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

It is seen that even if the lines are within 10 metre of contact wire they are subjected to
induced voltages exceeding 1000. This would lead to a continuous discharge across the spark
gaps with which telephone circuits are normally equipped and which have a nominal break
down voltage of about 100V dc.
When such bare conductors situated in the electric field are earthed through a person’s body
the resulting discharge current is proportional to the inducing voltage and the capacitance or
length of parallelism. If the parallelism reaches around 10 km the current could reach
dangerous proportions. Hence, it is not possible to contemplate normal operation of circuits
with bare overhead conductors over any significant length alongside an ac electrified railway.
Electrostatic effects decrease very rapidly when the separation between the inducing line and
the line receiving induced emf is increased. If separation is increased to 40m the voltage in
conductors placed parallel to 25 kV contact wire hardly exceeds 150V rms and the drawback
of continuous discharge across the spark gaps is immediately removed.
The CCITT gives the following formula (which is more conservative than the one cited above)
to arrive at the minimum separation between contact wire and the communication line to limit
the induced voltage to 300 Volts. The minimum spacing is given by
a = 1/3 E
Where E is the contact wire voltage. For 25 kV system this works out to 53 m.
In order to calculate electric induction due to an oblique exposure the distance a is replaced
by the geometrical mean a1a2 between the distances at the ends in the formula.
The calculation takes into account ideal conditions i.e lines parallel to earth’s surface and
mostly to one another, free from additional capacitances and pure sinusoidal alternating
current. In practice these conditions are never fulfilled. Line sags reduces average height and
additional capacitances occur between wires and poles including capacitance of insulators.
Further, roughness of earth’s surface, vegetation, buildings, etc. result in reducing the effective
height of conductors. The combined effect of all these is to increase capacitances to earth by
20% and mutual capacitances between conductors get reduced. Hence, measured values of
electric induced open circuit voltages are usually smaller than the calculated values. However,
high harmonics, even with a small amplitude may increase considerably, the electric induced
short circuit current.
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