NR NR/L3/SIG/11303/2P30 ISSUE 2 PDF

Introduction

The end of a jointless track circuit is created by connecting components to the rails to form a tuned circuit. Insulated rail joints are therefore not required in plain track. Continuously welded rail can therefore be used.

The track circuits use alternating current (a.c.). Frequencies are generally in the middle to upper audio band. Each manufacturer's make of track circuit uses a set of carefully chosen frequencies. One track circuit is separated from the next because each is allocated a frequency different from its immediate neighbours. This reduces the possibility of the current from one track circuit falsely energising another.

The a.c. on the rails has to be generated and detected. The units that do this are usually referred to as the "Transmitter" (Tx) and "Receiver" (Rx) respectively. The Tx is at the feed end of the track circuit, and the Rx controls the TPR, usually through an intermediate TR.

Important points to remember about these track circuits are:

• All components (and the connections between them) are carefully chosen to suit the frequency of the track circuit.

• Do not alter:

   • any piece of equipment from the designed types and values.

   • the way in which equipment is connected.

   • the length or type of cable connecting equipment.

• This particularly applies to equipment on or near the track.

• A.C. can leak across into nearby circuits which have inductance or capacitance between them but no direct connection. To reduce this leakage, wires may be screened or in twisted pair. Certain components are earthed. Items of equipment may also be positioned apart. Carefully follow any special requirements shown on the installation drawings. If for any reason designed layouts cannot be followed, seek advice before making any change.

• Unlike insulated rail joints, the division between two adjacent jointless track circuits is not at a clear cut position. One track circuit overlaps the next in a "shunt zone". As a train passes through this zone, the shunt effect on one track circuit diminishes and on the other, it increases. The position of the rail connections in relation to signals, points, fouling points, etc., is therefore very important. Do not make changes to the designed positions. If there are problems in doing what is asked for, consult your supervisor.

• Much of the equipment contains electronic components. Handle and store the units carefully. At installation, make sure that specified lightning and other interference arresters are fitted and earthed as required by the installation diagrams.

When carrying out initial testing and setting up, follow precisely the procedures laid down for the particular type of equipment.

Remember that the equipment connected to the rails creates the "electrical block" at the end of the track circuit. Removing rail connected tuning units or bonds will remove the "block". This action might cause a working transmitter on one side of the disconnection to falsely energise a track circuit of the same frequency on the other side. So beware when making disconnections.

Certain types of track circuit, (jointless, jointed or overlay) are incompatible with other types. This means that it is not safe to have certain types butting up together. In some cases it even matters whether it is the feed or relay end of the other track circuit which is adjacent. So do not make any alteration to designed track circuit arrangements, even under temporary or emergency conditions, without first seeking the advice of your supervisor.