March 5, 1915. THE COLLIERY GUARDIAN. 491 insulated sleeves so as to prevent birds operating the protective gear. Steel poles will be used, and the average length of span on straight runs will be 120 yds. For railway and road crossings a special construction will be employed with double insulators and heavy section screening (fig. 8). A continuous earth wire of 7/13 S.W.G. section hard drawn copper will connect the metal work of the entire system, each transmission pole having a galvanised iron earth plate electrically connected to the main structure of the pole. The end connections for the transmission line will consist at each end of not less than 300 yds. of O’lsq.in. split-phase paper insulated lead-covered double wire armoured cable, the armouring being con- deep, in well consolidated marl and clay, the earth plates resting on clay in damp positions, reached 2’6 and 2’7.ohms. If the conditions are not favourable a much higher resistance to-earth will be found, as the resistance to earth is directly affected by the nature and temperature of the surrounding strata and by the amount of moisture, unless precautions are taken to supply earth plates with moisture and periodically to impregnate the surrounding strata with salt. . Tests show that this treatment is effective in lowering the resistance of poor earth connections. Tests made on groups of earth plates in different localities, constructed on the lines recommended by the General Regulations, show that the size of the metal Tests were made on two earth plates, constructed as above described, buried 28 yds. apart in excavations 4ft. by 2 ft. by 8 ft. deep, the plates being surrounded by moist clay, which was reached 3 ft. from the surface, the upper layer of soil consisting of marl and mould well consolidated. Plate No. (1) was 45 yds., and plate No. (2) 60 yds., from a canal, in which the water was 2 ft. below the normal ground level. The resistance of these earth plates was measured with alternating current at 50 periods. Fig. 10 shows the positions of these earth plates and the potential gradient in the vicinity of the plates when a current of 100 amperes was passing. The tests show the desirability of burying the earth plates at considerable depth, so as to avoid too great a potential gradient on the surface. In order to test the variation of resistance with vary- ing current densities, alternating current at 50 periods Khynney I 2 ^Britannia iiddleDuffrym^ ^OuFFRYN^k pBARGOE! MW u REFERENCE RditocLijs shown thus Rivers «• - Canals • « == Collieries » » c Woo-voltlMnsinlssion Line- —- WCZU4C UF UUL.C.O A £ . . c * — £ $ 7.- * ’°M,es Pontypridd Fig. 9.—Route of 20,000-volt Transmission Line. tmmed I ________Mould J- C/dg------------, C&st-iroii p/pe\''-'-—Mould tr Cid#. Action taken oh line AJB. C/ag l/hur-l 1'.^, llll.s III1 In 'll' ^x 11 i' JW 20______25 [< 50 A NOTE:- • Denotes surface reading 0 •• <• + depth reading jjii \ \ \ *£||( 20 200' Plan of Site 5 /0 Scale of Yards T Volts 2OO too Volts JOO 0 2/0/2545678 ^O’feetdeao'fPad of rr— — — = = =+ St/MiteelE E EEEHi = E 11 nt 1 1 L-m I I I--I- I-I-+ I--1- Illi- I--I I -I- I I-tS-Ue 111 I I 20151871161614/512II /OS 8766452/0125456 7 8 3 10 // /2 !514/512 // /Q 3 8 7 65 Yards Curve of Voltages taken along line A A Fig. 10.—Location of Earthplates Nos. (1) and (2), and Potential Gradient in Vicinity of Earthplates when Passing 100 Amperes at 50 Periods. nected to the continuous earth wire at each end and to the main earth connections at the Middle Duffryn power station and Britannia Collieries. Figs. 1 and 2 show the general network of the Aber- dare and Rhymney Valleys, and fig. 9 the route of the 20,000-volt transmission line. With the completion of the latter, the Britannia Colliery will be fed from three power houses by three independent routes. The annual load factor of the entire undertaking will be as high as 55 to 60 per cent., while the reserve plant in either valley will be available for the whole undertaking. (6) Distribution Underground. — The multi-core armoured cable system has been used by the Powell Duffryn Company since 1903, with the cable sheaths earthed. (c) Earthing Connections. —■ To improve existing systems a separate earth cable is permissible, but to prevent the earth connection being opened it is advisable —in order to give the necessary conductivity—where the section of the armouring is insufficient, that the earth conductor should form an integral part of the cable :—• (1) By using the copper sheath under the lead; (2) As an additional conductor (i.e., a four-core cable for three-phase working); so that breakage or corrosion shall not affect the earth connection. (d) Earth Plates.—While recognising the difficulty of specifying how an efficient earth connection should be made, the author agrees that the main “earth” must be placed at the surface of a colliery, owing to the difficulty of constructing and maintaining efficient earth plates underground. With regard to the type of “ earth ” at the surface, it is desirable, instead of leaving this point to be settled individually, that the usual power station practice should be followed, in which several earth connections are made by a copper ring main connected to steam condensers, circulating water pipes, feed pipes, and other metal work in direct connection with earth, or, in cases where it is not pos- sible to make such an earth connection, that the contact area of each coke bed should be materially greater than that specified in the General Regulations issued under the Goal Alines Act, 1911, the coke l ed being carried down not less than 8 ft. below ground level. From tests on main station “ earths ” constructed on the lines indicated, the measured resistance to earth with alternating current at 50 periods varies in repre- sentative cases from 0’01 to 0’03 ohm. Tests made on earth plates constructed on the lines recommended by the General Regulations, the plates being buried in excavations 4 ft. by 2 ft. by 8 ft. deep in clay under favourable conditions as to moisture, vary from 1’8 to 2’22 ohms, while the resistance of similar earth plates in another district in excavations 4 ft. by 2 ft. by 6 ft. 5oo 200 6oo 400 ioo 0 20 40 60 80 100 IZ0 140 100 180 ZOO ZZO 240 Amperes 260 280 300 Fig. 11.—Pressure between No. (2) Earthplate and Earth when Alternating Current at 50 Cycles is Passing. Readings taken 1 minute after pressure applied. ’Ohms Minutes Fig. 12.—Fall in Resistance of No. (2) Earthplate to Earth when Passing Alternating Current at 50 Cycles, 600 Volts for 2 Hours. 30 40 50 60 70 80 90* 100 no 120 2-4- 2-2, 2-0 ,1-8 *4 1-2 1-0 s earth plate is comparatively unimportant compared with the area of the coke bed. With a plate of 8 sq. ft. area surrounded by 12 in. of coke, the surface contact of the coke with the surrounding strata is about 40 sq.ft., which results in the resistance between the earth plate and the outer surface of coke being approximately 0’5 per cent, of the total resistance to earth, with the coke bed surrounded by clay. was passed between No. (2) earth plate and the station “ earth.” Fig. 11 show the readings taken one minute after pressure had been applied. To test the ability of these earth plates to carry current for a considerable period, alternating current at 50 periods was passed between No. (2) earth plate and the main station “ earth ” at a pressure of about 600 volts for two hours. The readings are plotted in fig. 12. The gradual fall in resistance over a period of two hours when current is passed through the earth plate is due to the decrease in resistance of the clay surrounding the coke bed owing to the temperature rising. These tests on earth plates show that if a main “ earth,” consisting of two earth plates constructed under favourable conditions on the lines recommended by the General Regulations issued under the Goal Mines Act, 1911, had been called upon to carry a current of 500 amperes for a short period, such as that necessary for a circuit breaker to operate, the potential at the earth plate would have been not less than 500 volts above earth. Any such pressure on the earth plates, although of momentary duration, is dangerous, as it would result in the cable sheathings and any metal connected to the main “earth” being raised to a dangerous pressure. It is therefore essential that a different type of main ‘ ‘ earth ’ ’ should be used from that recommended by the Regulations, or else that the number of earth plates be increased. (e) Earthing of System.—The general safety has been much increased by the Regulations issued in 1913, but the author suggests that when the Regulations are next under revision Regulation (124) . should be amended so as to make it compulsory to earth the neutral point of all polyphase systems. The main advantage of working with one point definitely earthed is the impossibility of continuing working with a definite fault on the . system. While working with a fault on the system is disallowed by Regulation (124), the possi- bility of continuing working with one fault on the system is still considered an advantage by some of the advocates of an unearthed system. Switchgear. (a) Power Stations and Substations.—The switch- gear conforms to the standard practice in power stations. Fig. 13 shows the method' of grouping the busbars, generators, and feeder switches at the Middle Duffryn power station. The switchgear, which is erected in a separate building, is operated electrically by remote control from a 110-volt battery. The busbars are arranged as a ring, which can be split by two 2,000-ampere oil break motor operated switches. For repairs, each half of the busbar ring can be subdivided by unbolting links. Ten feeder switches are provided, the ends of each ring feeder being con-