224 THE COLLIERY GUARDIAN. January 31, 1913. the temperature at 300 kw. remaining well below the limit. The underground switchboard which controls the two pumping sets shows some distinct features, the most important being the arrangement that makes the handling of each double motor-set practically fool-proof. Each pumping set is controlled by one main automatic oil-switch and one extra switch for the stator of the second motor. Although both motors have been built exactly alike, with slip-rings on the rotor, one of them (as a rule the inner one) has these always short-circdited. The arrangement of building both motors exactly alike has been decided upon in order to secure an equal division of the load between the two. The outer motor has its slip-rings connected to an oil-immersed rotor- starter; and the triple-pole oil-switch fuse of No. 2 Fig. 5.—Diagram of Underground Switchboard Connexions at D*ver Colliery. iNOohlKG-FEEDER CONTROL oonnkxi K>*. HIGH AND I TENSION TRANBFORMER FAN-MOTOR CONTROL-PILLAR FEDERATE FROfc MAIN •WHOHIOARO MECHANICAL interlocks. PREVENTS BOTjl PUMPS BEING RUN FROM ONE FEEDER. ,/>REVENT CURRENT BEING MAPS OR BROKEN ON THE AIR-BREAK ISOLATING SWITCHES. PREVENT THE INSPECTION-DOORS OF THE PUMP-CONTROL SECTORS OF THE BOARD* BEING OPENED WHEN ANY METAL IS ALIVE. OR THE SECTIONS BEING MADE ALIVE WHEN THS DOORS ARE OPEN. PREVENT THE SHORT-CIROUITBO ROTOR-MOTORS BEING THROWN ON TO IHE MAINS UNTIL UP TO SPEED, BY LOOKING THE SWITCH-FUSES OPEN UNTIL THE STARTERS ARE IN THE FULL-RUNNING POSITION ENSURES THAT THE OIL-SWITCH FUSE SHALL BE USED AB THE MAIN LIGHTING SWITCH, ANp NOT THE OIL-BREAK CHANGE-OVER SWITCH. • PREVENTS THE INSPECTION-DOORS OF THE LIGHTING SECTION tU> THE BOARO BEING OPENED WHEN ANY METAL IS ALIVE. OR THE SECTION SEIKO MADE ALIVE WHEN THE DOORL REMAIN OPEN. tLEOTRIOAL INTERLOCKS. Contacts a1 ano maos when the oil-swiTches are closed. Contacts b' ano (j» made when the twiTOH-Fuses are open. Contacts c ano q- made whe rotor juarjsrs are in "off- position. Turns YZ on interlock relays holo contacts, olosep. but Turns ZX are required to close the contacts. AMMETER VOLTMETEft Scale, 3 Feet to 1 Inch. Fig. 7.—Plan OIL- IWITOI FUSE OIL- SWITCI 'FUSE OIL- I EWSAN FUSE OIL-BREAK < IHANGE-OVEII SWITCH © © AIR-BREAK CHANGE-OVER ISOLATING LINKS OIL-BREAK CHANGE-OVER SWITCH OIL-BREAK IHANGE-OVEI SWITCH Figs. 6 and 7.—Pump-control Underground Switchboard. motor is interlocked with the rotor-starter in such a manner that it cannot be put in, unless the starter shows “ full on ” position. This gear enables the pumps to be started up in the following manner:—The oil-break automatic switch is first put in, and the stator of No. 1 motor excited. The starting rheostat is then operated until the motor is up to speed, when the switch controlling the stator circuit of the second motor can be put in. Further interlocking devices are provided in the form of electricaliy-operated relays in connection with the trip-gear on the oil-break automatic switch and the starting rheostat. By reference to the diagram of connections (fig. 5), it will be seen that in the event of the current being cut off on the surface, the oil-break automatic switch is tripped, and cannot be put in again unless the switch controlling the stator circuit of No. 2 motor is out and the starting rheostat put to starting position. This same inter- locking system guards against the possibility of the operator, through ignorance, attempting to operate the various switches in improper sequence. As it was intended to add a third cable to the two original cables at an early date, the switchboard (figs. 6 and 7) has been arranged to control three incoming feeders. These are connected to two air-break double- throw isolating switches, in the manner shown on the diagram of connections (fig. 5). It will be seen that No. 1 cable is connected to one side of one of the isolating switches, No. 2 cable to one side of each isolating switch, and No. 3 cable to the opposite side of the second isolating switch. The two switches are interlocked in such a manner that it is not possible for both to be thrown over to No. 2 cable at the same time. This arrangement makes it further impossible to put pressure on any one cable from below—a point which should^always be considered in such installations. The framework of the switchboard has been con- structed in sections to facilitate transport. Sliding doors are arranged along the front, so that the whole interior of the enclosure is accessible. These doors are, however, so interlocked that it is not possible to gain access to the interior of the board unless all current- carrying parts have been isolated from the feeders. For supplying current to the lamps in the lodge-room and pump galleries, a small three-phase oil-cooled transformer is arranged within the switchboard enclosure, and controlled by means of an oil-break switch-fuse. A triple-pole air-break throw-over isolating switch is also provided to enable the transformer to be connected to either of the feeders. It is intended to add another feature to the under- ground switchboard, which has been indicated on the diagram of connections (fig. 5)—namely, an arrangement to keep the pump-motors dry while they are standing still. A transformer will be installed near the under- ground switchboard with a secondary pressure of about 190 volts, calculated to generate a current of 16’5 amperes in each phase, and a temperature-rise above the surrounding atmosphere of about 40 degs. Fahr. The main oil-switch controlling each pumping set will have a change-over arrangement from high to low pressure. The attendant will therefore only have the choice of running each double set of motors, or of keeping it dry, unless for special reasons he disconnects the group from the electrical pressure altogether. Figs. 5 to 7, showing the general arrangement of the switchboard and the diagram connections, will explain all these points. Before this installation was completed, it was decided to increase the capacity of the generating plant to such an extent that both pumps could be worked together. For this purpose a 600 k w. mixed-pressure turbine and a third cable were purchased, the latter as a stand-by for the two others. The turbine was designed to take the exhaust-steam from the two high-speed generating sets, and from a compressor, which is also installed in the power-house. Attached to this turbine is an ejector condenser and a water-cooling plant on the spray system, with a centrifugal circulating pump of a capacity of 1,800 gallons a minute. The pump is coupled direct to a 50-horse power high-tension motor, this arrangement avoiding all complications and losses of transformation. A feature of the low-pressure steam plant is the way in which the oil is extracted and regained from the exhaust steam. The oil is separated from the steam in the usual way, and drops into a closed tank which has a direct connection with the live-steam pipes. As soon as this tank is filled with oily water, a floating lever opens the steam valve, and the oil is pressed into a clearing vessel. The admission of the steam is automatically stopped as soon as the tank is empty. This arrange- ment replaces the oil pump which is usually installed in connection with low - pressure installations, and requires no attention whatever. Special precautions had to be taken to minimise the risk attached to the bringing down of valuable machinery in a pit which was continuously in danger of flooding. It was therefore decided to keep the water out of the pit by means of the cages while fixing the first cable and the first pumping set, and to do without any starting gear until the second pumping set and the other cables had been brought down. In accordance with this programme, the first cable was fixed and jointed in the middle of the shaft. Unfortunately, when the cable was unsealed at the bottom, in order to connect it to the double motor, a test showed that one of the cores had a very low resistance to earth. The fault was located to the joint in the middle of the shaft, and an attempt made to rejoint the cable; but, owing to the dampness of the atmosphere, which was increased through the above-mentioned method of water lifting, this attempt was unsuccessful. In this emergency it was decided to earth that particular core effectively in order to remove any pressure on the weak spot, and dynamos, cable, and motors were run with one leg earthed. The writer thinks that this is the first time that such a step has been taken deliberately in a mining installation, and he would like to remark here that it would not have been possible to take it, if the much-advertised practice had been followed of earthing the neutral point of the system. It might be advisable to consider the described eventuality in similar cases before adopting some of the newly-developed safety devices which require the earthing of the neutral point. The starting was effected by running one of the generating sets, fully excited, up to speed, the pump following in synchronism. The only switchgear in the pumphouse during these initial operations was an oil switch for disconnecting the cable from the motor terminals. This example shows that even in a pumping plant of moderate dimensions a number of special circumstances may arise which require a good deal of technical con- sideration. Amongst the features of the described installation some are probably more or less novel in mining work, especially the variation of the output of the centrifugal pumps, the increase in the load of the alternators by means of air-blades, the fool-proof com- bination of switchgear for the starting of two motors coupled to the same pump, the drying of high-tension motors by means of low-tension current as a regular operation, and the deliberate earthing of one leg of a three-phase high-tension system.