Patent GB-190620542-A (GB)
Title: Improvements in and relating to Signal Systems for Railways and the like.
Filed: 1906-09-15
Granted: 1907-08-08
Abstract: 20,542. British Thomson-Houston Co., [General Electric Co.]. Sept. 15. Automatic systems; electric railways, signalling on.- In an automatic electric-railway signalling system, both track rails are electrically continuous and serve as returns for both power and signalling currents, which are of different character, and the signalling-current in each block differs from that in adjacent blocks. Fig. 2 shows the preferred arrangement of circuits. The power circuit may be continuous or low-frequency alternating, and the signalling-current is higherfrequency alternating of two frequencies supplied from generators D<1> D' and line - wires d, d<1>, d', through transformers E<1> E<2> E<3> E' to the track. These, transformers have an air gap to prevent saturation of the iron by excessive currents due to power currents and defective bonding, and a resistance e is inserted in the secondary circuit to decrease the power current. These arrangements also prevent damage by excess current due to short-circuiting the transformers by train wheels. The relays employed are of any type which respond only to one frequency of alternation, and in the example are of the moving and fixed coil type, the moving coil being energized from the track rails through an ohmic resistance f to prevent damage by power current due to defective bonding. The coils of two relays are connected in parallel at a point midway between the transformers, one relay responding to the current from the transformer to one side and one to the transformer at the other side. The local-circuit contacts of the two relays are connected in series, to the signal-operating mechanism. A train H, at the position shown and moving in the direction of the arrow, short-circuits the relay F<6> energized from the transformer E<4> and allows the signal G<3> to go to "danger." When the train has passed the point where the relay is connected to the track, the relay F<5> energized from the transformer E<3> is deenergized and so keeps open the circuit of the signal G<3> until the train passes the point where that transformer is connected to the rails. The high reactance of the steel rails and the proper proportioning of the signal devices are sufficient to prevent interference with one relay by current from other transformers of the same frequency, and also to prevent actuation of relays by trains at more than a short distance from the end of a block, that is to say the relay F<4> is not de-energized by the train H until it has reached a point quite close to the point where the transformer E<3> is connected to the rails, and the relay F<5> is energized when the train has passed only a short distance past that point. This overlap of the sections is practically negligible. Another arrangement is also described, in which single relays are connected to the track rails at the same point as the transformers feeding the adjacent relay. Adjacent transformers supplying current of the same frequency are connected to the rails in opposite. phase so as to prevent interference, as they tend to open the local circuit at other relays than the. one they are intended to energize to close its circuit.