May 10, 1918. THE COLLIERY GUARDIAN 943 should be checked as regards its position. Otherwise the corresponding exciter voltage for a given field current will not be as usual, if tbe rheostat has been altered, and it might give the impression that the readings were unusual and that there was trouble in the generator field. Shunt Rheostat.—This rheostat regulates the voltage of the exciter, by introducing a variable resistance in the exciter field. The voltage of the main generator depends upon the position of this resistance. In connection with colliery work the load is usually a very fluctuating one, combined with a poor power factor, the result being that the generator field has to be varied between wide limits in order to maintain a constant pressure at the bus bars. This is practically an impos- sibility with a hand-regulated rheostat, as the variations occur so rapidly. Automatic Voltage Regulator.—In order to get over the difficulty of voltage regulation it is customary to instal an automatic regulator. This may take a number of forms, but consists of an automatically operated shunt, or exciter field rheostat. The working of such apparatus necessitates a separate potential transformer, which supplies the motive power to the regulator, and the pressure of which also regulates the exciter voltage and therefore the main field exciting current. Emergency Connections. While dealing with the generator panel, it is necessary to draw attention to the necessity of being able to supply an important circuit, such as the ventilating fan or some other plant, direct fr< m one of the generators, while the bus bars of the switchboard are “dead” for cleaning or repairs. A suitable connection can be made to enable the generator to feed a circuit direct, by installing a change- over switch, which enables current to be supplied from a point between the generator oil switch and its isolating switch. The change-over switch, whilst having only one operating handle, really consists of two switches, so arranged that when one opens the other closes, though during the operation the circuit is not broken, i.e., the one switch makes contact before the other is quite open. The change is thus made without an interruption in the circuit that is being changed over. Generators. High Tension Distribution. Transformers. H.T. 3,00'0-Volt Bus. Bars. 600-Volt Bus Bars. Low Tension Distribution. Transformers. L T. Fig. 10. 'Oil Su^/tches. A 0cl SujitchV) Figs. 8 and 9 show diagrams of connection of such an arrangement. Fig. 8 represents the fan and generator running normally through the bus bars, whilst fig. 9 shows the fan being fed from the generator while the bus bars are “ dead.” The diagrams are shown in single lines to simplify the connections. Such an arrangement should be worked under proper instruc- tions placed on the switchboard, together with a diagram of connections. Assuming the plant is running normally as in fig. 8 and that it is desired to isolate the bus bars for repairs, the sequence of operations is as follows:—The arrange- ment would be on one generator panel only, so that machine would have to be running alone before isolating the bus bars. All other switches would be open and the turbine running on atmosphere in cases where the auxiliaries were electrically driven. The change-over switch is first turned over, which invariably removes the supply from the fan switches. The fan switches may then be opened, and finally the generator isolator, which leaves the bus bars “ dead.” In order to change back to the bus bars, the generator isolator is closed, then the fan isolator and oil switch, and finally the change-over switch is replaced in its normal position and the fan is once more connected to the bus bars in the usual way. It will be noticed that when the generator isolator is open, the instrument potential transformer is not in circuit, so that the generator voltmeter and ammeter alone must be relied upon for taking readings. The arrangement just described is useful if the fan switch should become hot, as it can be isolated by the change-over switch, the bus bars being still kept alive. Feeder Panels. All feeder panels should have an ammeter and an indicating wattmeter. These, together with the bus bar voltmeter reading, enable the power factor of the particular feeder to be calculated. Three trip coils, with inverse time-limit relays, should be provided to take care of heavy overloads without opening the circuit breaker. Transformer Panels. High Tension.—These should be equipped similar to feeder panels. Low Tension.—An. ammeter and voltmeter will usually suffice for these panels, with the usual tripping gear. The power taken by the transformer can always 'Be checked on the high-tension side. Operating Diagram of Connections. The ordinary diagram of connections found on some boards are often very complicated, and should not be used as operating diagrams. The actual operations are expressed in terms of switch handles rather than con- nections, so that a single-line diagram, as shown in fig. 10, with all switches neatly numbered and labelled, will secure safety in operation, and can be taken in at a glance. Limitation of Switching. Switching operations should always be reduced to the minimum. Unnecessary switching only tends to increase maintenance costs, and incidentally increases the chances of switch failure. Power switch gear is always designed for a quick break, and on high-tension gear the break is long. Consequently, when a switch is tripped, the shock is severe. There is always a possibility of insulators frac- turing or castings giving way, and therefore the tripping of switches should be reduced as much as possible. No switch should, on any account, be tripped while its oil tank is removed or dry, the oil greatly relieving the shock to the switch as it opens. Maintenance of Switchgear. Generator Switches.—These ought not to get out of repair through burning of the contacts, so long as synchronising is properly carried out. * The load should be removed before a generator switch is opened. A. bad synchronise is sufficient to wreck a switch completely, especially if the running generator is carrying a heavy load .at the time. Distributors.—These should be rarely called upon to open on load, unless they are tripped by overload* or short circuit. Switchgear connected to overhead lines has to work under abnormal conditions, especially in windy weather, when lines are liable to blow together and cause dead short circuits. Such switchgear should be examined at every opportunity. Lightning, too, may cause trouble due to insulation breakdowns, but this is usually taken care of by properly adjusted lightning arresters. Transformer Switchgear.—When a number of trans- formers run in parallel it is necessary to have just as many transformers in circuit as will enable them to be working at or near full load, this being the most efficient method of working. To preserve these conditions it may be necessary to switch units in and out fairly frequently, and consequently the switches may require a little more attention than others. It must be remem- bered that load cannot be shifted from one transformer to another as in the case of generators ; and trans- formers if properly designed to run in parallel will share the load between them, whether large or small. Thus, if two transformers are running in parallel and the. load is reduced sufficiently to provide one of the units with full load, then in order to remove one unit, it is necessary for the switch to break a current equal to half-full load of one transformer. General Examinations. Exactly how often switchgear should be examined, cannot be laid down, as it obviously depends on the amount of work in the way of opening under load or short circuit which switches have to withstand. A switch that opens under a bad short circuit, should be opened and thoroughly examined at the earliest oppor- tunity. There is always the possibility of the contacts being burnt so badly as to impair the contact and produce heating when the switch is closed. Any signs of heating in connection with switchgear should receive immediate attention as there is always a possibility of fire, should an explosion of gases occur inside the switch, and the hot oil blown out of the tank and become ignited. Switch terminals and contacts should always receive attention, as after years of satisfactory service they will sometimes mysteriously get hot and cause considerable trouble. Where the atmosphere is humid it is unwise to leave the oil too long in the switch tanks without changing. Exactly how often it should be changed depends on local conditions. If a switch tank gets hot and all the terminals are apparently of the same temperature, moisture in the oil is indicated. It should be immediately changed, which done, the trouble usually disappears. Log. Finally, a proper log book should be kept, giving half-hourly readings of all meters and also units gen- erated. The opening and closing of every switch should be entered in the log, with full particulars as to why such switching was carried out. In this way a constant check of switching operations is obtained enabling efficient supervision to be maintained and the necessary examinations carried out as required. NEW MAXIMUM PRICES FOR FRENCH COALS. The French Minister of Munitions has fixed the following maximum prices for coal at the pits mentioned. The figures given in parentheses immediately preceding the prices relate to the average maximum percentage ash content of the coals. Soc. des Mines de Houille de Blanzy. Brights.—Large^screened (1), above 55 mm. (15 per cent.), 46 fr.; (2) above 55 mm. (20), 39 fr.; large washed beans, 25-55 mm. (18), 45 fr.; small washed beans, 10-25 mm. (16), 39 fr.; washed smalls, 0-22 mm. (17), 35 fr.; 0-10 mm. (16), 33*50 fr.; washed smithy small, 0-22 mm. (15), 38*50 fr.; unwashed small, 0-22 mm. (20), 27 fr. • through and through regraded (20), 32 fr. j through and through (25), 28 fr. Gas Coals.—Washed small, 0-22 mm. (17), 36 fr.; 0-10 mm. (16), 35 fr.; unwashed small, 0-22 mm. (20), 29 fr.; through and through (20), 33 fr. Lean Coals.‘—Cobbles, 50-90 mm. (15), 47 fr.; large screened (1), over 55 mm. (20), 38 fr.; (2), over 55 mm. (28), 31 fr.; large beans, 25-55 mm. (20), 45 fr.; small beans, 10-25 mm. (20), 40 fr.; washed small, 0-10 mm. (16), 32 fr.; unwashed small, 0-22 mm. (30), 25 fr.; through and through regraded (25), 28*50 fr.; through and through (30), 25 fr. Washery Products.—Screened brights (3), over 55 mm. (30-35), 27 fr.; small brights, 10-55 mm. (below 30), 27 fr.; lean small, 10-55 mm. (below 30), 27*50 fr.; small (3), 0-55 mm. (35-40), 21 fr. * washery sludge (25-30), 19*50 fr. Briquettes.—Blocks (12), 56 fr.; lean ovoids (18), 56 fr. Soc. des Houilleres et Chemin de. Fee d’Epinac. Coals.—Cobbles, over 60 mm. (18), 53 fr.; nuts, over 30 mm. (22), 52 fr.; large washed beans (1), 30-60 mm. (18), 54 fr.; (2), 30-60 mm. (30-32), 41 fr.; washed nuts (1), 15-30 mm. (18), 48 fr.; (2), 15-30 mm. (30-32), .36 fr.* washed small, 0-15 mm. (17), 40 fr.; through and through washed and regrade*! (20), 45 fr. ; rough screened (25), 40 fr.; bright, 25-27 per cent, volatiles (25), 35 fr.; lean, 18 per cent, volatiles (28), 43 fr.; Dinay-St.-Leger through and through (32), 33 fr. Washery Products.— Small (30-35), 23 fr.; washery sludge (30-35), 20 fr. Briquettes.—(17) 63 fr. Cie. des Mines de Perrecy. Lean Coals (10-12 per cent, volatiles).—Cobbles, 60-80 mm. (25), 54 fr.; nuts, 30-60 mm. (20), 51 fr.; 20-30 mm. (22), 49 fr.; 18-25 mm. (32), 31 fr.; beans, 5- 18 mm. (32), 28 fr.; small, 0-18 mm. (30), 27 fr.; dust, 0-5 mm. (30), 22 fr.; through and through, 50 per cent, cobbles (30-35), 30 fr. Soc. des Houilleres de Ronchamp. Coals with 20 per cent. Volatiles.—Large, over 50 mm. (18), 50 fr.; washed nuts (1), 25-50 mm. (18), 47 fr.; washed beans (2), 15-25 mm. (18), 47 fr.; washed peas (3), 9-45 mm. (18), 47 fr.; washed small, 0-9 mm. (12), 47 fr.,* slaty coal, 0-50 mm. (35-40), 23 fr. Soc. des Mines du Cros. Smalls, over 25 mm. (25), 47 fr.; rough small (1), (25), 42*50 fr.; (2) (40), 31*50 fr.; washed slack, 6- 25 mm. (18), 45 fr.; washery sludge (35), 18 fr. Soc. des Houilleres de Saint-Etienne. Coals.—Cobbles, over 35 mm. (12), 57*50 fr.; picked cobbles, 35 mm. (without guarantee), 30*50 fr.; smithy nuts, 10-35 mm. (10), 57*50 fr.; washed smithy nuts, 10-35 mm. (10), 53 fr.; washed steam nuts, 10-35 mm. (16), 47*50 fr.; washed smithy small, 0-10 mm. (10), 49 fr.; washed coking small, 0-10 mm. (16), 40*50 fr.; washed small (1), 0-10 mm. (16), 40*50 fr.; washed coking small slack, 0-12 mm. (20), 40*50 fr. ; (1), 0-12 mm. (20), 40*50 fr.; (2), 0-35 mm. (25), 37*50 fr.; screened slack, over 10 mm. (15), 53 fr.; through and through smithy (10), 55 fr.; through and through (1) (18), 47*50 fr.; (2) (25), 43*50 fr. Washery Products.—Slack, 0-35 mm. (30-45), 23*50 fr. ; washery sludge (25), 27 fr. Briquettes.—Blocks (13), 65 fr.; ovoids (22), 61*50 fr. Soc. des Mines de la Loire. Large (10), 57 fr. ; gas nuts, over 45 mm. (10), 55*50 fr.; nuts (1), 45 mm. (10), 54*50 fr.; (2), 35 mm. (25), 40*50 fr.; washed gas nuts, 30-45 mm. (18), 50 fr.; ordinary washed nuts, 30-45 mm. (18), 49 fr.; 12-30 mm. (16), 44 fr. ; washed smithy small, 6-45 mm. (10), 55 fr.; 0-12 mm. (8), 46*50 fr.; washed gas small, 0-12 mm. (15), 43 fr.; washed small (1), 0-12 mm. (15), 40*50 fr. ; (2), 0-12 mm. (28), 25 fr.; washed gas slack, 0-35 mm. (15), 41*50 fr.; washed slack (1), 0-35 mm. (15), 40*50 fr. ; screened slack, over 12 mm. (15), 48 fr.; through and through smithy (10), 52 fr.; through and through gas (15), 43*50 fr.; through and through (1) (15), 42*50 fr. Washery Products.—Washed small (3), 0-35 mm. (30-40), 20 fr.; washery sludge (I), 24*50 fr.; (II), 20*50 fr. Briquettes.—0-35 mm. (15), 61 fr. Soc. des Houilleres de Montrambert et de la Beraudiere. Coal.—Cobbles, over 40 mm. (10), 58 fr.; unwashed nuts (1), over 40 mm. (20), 42*50 fr.; washed nuts (1), 25-50 mm. (15), 52 fr.; (2), 25-50 mm. (22), 28 fr.; smithy* beans, 15-25 mm. (10), 52 fr.; washed slacks, 0-15 mm. (15), 42*50 fr.; fine slack (1), 0-15 mm. (15), 41*50 fr.; (2), 0-15 mm. (22), 34*50 fr. ; coarse slack (1), 0-40 mm. (15), 43 fr.; (2), 0-40 mm. (22), 34*50 fr.;