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

Reduction factor Kc represents the relationship of the emf developed between a cable
conductor and its sheath to the emf which appears between an insulated conductor and earth
in the absence of sheath. This reduction factor depends on character and dimensions of
sheath and armouring and magnetic properties of the metal used for armouring. This improves
as the frequency of the inducing current increases.
For cables located at a little distance from the track Kc is smaller under heavy inductive
conditions like short circuits than under less severe conditions like normal operation.
Screening of cable sheaths can be improved by reducing the dc resistance of sheath and by
increasing the mutual inductance between sheath and wires. Aluminium sheath has resistance
of about 1/7 th of similar sized lead sheath. As against screening factor of 0.8 for lead sheathed
cable for A1 sheathed cable it is 0.16. If the inductance is increased by steel tape armouring.
Screening factor gets reduced to 0.025 and 0.2 respectively.
The presence of metal work electrically connected to earth near the track, or the circuit
subjected to induction exerts a reducing effect, which may be considerable especially when
several cables are buried in same trench as they shield each other to certain extent.
8.6 RAIL CURRENTS
The rails form a conductor with rather uncommon qualities. The resistance is very small and
the leakance very large. The attenuation is so large that the return current is diverted
completely to earth after about few kilometers and with higher frequencies even sooner. If the
distance between feeding and loading points is large enough and if the track is homogenous,
the rail current divides equally in both directions at both points without any preference for the
‘inside’ direction. Part of the current penetrates deep into the earth and some leaves the earth
to find a path in cable sheaths, metal pipes and other similar conductor parallel to the track.
Near the feeding point the whole of the current returns to the earthed end of the traction
transformer winding through the rail/earth mat of the feeding point.
If the return current could be wholly retained in the rails the inducing effect on an adjacent
telephone line would be that from a comparatively narrow loop formed y the overhead wires
and rails and would be relatively small. In practice the load current rapidly leaves the rails for
earth as shown in fig. 8.4 (b) which is applicable to an electrically long section where the rails
continue for some distance on either side of the section.
The rails will however themselves be subjected to an induced voltage from the overhead wire
which will cause a current to flow in the rails virtually in the opposite direction to the contact
wire current as shown in fig. 8.4 (c).
In the centre of a long section the value of this current will be uniform and is equal to the
induced voltage divided by the series impedence of the rails.
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