Patent GB-458356-A (GB)
Title: Improvements in or relating to railway track circuits
IPC: B61L1/18
Filed: 1935-05-17
Abstract: 458,356. Railway signalling systems. GENERAL RAILWAY SIGNAL CO., Rochester, New York, U.S.A.-(Assignees of Powell, W. T. ; 413, Browncroft Boulevard, Rochester, New York, U.S.A.) May 17, 1935, No. 14552. Convention date, July 28, 1934. [Class 105] To compensate ballast variations a source of current separate from the track circuit provides power to operate under control of the track conductance, a variable feed resistance in the track circuit, but such power current is cut off when an abrupt change in track conductance occurs as on entry of a train. The control is effected by the unbalancing of a Wheatstone bridge one branch of which has the track circuit and the variable resistance a as its two arms while the other branch has as its arms an impedance b and a resistance c. The detecting means comprises a sensitive polar relay P and a relay P<1> which only responds to a substantial current. A gradual change in the track conditions unbalances the arrangement so causing a current to flow one way or the other through the detecting relays P, P<1>. If the ballast resistance decreases, the unbalancing of the bridge is such that a current flows through the detecting relay P to set its contact 12 to the right whereby operating current is applied to the right-hand winding of the motor M which through reduction gearing moves the arm of the resistance a counter-clockwise to reduce the resistance in circuit and thus the bridge is automatically re-balanced thereby again applying a normal voltage to the track relay T. When a train enters there is a sudden and considerable unbalancing of the bridge and sufficient current flows through the detecting relays P, P' to cause the relay P<1> to lift ; the opening of the back contact 11 cuts off the operating current to the motor M and thus no automatic compensation arises. The motor circuit is broken, when the end limits are reached, by means of a cam contact-breaker CM. Specification 428,751 is referred to. The Specification as open to inspection under Sect. 91 comprises also an arrangement, Fig. 3 (Cancelled), in which compensation for ballast changes is effected by providing on the track relay, a second winding through which the full feed current flows. With increasing leakage the reduced current in the main winding of the track relay T is compensated by the increased current in the second winding. To prevent such compensation when a train enters there is provided in the feed line 15, a Wheatstone bridge arrangement of non-inductive resistances a, b, and inductive impedances c, d. The action is that on the entry of a train the suddenly increased current component flows at first only through the non-inductive arm a, the polarized detector relay P, and the non-inductive arm b, to the feed line 16. The polar relay P sets its contacts to the left whereby a relay AX is picked up and thereafter maintained energized and closes its contact 23 to shunt out the compensating winding of the track relay T. On the departure of the train the relay P is energized in the opposite sense and its contacts being set to the right energize a relay G which at its contact 22 breaks the holding circuit of the relay AX, and thus the shunt 23 is removed from the compensating winding of the track relay. This subject-matter does not appear in the Specification as accepted.