive coupling is present if the interference can be suppressed by resting the earthing
connections or by replacing the earth return by a metallic return conductor well away from the
existing one. Electric or magnetic coupling is present if the interference can be suppressed by
a displacement of the line well away from the inducing line without any alteration of the
earthing connections.
8.4 ELECTROSTATIC INDUCTION :
Electric induction occurs due to capacitive coupling. Discussion hereunder is confined to the
systems of lines which are long in relation to all dimensions perpendicular to the length
(distance, diameter), the lines are assumed to be parallel with the surface of the earth and
with each other.
Consider line I at voltage U1 with the frequency f Line4 is insulated (see fig. 8.2). The mutual
capacitance K14 and the earth capacitance K40 are in the series. The voltage U 1 produces
a charging current.
U1jw - ___K 14 K 40 __ = U1 JWK14 as K 40 >>K 14
K 14 + K 40
Voltage on line 4 is U4 = U1 K14 K + K 14 40
= U1 K 14 /K 40
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