Abstract:
A railroad track circuit providing for the positive detection of broken rails despite the presence of factors that would otherwise preclude such detection is disclosed. These factors include sneak paths arising from the presence of negative return cross-bonding as applied between parallel tracks in electrified territory. Broken rail detection is ensured through the provision of two track relays, or devices that function as track relays, uniquely arranged so as to render the track circuit immune from these factors.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and claims priority to U.S. Provisional Patent Application No. 61/671,301, filed on Jul. 13, 2012, the disclosure of which is incorporated herein in its entirety by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The invention pertains to a railroad track circuit providing for the positive detection of broken rails despite the presence of factors that would otherwise preclude such detection. Prior art railroad track circuits that monitor for broken rails have been negatively affected by sneak paths that arise from the presence of negative return cross-bonding as applied between parallel tracks in electrified territory. 
       BRIEF SUMMARY OF THE INVENTION 
       [0003]    The railroad track circuit of the present invention provides accurate broken rail detection, which is ensured through the provision of two track relays, or devices that function as track relays. These devices are uniquely arranged so as to render the track circuit immune from sneak paths that interfere with the function of the prior art track circuits. 
         [0004]    The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  shows a prior art arrangement and operation of a track circuit; 
           [0006]      FIG. 2  shows a prior art de-energization of a track circuit due to the presence of a train; 
           [0007]      FIG. 3  shows a prior art de-energization of a track circuit due to the presence of a broken rail; 
           [0008]      FIG. 4  shows the arrangement of impedance bonds and flow of traction return current in a prior art track circuit; 
           [0009]      FIG. 5  shows a typical cross-bonding arrangement as applied in multiple-track territory per the prior art; 
           [0010]      FIG. 6  shows sneak paths caused by cross bonding in multiple track territory per the prior art; 
           [0011]      FIG. 7  shows sneak paths caused by grounding between adjacent power substations per the prior art; 
           [0012]      FIG. 8  shows the normal flow of current through a track relay per the prior art; 
           [0013]      FIG. 9  shows the flow of current through a track relay in a track circuit with a broken rail via a sneak path per the prior art; 
           [0014]      FIG. 10  shows the invention&#39;s arrangement of two track relays providing immunity from sneak paths; 
           [0015]      FIG. 11  shows the normal flow of current through the two track relays of the present invention; and 
           [0016]      FIG. 12  shows the flow of current through one of the two track relays of the present invention in a track circuit with a broken rail. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    The track circuit was invented in the 1870&#39;s. Its function is to detect whether a defined length of track, or “block”, is clear of trains, thereby allowing following trains to proceed safely at high speed.  FIG. 1  illustrates a typical track circuit. A track circuit includes the two running rails  1 . An energy source  2  connected to one end of a block isolated by insulated rail joints  3 . A relay or equivalent device  4  is connected to the end opposite the energy source  2 . Track circuit current  5  flows from the power source  2  through the rails  1  to the track relay  4 , thereby energizing it. 
         [0018]      FIG. 2  illustrates track circuit operation in the presence of a train. The train wheels  6  short-circuit the current  5  away from the relay, thereby de-energizing it.  FIG. 3  illustrates track circuit operation with a broken rail. The break  7  blocks the flow of current  5 , thereby de-energizing the track relay  4 . An energized track relay thereby ensures that the block is both clear of train and that it contains no broken rails. These conditions being met, safe train operations could be ensured. 
         [0019]    The subsequent development of electric propulsion for trains presented complications for track circuits because the rails were now also required to provide a return path for propulsion current back to the substations.  FIG. 4  depicts the provision of impedance bonds  8 , an equalizer bar  9 , and side leads  10  to provide such a path for the propulsion current  11  to return to the substation(s) that generated it. Impedance bonds typically consist of two concentrically but oppositely wound copper coils arranged so as to provide nearly zero impedance to the return propulsion current while presenting an impedance of several ohms from rail to rail. The impedance to the return current is near zero because the opposing orientation of the two windings effect a cancellation of the magnetic fields induced by the equal currents flowing in each. This is referred to as impedance bond “balance.” The power source  3  is adjusted so as to overcome the rail-to-rail impedance to a degree sufficient to energize the track relay. 
         [0020]    Where there are multiple tracks or other complex track arrangements, the return paths must be interconnected via “cross-bonding.”  FIG. 5  illustrates the concept of cross-bonding in which the equalizer bars of two tracks are connected by cross bonds  12 . The cross bonds  12  are, in turn, connected to the substations  13 . The presence of cross bonds gives rise to a significant problem in connection with broken rail detection. This is because when a rail is broken, the combination of the cross bonds and parallel tracks comprise a “sneak path” whereby the track circuit energy effectively flows around the rail break rather than being blocked by it.  FIG. 6  illustrates the sneak path  14 . The current flows through the impedance bonds within the track circuit having the break  15 , then through the impedance bonds of adjacent track circuits  16  and the cross bonds  12 . Because this current flows through the impedance bonds of adjacent track circuits  15  in balanced mode, these impedance bonds present virtually no impedance to this current. Therefore, such a sneak path can exist even if many track circuits intervene between cross bonds  12 .  FIG. 7  illustrates a similar sneak path that may be caused by the ground impedance  17  between adjacent substations  13  which are intentionally grounded. 
         [0021]      FIG. 8  details the flow of track circuit current  5  through the track relay  4  in an unoccupied track circuit in the absence of a broken rail, i.e., the normal condition. In contrast,  FIG. 9  depicts the flow of track circuit current  5  in the presence of a broken rail  7  through the track relay  4  via the sneak path created by the cross bonding  12 , the equalizer bar  9 , the impedance bond  8  and a side lead  10 . In this circumstance, the track relay  4  would be falsely energized despite the presence of the broken rail. This would give rise to a hazardous situation because the broken rail would not be detected. 
         [0022]    The present invention overcomes the limitations of the prior art described above by the unique arrangement of two standard track relays.  FIG. 10  illustrates this arrangement whereby the positive terminal of the first track relay  18  is connected to one running rail  20  while the negative terminal of the first track relay  18  is connected to the equalizer bar  9 . Further, the positive terminal of the second track relay  19  is connected to the equalizer bar  9  while the negative terminal of the second track relay  19  is connected to the other running rail  21 . 
         [0023]    Normal operation is illustrated in  FIG. 11  wherein track circuit current  5  flows through both the first track relay  18  and the second track relay  19  in series, thereby energizing both. The circuit interfacing to the signaling or train control system  22  is wired through normally energized, or “front”, contacts  23  and  24  respectively of the first and second track relays  18  and  19 . Such contacts are arranged in series so that the circuit is not completed unless both track relays  18  and  19  are energized. The presence of energy on this circuit at the interface to the signaling or train control system  22  corresponds to conditions where 1) the track circuit is vacant and 2) there is no broken rail within it. 
         [0024]    Operation in the presence of a broken rail is depicted in  FIG. 12 . The rail break  7  is located on the first running rail  20 . Track circuit current  5  flows through the cross bonding  12  and the equalizer bar  9  as it had in  FIG. 9 . Due to the rail break  7  no current can flow along the first running rail  20 . The impedance bond  8  and second track relay  19  now form a parallel circuit between the equalizer bar  9  and the second running rail  21 . Accordingly the track circuit current splits to run through the impedance bond  8  and the second track relay  19 . The reduced current flowing through the second track relay  19  may or may not be sufficient to energize it. However this is of no consequence. It is readily seen that because of the rail break  7  no current will flow through the first track relay  18 . Therefore, even in the event that the second track relay  19  is energized, the first track relay  18  is assured to be de-energized. The front contact  23  of the first track relay  18  is thereby assured to be open, thereby de-energizing the interface circuit to the signaling or train control system  22  and ensuring protection for trains. 
         [0025]    From the foregoing, it will be appreciated that various embodiments of the present disclosure have been described herein for purpose of illustration, and that various modifications can be made without departing from the scope and spirit of the present disclosure. Accordingly, the various embodiments disclosed herein are not intended to be limiting, with the true scope and spirit being indicated by the following claims.