General Information on Track Circuits - RGS Online

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<strong>General</strong> <strong>Information</strong> on Track Circuits
A disconnection in the series bonding will:
a) Reduce the current being fed into the track circuit.
Railway Group Approved Code of Practice
GK/RC0752
Issue Two
Date December 1998
Page J3 of 5
b) Reduce the voltage normally dropped across the feed impedance.
c) Increase the rail voltage at the feed end.
A short circuit will:
a) Increase the current fed into the track circuit.
b) Increase the voltage dropped across the feed impedance.
c) Reduce the rail voltage at the feed end.
4.2 Locating a Disconnection
The higher rail voltage can be measured at all positions along the track circuit
from the feed end up to the point of disconnection. On the relay side of the
disconnection the rail voltage will be very low. Fault location therefore entails
walking through and checking the rail voltage to identify the position of the step
change in value. The faulty bond etc, can be confirmed by measuring a voltage
across it.
4.3 Locating a Short Circuit
4.3.1 <strong>General</strong>
The extent of the change in feed end voltages and currents will depend upon the
type of track circuit and the physical position of the fault along the length of the
track circuit. Consult the relevant section concerned with the specific type of
track circuit, as appropriate. The general position is as follows:
a) At d.c. and power frequency a.c., the rail impedance is negligible, and the
resulting electrical circuit is constant, wherever the short circuit is located
within the track circuit. Thus, any voltage across the rails, permitted by an
imperfect short circuit, will be constant throughout the length, giving no clue
as to its physical position.
b) With audio frequencies and impulses, the rails have significant impedance,
and the effect of a short circuit will vary depending upon its physical position.
The closer it is to the feed end, the more it will increase the feed current and
decrease the rail voltage.
In either case, it is not possible to precisely locate the short circuit by simple
observation of rail voltage along the track circuit. Other methods available are
as follows:
4.3.2 Visual Inspection and Provocation
This is particularly useful where track circuit equipment is not responsible.
Examples of such a fault include signal wire or point rods touching the rails, and
faulty insulation in point connections etc.
Connect a meter across the rails and observe its reaction when the item of
equipment is provoked (eg. waggle the signal wire or stand on the rods, having
first ensured that they will not be operated). In the case of faulty insulations, a
hammer blow to the metalwork adjacent to the insulation will often produce sharp
changes of rail voltage. Alternatively, the insulation should be carefully
dismantled and reassembled.
R A I L T R A C K J3