General Information on Track Circuits - RGS Online

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<strong>General</strong> <strong>Information</strong> on Track Circuits
Railway Group Approved Code of Practice
GK/RC0752
Issue Two
Date December 1998
Page E27 of 52
15.4 Impedance Bond Interchangeability
When an impedance bond is changed for another type, it is usually necessary to
change some or all of the associated connections. Figure E25 is included as a
guide to the items affected. The preferred replacement is shown shaded.
As well as physical interchangeability, the replacement bond will need to be
sufficiently rated for the peak and continuous traction currents that occur on that
line.
Imbalance in traction return currents will tend to apply a proportion of any
interference content to track circuit receivers, which under particular conditions
could lead to a wrong side failure. One mechanism whereby imbalance is
detected, is due to saturation of the impedance bonds. As the difference
between the traction currents flowing through the impedance bond’s half
windings increase, the transformer action of the impedance bond becomes less
efficient as it is driven into saturation, thereby suppressing the track circuit
voltage. The operation of the track circuit will become unreliable, but some
protection against wrong side failure of the track circuit is provided.
The level of imbalance current required to saturate impedance bonds is roughly
proportional to their rating, with higher rated type 3 bonds able to withstand
greater imbalance current levels than type 2 bonds for example. This saturation
of impedance bonds leading to right side failure of the track circuit, can provide
useful protection against wrong side failure due to traction interference.
Therefore, care should be applied when replacing one type of bond for another,
as the tolerance to imbalance may be increased, but at the expense of detection
of situations where track circuits become exposed to traction interference which
could lead to wrong side failure.
Additionally, certain bond/relay combinations should be avoided as they may
create problems in correctly setting up 50Hz a.c. double rail track circuits. In
particular, G4 relays should not be used with type 2 or type 3 bonds. If the track
relay is a style CE391, the recommended replacement impedance bond is style
DE, but if this is not available, it will usually be necessary to change the relay and
base to a VT1. It is also recommended that the impedance bond at the other
end of the track circuit is changed in order to get a satisfactory adjustment,
particularly if a style B impedance bond is being displaced.
R A I L T R A C K E 27