150,766. Ateliers de Construction Oerlikon, Dawson, P., Fawdry, F. W., Luder, P., and W³thrich, G. May 3, 1919. Protective arrangements.-Faults in electric transmission or distributing systems are isolated by means of overload circuit breakers so arranged and connected that the fluctuations of the load current are prevented from operating them as long as the pressure in the system exceeds the minimum working pressure at which working current can be obtained from the system, but when, on fault, the main resistance-shunted circuit-breaker opens (whereby the pressure of the system is reduced), the circuit-breakers are free to operate under the influence of the limited fault current. In the arrangement shown in Fig. 1 for selectively isolating any of four parallel feeders f<1> to f' supplying power in an electric railway system and connected to auxiliary bus-bars h, h<1> one of which is connected to the supply bus-bar c, the feeders are protected by overload circuit-breakers g<1> to g<4>, the trip coils of which are energized by current transformers in the feeders through inverse time-element overload relays p<1> to p<4>. The trip coils are, with normal voltage on the feeders, short-circuited or open-circuited by a ' potential block relay ' q actuated through a solenoid by a potential transformer connected to the auxiliary bush-bar and having a time-lag to the ' off ' position. The interlock between the voltage of the feeders and the circuit-breakers may, however, be obtained by known electro-mechanical means. The feeders are supplied through a main overload automatic circuit-breaker k which is shunted by a resistance or a reactance m and through an auxiliary overload circuit-breaker l which may be mechanically interlocked, or as shown, electrically interlocked by the contacts n on the moving arm of the main circuit-breaker, so that the auxiliary circuit-breaker opens on overload after the main circuit-breaker. The heavy current arising from a leakage from a trolly wire or feeder causes the main circuit-breaker k to open (introducing the resistance &c. m into the supply circuit) and also causes the subsequent opening of the auxiliary switch l. These switches open before the interlock of the delayed action potential block relay on the relays is removed. To isolate the faulty line, the switch l is closed and retained closed by short-circuiting the trip circuit temporarily at the switch s. The fault current is thus limited to a non-dangerous degree and the voltage at the bus-bars h, h<1> reduced so that the interlock on the relays is removed by the fall of the core of the potential block relay. Consequently, the inverse time-element overload relay in the faulty feeder operates selectively to cut out the faulty feeder. When the isolation is completed, as indicated by an ammeter A, the main circuit-breaker k is closed and the resistance m cut out. In order that an abnormally high consumption of current by a train, unaccompanied by leakage, shall not, after the circuit-breaker k has opened and the resistance &c. m been inserted, result in the isolation of the feeders unnecessarily, use is made of the usual low-voltage devices on the trains to disconnect the motors when the voltage falls below a certain minimum, and the voltage setting of the potential block relay is lower than that of these low-voltage devices. Although the circuit-breaker k opens with overload unaccompanied by leakage the auxiliary switch l does not so open, as the amount of current necessary is such as, by reason of the resistance m, causes the voltage to drop to such an extent that the low voltage devices on the train open first. The fact that the switch l remains open indicates that an overload unaccompanied by fault exists on the system. Instead of reducing the leakage current by a resistance or reactance m, the circuit-breaker k may switch the supply on to an auxiliary source of lower voltage or on to auxiliary bus-bars supplied from the main bus-bars through a reactance. The arrangement described for one trolly section may be extended to any number of trolly sections, and intermediate switch cabins may be located between the supply point and the remotest feeding end of the trolly wires for the purpose of duplicating the number of sections. To complete the protection with this arrangement, reverse-current devices may be employed on the supply side of inter. mediate cabins. Wattmeter-type relays may be employed to control the circuit-breakers and, in this case, the potential coil of the wattmeter relay is switched on at low pressure and off at high pressure by an auxiliary contact on the potential block relay.