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September, 1933

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In the course of a recent visit to Newcastle we have been shown many items of interest to colliery people; some of these form the subject of the following article and others will be dealt with subsequently. The firm of A. Reyrolle & Co. Ltd. has a reputation for sound work incorporating pioneer ideas and its name needs no introduction in colliery electrical circles. The total metal enclosure of both large and small switchgear has always been a proposition associated with Reyrolles, who offer excellent designs ranging from small air-break distribution gear to the highest-voltage switchgear with proved rupturing capacity. In the mining field they produce flameproof switchgear of various kinds, a special low-voltage face-lighting system, gate-end control gear and flameproof plugs and sockets, while the associated firm of J. H. Holmes & Co. Ltd. who incidentally, celebrate this year the jubilee of their establishment as a private firm-build flameproof induction motors.

Developments in gate-end switchgear have resulted in designs which now provide all-in protection for a.c. motors driving coalcutters, conveyors, loaders, drills and so on, as well as for their cables and pilot-wires and for their operators. Each unit of the air-break type incorporates busbars, an isolating switch for isolating the main switch, and automatic protective equipment. The external casing is made of welded steel and is built so as to form two separate chambers, of which the upper contains the busbars and the isolating switch, and the lower the circuit-breaker and the automatic protective equipment. Over-current protection, using magnetic trips with time-limit dashpots, is supplied as the standard type, and provision is also made for including, when required, a core-balance transformer and relay for earth-leakage protection, and a special motor-starting dashpot. The main switch is relay-operated, and can be arranged for push-button or remote control. The isolating-switch is direct-operated by a handle fitted at the front of the unit. Removable covers, one at the top of the busbar chamber, and another at the front of the lower chamber, give access to all the parts, but shrouded bolt-heads and positive interlocks prevent their removal unless the right conditions exist. The busbars are connected at one end to a flit-plug, which is bolted to the upper chamber and takes the incoming supply cable. When two or more units are mounted together on a common skid, the separate busbar chambers can be bolted together to form one continuous chamber. The outgoing supply is led away through a British Standard plug.

A scheme has been evolved for the control of such a gate-end switch from the coalcutter that it serves, and the diagram shows this arrangement. An auxiliary transformer, with earth shield, is provided in the gate-end box. Its secondary circuit is completed only when the trailing cable plug couples together two parts of a split control pin, and the coalcutter is definitely earthed through one pilot, which is not shown in the detail diagram. As soon as the plug is fitted the coil or relay B is energised in series with one leg of coil A. The armatures of both coils lift, but the closing coil of the main gate-end switch is not energised until the coalcutter control is moved, so that the other leg of relay A is fed in opposition to the one already energised. By means of these arrangements and the mechanical interlock between the armatures of A and B it is thus ensured that the gate-end switch automatically opens in the event of a break in the pilot or earth circuit; it also trips if there is a short-circuit between pilots or even a contact between them possessing resistance. If the supply to the coalcutter should fail owing to a failure of the main supply or owing to the removal of the trailing cable at the gate-end switch, the switch opens. It does not reclose when the main voltage reappears or the trailing cable is reconnected, until the operator opens the coalcutter controller and then recloses it.

The face-lighting system comprises a self-contained air-break switch-and-transformer unit for controlling and stepping-down the supply voltage to 25 volts, together with the lamp-fittings, which are made up in groups of three and connected by flexible cable with about 18 ft. between adjacent fittings. The three lamps of a group are connected each across one phase, and so the load on the supply is always balanced. The separate phases of the supply are controlled by separate single-phase contactor-switches mounted in the switch-and-transformer unit. The contactors are fitted with thermal over-current trips so arranged that, when a fault occurs on any one phase, only that phase is disconnected. Flameproof plug-and-socket connectors are used to couple groups of lighting fittings together and to the switch-and-transformer unit. Safety-interlocks provide for such contingencies as faults in cables, broken lamp-glasses, and unauthorised tampering with the fittings. The plug-and-socket connectors can themselves be used to break the circuit, because they incorporate a two stage delayed-withdrawal feature; this necessitates two separate and distinct movements which prevent the plug from being disconnected abruptly. and thus any sparking that may occur is confined within the flameproof enclosure. It is unnecessary to stress the advantages of better face-lighting, and this system appears to posess the virtues of an easy extension and reerection to follow the face.

Easy transportation is a cardinal feature of one type of Holmes flame proof induction motors, the casing being of plain cylindrical form, without any protuberances, so that the motor as a whole can be rolled along inbye. A separate cradle is provided to allow quick and stable fixing in situ, and the electrical connections are made through a flameproof plug-and-socket on one end-shield. The rotor is of the indestructible type and the bearings are dust-proof. Mica is used for the main insulation and the copper is liberally proportioned, so that it is claimed that this type of motor is peculiarly suitable for the arduous conditions of underground working.

Other interests of the Reyrolle firm include the Bushing Company (jointly with the General Electric Co. Ltd.), which manufactures bakelised paper and other insulating materials. The Metalfilter is a form of filtration apparatus which has already been described in our columns, and the domestic side, including water heaters, radiators and the Eureka vacuum cleaner, is not of direct concern to this journal.

Drawings and Photographs accompanying the article