In addition to the vital function of train detection, a track circuit is also expected to provide an indication of a broken rail within its boundaries.

Ability to detect a broken rail in a double rail track circuit is a result of the failsafe design principle, which interrupts the flow of track signal current and drops the track relay in the presence of any credible component failure. The overall result of such an event is identical to that of track occupancy, which causes the associated signals to be set to the most restrictive aspect.

In the case of electrified territory, the ability to detect a broken rail may be compromised by the presence of crossbonds between parallel tracks. These additional connections, which are intended to minimize potential differences in the negative return system, also provide a sneak path for track signal current to flow from transmit to receive ends of the track circuit under broken rail conditions. In most cases involving 2 parallel tracks, application of a simple rule of thumb (described in this paper) regarding crossbond spacing and track circuit length is sufficient to ensure broken rail protection. In situations which are marginal, or where complex track work is involved, a more detailed analysis is necessary. Evaluation of the broken rail protection in such cases requires that the sneak path impedance be calculated and compared to the normal track circuit impedance. A formal method for this impedance calculation, based upon the classical circuit analysis techniques, is presented.

To completely characterize the behavior of the track circuit under broken rail conditions, the operational characteristics of the track circuit must also be considered. In this paper a digital jointless AF track circuit, operating in the 9.5–20.7 kHz range, is used as a basis for this discussion. The operational characteristics of the S-bond, O-bond, as well as the role of overdrive are considered in the context of broken rail (BR) protection.

The mathematical methods described for the simplified examples in this paper can readily be expanded to include any number of parallel tracks.

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