Introduction. — Murton Colliery (Fig. 12), the property of the South Hetton Coal Company, Limited, is situated on the east side of the Durham coal-field, the shafts having been sunk between the years 1838 and 1842. The seams worked are the Main Coal, Low Main, and Hutton, the output being about 4,000 tons per day.

There are three pits, the West, Middle and Polka, and East and New (Fig. 1, Plate IV.). The West Pit, or upcast, is entirely a ventilating shaft, and the Middle and East Pits are the coal-drawing shafts.

Ventilation was carried out by means of an underground furnace, assisted by the heat from twenty underground boilers, the quantity of air circulating being 500,000 cubic feet per minute.

The Old Steam-plant. — The underground pumping and primary haulage-plant was driven by steam generated from twenty boilers, situated underground, as follows

(a) Polka Pit, Main Coal Seam. — Two externally-fired cylindrical boilers, 40 feet long by 5 feet in diameter, with a steam-pressure of 40 pounds per square inch and two tubular boilers of Cornish type, 28 feet long by 6 feet in diameter, with a steam-pressure of 60 pounds per square inch.

(b) Middle Pit, Low Main Seam. — Six Babcock-and-Wilcox water-tube boilers, 22 feet long by 4 feet in diameter, with a steam-pressure of 110 pounds per square inch two tubular boilers of Lancashire type, 30 feet long by 7.5 feet in diameter, with a steam-pressure of 40 pounds per square inch; and four externally-fired cylindrical boilers, 40 feet long by 5 feet in diameter, with a steam-pressure of 40 pounds per square inch.

(c) East Pit, Hutton Seam. — One tubular boiler of Cornish type, 28 feet long by 6 feet in diameter, with a steam-pressure of 60 pounds per square inch; and three externally-fired cylindrical boilers, 40 feet long by 5 feet in diameter, with a steam-pressure of 50 pounds per square inch.

These boilers were connected together, and supplied steam to the whole of the pumping- and haulage-plant underground.

Haulage-plant. — There were five main haulages underground. particulars of which are given in Table I.

Pumping-plant. — This plant consisted of three Hathorn-Davey differential pumping-engines, with cylinders 32 inches in diameter and 60 inches stroke, and 8-inch double-acting rams, forcing from the Main Coal Seam to the surface, a vertical height of 1,224 feet; and one Evans pump, with cylinders 14 by 8 inches and 24-inch stroke, forcing from the Hutton Seam (bottom level) to the Main Coal Seam standage, a vertical height of 260 feet.

The whole of this plant was fairly efficient, but, owing to its age, the low pressure of many of the steam-boilers, and the long ranges of steam-pipes, it was costly to work and maintain.

Electrification. — After the establishment of the Durham Collieries Power Company, with their generating station at Philadelphia, a distance of 8 miles from Murton Colliery, it was decided to dispense with the whole of the boilers underground, substitute electric power, convert the haulages to electric drive, put down ram pumps driven by motors, and do away with the underground furnace.

The current is three-phase alternating with a periodicity of 40 per second, and a voltage of 5,500, and is supplied through duplicate cables to a sub-station on the surface (Fig. 13). This sub-station is equipped with three switchboards, namely :—

The No. 1 or high-tension switchboard (Fig. 14) receives the current from the cables, and distributes it through the separate panels to (1) the fan-circuit, (2) the four transformers for underground work, and (3) the transformer to the surface screening machinery.

The No. 2 or medium-tension (2,000 volts) switchboard (Fig. 15) receives the current from the low-tension side of the four transformers for underground work, and distributes it through separate panels to the three shaft-cables, connected to a distribution-board underground in the Main Coal Seam.

The No. 3 or low-tension (500-volt) switchboard receives the current from the low-tension side of the fifth transformer, and distributes it to the motors for driving the screening machinery at the surface.

The 5,500- and 2,000-volt switchboards (Figs. 14 and 15) are built of granite-concrete and are of the cellular type, with enclosed bus-bars, each panel being equipped with an oil-break switch, overload relay, time-limit fuse, and isolating switches to separate the switch-gear of any panel from the bus-bars.

From the surface sub-station the whole of the system may be divided into three circuits (1) the high-tension circuit (5,500 volts) for ventilating fans ; (2) the medium-tension circuit (2,000 volts) for underground power for haulages and pumps ; and (3) the low-tension circuit (500 volts) for screening machinery at the surface.

(1) High-tension Circuit for Ventilating Fans. The current is transmitted through two 19/15 double-armoured dialite cables to the switch-panels in the fan-house (Figs. 16, 17, and 18). The switch-panels are of special make for high-tension work, the bus-bars and switch-connections are enclosed in granite-concrete cells, and each panel is arranged to work one or both motors. The fan-installation (Figs. 16, 17, and 18) consists of two duplicate single-inlet Capell fans, each capable of passing 300,000 cubic feet of air, with a 4-inch water-gauge, at a speed of 200 revolutions per minute, and are arranged with a rope drive (Fig. 17) consisting of twelve cotton ropes, each 1.5 inches in diameter. The fans are driven by two 12-pole induction motors (Fig. 18) of the slip-ring type, with a synchronous speed of 400 revolutions per minute, each machine being capable of developing 400 brake-horsepower and sustaining an overload of 25 per cent. for 1 hour, with a temperature rise of 80° Fahr. and a maximum overload of two-and-a-half times the full load. The efficiency and power factor is as follows — Efficiency at half-load, 88.2 per cent.; three-quarter load, 92 per cent.; full load, 92.8 per cent. ; one-and-a-quarter load, 92.4 per cent. Power-factor at half-load, 80 per cent.; three-quarter load, 87 per cent.; full load, 89.5 per cent.; one-and-a-quarter load, 90.2 per cent. Air-gap, 0.07 inch radially.

The weight of each motor, without pulley and slide-rails, is 11 tons 15 hundredweight 2 quarters 24 pounds. The slip-rings are fitted with a short-circuiting device of the clutch type, and the winding of both rotor and stator is of the enclosed slot type and formed coils. To each motor is attached a liquid starter, the electrodes of which are worked by a balanced-pulley arrangement.

(2) Medium-tension Circuit. — The current is distributed to the underground distribution switchboard in the Main Coal Seam (Fig. 19). The power of this circuit is transmitted through three 37/14 double-armoured dialite cables, which are carried in the shaft by wooden cleats resting on wrought-iron clamps attached to steel-wire ropes suspended in the shafts of the Middle and East Pits. The cables feed on to the bus-bars of the switchboard in this distribution house, which has the same equipment as the switchboard at the surface sub-station. The current is then transmitted through separate panels to the motors at each pumping and haulage station.

Electric Underground Haulage Stations. — Six three-phase induction haulage-motors have been installed, representing a total rating of 1,550 brake-horsepower, particulars of which are given in Table II. —

(a) The Hawthorn Haulage (Figs. 2, 3, and 4, Plate V.) in the Hutton Seam is one of the largest underground haulages known. The distance to the farthest lauding is 13,464 feet, the average gradient being 1 in 33.5. and the maximum 1 in 9 against the load. The load consists of 60 tubs, the total weight of the set being 45 tons, and the maximum speed is at the rate of 16.5 miles per hour. In addition to the coals, sets of empty tubs are run for the purpose of conveying the men and boys in- and out-bye at a speed of 6 miles per hour, an auxiliary motor being used operating the same drums.

The motor (Fig. 20) is attached to the old haulage-drums by double-reduction gear, the gear in the first motion being machine-cut cast-steel wheels. The motor is of the protected ventilated type, with a capacity of 400 brake-horsepower, working at a speed of 285 revolutions per minute.

The auxiliary motor is connected to the shaft of the 400 horsepower motor with a clutch, the gear operating the clutch also working a change-over switch, which cuts off the current from the stator of the 400-horsepower motor and switches it on to the auxiliary motor when required. This arrangement entirely prevents the possibility of the current being switched on to one motor whilst the other is in gear. The auxiliary motor has a capacity of 200 brake-horsepower, and works at a speed of 575 revolutions per minute.

The controller is of the liquid type, having a high-tension reversing switch, and three movable electrodes operated with a wheel-gear by the attendant. The cooling is done by water from the shaft-tubbing passing through a coil above the electrodes. The switch-panel is fitted with an overload relay and a wattmeter.

Careful tests have been made with this haulage, and load-curves (Figs. 5 and 6, Plate V.) designed from the observations taken show a maximum load of 610 horsepower.

(b) The Low Main Haulage deals with all coals on the east side of the Low Main Seam. The most important landing is 9,000 feet from the shaft, the average gradient being 1 in 180, and the maximum 1 in 30 against the load. The load consists of 60 tubs, the weight of the set being 45 tons, and the maximum speed is at the rate of 18 miles per hour.

The haulage is a converted one, the motor being attached to the old haulage-drums by double-reduction gear (Fig. 21). The motor is of the induction ventilated type, with slip-rings. The rating is 300 brake-horsepower, with a speed at full load of 390 revolutions per minute. The controller is of the same type and size as that on the Hawthorn haulage.

(c) The Hallfield Haulage in the Middle Pit works the south side, or Shotton Low Main Seam, and runs into a landing 3,900 feet from the shaft, the average gradient being 1 in 234, and the maximum 1 in 185 with the load. The load is 35 tubs, the weight of the set being 26.5 tons, and the maximum speed is at the rate of 10 miles per hour.

The motor is of the same type as the former haulage, having a capacity of 150 brake-horsepower. The speed at full load is 380 revolutions per minute, and is applied to the drums of the old haulage by double-reduction gear.

The controller is of the metallic type, with which is combined a high-tension reversing switch. The secondary switch is connected to the rotor circuit, which is two-phase wound, and consists of six fixed and two movable contacts, each fitted with four fingers, the tips of which are fitted with sparking-pieces. The whole of the contacts operate in oil.

The resistance frames comprise main, multiplying, and wire resistances, which are enclosed in well-ventilated cases, and placed as near to the controller as possible. The method of control is as follows : — The first movement of the wheel closes the primary switch, and allows the current to enter the stator ; in this position the fingers are all clear of the contacts, with all the resistances in series. The second movement is on to the first contact with the first fingers, and this cuts out the wire resistance. The third movement cuts out the multiplying resistance, and leaves the main resistance in series. The fourth movement brings the second fingers on to the first contact, and puts the first step of the main resistance in parallel with the multiplying resistance, whilst the fifth and sixth steps of the main resistance are in series with the multiplying resistances. The movements over the six contacts result in cutting out or putting in sections of resistance, working the section in parallel or series according to the speed required, and this arrangement gives a range of 25 speeds to the motor.

(d) No. 1 Main Coal Haulage (Figs. 7, 8, and 9, Plate VI.). — This haulage works the south-east side of the Main Coal Seam, the most important landing being 9,900 feet from the shaft. The average gradient is 1 in 67, and the maximum 1 in 25 against the load. The load consists of 65 tubs, the weight of the set being 50.25 tons, and the maximum speed is at the rate of 18 miles per hour. The drums are 7 feet in diameter.

The motor is of the same type as the Low Main haulage-motor, with a rating of 300 brake-horsepower, and a speed at full load of 380 revolutions per minute. Figs. 22 and 28 show the arrangement of the drums and gear, and Figs. 10 and 11 (Plate VI.) load-curves designed from this haulage. The controller is of the liquid type.

(e) No. 2 Main Coal Haulage. — This haulage has been converted, and works the north side of the Main Coal Seam, the chief landing being 5,280 feet from the shaft. The average gradient is 1 in 70, and the maximum 1 in 19 against the load. The load consists of 40 tubs, the weight of the set being 30 tons.

The motor (Fig. 24) is connected directly on to the end of the main shaft, and is of the same type as those described in the former haulages, the rating being 200 brake-horsepower, and the speed at full load 165 revolutions per minute. The controller is of the metallic type.

The maximum horsepower developed during the run is 275, and the speed 11.5 miles per hour.

Pumping Stations. — The feeders of water to be pumped from the mine are 100 gallons per minute from the Hutton Seam and 200 gallons per minute from the Main Coal Seam. The water from the Hutton Seam is pumped up to the large standage in the Main Coal Seam, where the water from that seam is collected, the whole then being forced 1,224 feet to the surface.

The Hutton Seam pump, which forces 260 feet to the Main Coal Seam standage, is a three-throw rain-pump, with a capacity of 200 gallons per minute. The rams are 8 inches in diameter, with a 9-inch stroke, the speed of the crank-shaft being 43 revolutions per minute. The driving arrangement consists of double-reduction spur-gear; the first reduction being machine-cut cast-iron wheels. The pump is driven by a 30-brake-horsepower induction-motor of the enclosed type, with slip-rings, enclosed in a gas-tight case, and runs at a speed of 480 revolutions per minute.

The main pumps, which force from the Main Coal Seam standage to the surface, a height of 1,224 feet, consist of two three-throw ram-pumps, the rams being 7 inches in diameter, with a 12-inch stroke. The capacity of each pump is 200 gallons per minute, the speed of the crank-shaft being 45 revolutions per minute. The driving is done from each end of the crank-shaft with machine-cut cast-iron spur-wheels from an intermediate shaft, this being driven from the motor by eight cotton driving ropes, 1.25 inches in diameter. The pumps are driven by two induction-motors of the slip-ring protected type, each having a capacity of 120 brake-horsepower at a speed of 385 revolutions per minute. The starting arrangements consist of two starting rheostats, with resistance and contacts immersed in oil. Thorough tests have been made with these pumps, and careful measurements of current and water show an overall efficiency of 73 per cent.

The Golightly Drift pump (Fig. 25), for supplying water for boiler purposes at the surface is situated in what is known as "Golightly's" Drift, 790 feet below the surface. The standage is fed by water from behind the shaft-tubbing. The pump is of the three-throw ram type, the rams being 5.75 inches in diameter with a 9-inch stroke, and the capacity of the pump 150 gallons at 47 revolutions per minute. The pump is belt driven, worked by an induction-motor of the slip-ring type, which has a capacity of 60 brake-horsepower at a speed of 460 revolutions per minute.

(3) Low-tension Circuit. — This circuit is for the surface screening machinery at the Polka and Middle and East Pits. The current is transmitted through two 37/12 main cables to two small distribution switchboards, from which it is transmitted through separate cables to each motor-panel.

The Polka Pit circuit consists of five motors, namely, one 50-brake-horsepower induction-motor, working at a speed of 500 revolutions per minute, for driving three shaking-screens, three cleaning-belts, three tipplers, two creepers, and one elevator; one 20-brake-horsepower induction-motor, working at a speed of 580 revolutions per minute, for driving the nut-screening apparatus (Fig. 26); one 1 5-brake-horsepower induction-motor, working at a speed of 580 revolutions per minute, for driving the apparatus elevator; one 10-brake-horse-power induction-motor, working at a speed of 770 revolutions per minute, for driving two nut-cleaning belts (to this motor is attached a Baker reduction-gear, which reduces the speed from the motor to 12 revolutions per minute); and one 20-brakehorsepower induction-motor, working at a speed of 580 revolutions per minute, for driving the machinery in the work-shops.

The East Pit circuit consists of three motors, namely, one 20-brake-horsepower induction-motor, working at a speed of 580 revolutions per minute, for driving the East Pit shaking-screen, cleaning-belt., tippler, and creeper; one 20-brake-horsepower induction-motor, working at a speed of 580 revolutions per minute, for driving the Shotton Low Main Seam screen, cleaning-belt, tippler, creepers, and nut-screening apparatus and one 20-brake-horsepower induction-motor, working at a speed of 580 revolutions per minute, for driving the East Pit nut-screening apparatus, elevator, and conveying-belt.

The whole of these motors are of the slip-ring type, totally enclosed, and dust-proof Each motor is equipped with a switch-panel, fitted with an oil-break switch, a wattmeter, and a starting switch with cast-iron grid resistances.

The whole of the plant described has been in full work since August, 1908, and is giving good results.