198007 – Haberlin – A.C. Immune D.C. Track Circuits with Particular Reference to DML-Style TR27A

A.C. immune track-circuits manufactured by Delairco M.L. Engineering Pty. Ltd. Use a track feed set (TR27A) designed to comply basically with British Rail Specification BR 867 and are arranged to operate a track relay in accordance with BR Spec. 966 Appendix F2.

Both the relay and the feed set are A.C. immune. A 750 volt A.C. immunity of the relay is achieved magnetically with the aid of a conventional copper slug and magnetic shunt, whilst A.C. immunity of the feed-set is basically accomplished with the use of a series connected choke providing a specified immunity of at least 450 volts 50 Hz. See Fig. 1.

Figure 1 shows the basic arrangement of the track circuit applied to a single-rail return A.C. electrification scheme. It should be particularly noted that a requirement of Spec. BR 867 is that no adjustable resistors or similar continuous1 y adjustable devices are permitted. Only one connection giving two preset outputs is permitted to allow for alternative conditions i.e.
a) 4 ohms to infinity ballast resistance, or
b) 3 ohms to some maximum ballast resistance to be specified by the supplier.

The normal output used would be the former one, the latter only being used when ballast conditions are particularly low and the maximum ballast resistance condition is known. Because the feed- unit does not embody means of adjustment by the user, much more onus i s placed on the designer to ensure that a71 component specifications of the feed-set are optimised to cover all conditions of A.C. Immunity, variations of supply, environment, ballast, lead resistances, relay operating Characteristics, and component tolerances. With all this a minimum train shunt of 0.5 ohm is to be obtained under any combination of conditions. The normal output used would be the former one, the latter only being used when ballast conditions are particularly low and the maximum ballast resistance condition is known. Because the feed- unit does not embody means of adjustment by the user, much more onus i s placed on the designer to ensure that a71 component specifications of the feed-set are optimised to cover all conditions of A.C. Immunity, variations of supply, environment, ballast, lead resistances, relay operating Characteristics, and component tolerances. With all this a minimum train shunt of 0.5 ohm is to be obtained under any combination of conditions.