THE COLLIERY GUARDIAN AND JOURNAL OF THE COAL AND IRON TRADES. Vol. CVIII. FRIDAY, JULY 24, 1914. No. 2795. SWITCHGEAR FOR MINES. By ERNEST KILBURN SCOTT, Assoc.M.Inst.C.E., M.I.E.E. A paper (slightly abridged) read before the Midland Institute of Mining, .Civil, and Mechanical Engineers. In the following paper the writer proposes to draw attention to a number of novel features in switchgear design. In order to limit the length of the paper, metallic resistances and liquid switchgear are not dealt with. A difficulty with which the switchgear maker has had to contend is that of obtaining full information as to the work to be done. In heavy motor work. especially, he should know as much about the duty required as does the designer of the motor. Taking, for example, a haulage gear—it is desirable to know whether it is for main and tail or for endless rope haulage, and, if the work is intermittent, the number of journeys desired in a given time. If a “ creeping ” speed is required, then this should be specified, for on it depends the dis- sipating capacity of the resistance. If there is likely to be much “ inching,” that also should be mentioned, as it is very destructive to switch contacts. ‘If the switch- gear is not suitably designed for “ inching,” the attendant may find that he cannot run the haulage pro- perly on the first contact, and he may acquire the dangerous habit of running on an arc between the con- tacts, the arc acting as a kind of resistance. Materials. Although copper is generally employed for switch contacts, etc., it is not necessarily the best metal for the purpose, for it has been observed that after frequent switching of heavy current, copper contacts require more dressing or filing than those made of hard brass. In order to protect the current carrying parts against the effects of bad atmosphere, some makers (as, for example, the Union Electric Company) coat the ter- minals, etc., with nickel. When large alternating currents have to be carried by the terminal bolts, etc., “ skin effect ” has to be reckoned with. Messrs. John- son and Phillips meet the difficulty by cutting four shallow slots longitudinally in the terminal bolts. Carbon sparking tips are usually made of hard non- graphic carbons, but when necessary for them to be specially refractory, it is preferable to use a graphitic, or even an electro graphitic carbon. It is of interest to note that with direct current there is a marked difference in the rate of wear of the positive and negative tips; the position carbon wears away quicker than the negative. It is becoming the practice for firms so to design their switchgear that the same size of carbon tips is used for all sizes, the larger switches having several tips in parallel. Arc shields are usually made of a mixture of asbestos, silicate of soda, and Portland cement, and they should be as non-hygroscopic as possible. Porcelain insulators for switchgear should have a smooth outer surface : for, although corrugations and “ petticoats ” give longer surface insulation, they are apt to accumulate dust and moisture. The body of the insulator should be non-hygroscopic, so as to act as a good insulator quite independently of the glaze. Connecting cables for high tension currents should be ozone proof. Ordinary rubber is quickly affected, but can be rendered ozone proof by the incorporation of mineral matter to prevent oxidation. Glass beads are usually employed to protect mechanically the smaller wires, but, whilst they serve adequately their purpose, their employment cannot be said to be desirable. Bitumen is generally used for sealing glands and for filling boxes. Paraffin wax has been suggested, because it melts at a lower temperature, remains liquid much longer, and is a better insulator than bitumen; it has no effect on the cable insulation, and can be easily cut out of a box if necessary. Springs are subject to corrosion, loss of elasticity, and crystallisation. Mr. Ellison tins the steel, and fixes the ends by means of gunmetal eyes which screw into the ends of the springs. Ball bearings are used in the larger controllers, in order to reduce friction; but a peculiar feature is that they do not seem to work so well with alternating current as with direct current apparatus. If current passes across a ball race, there is a pitting action on the balls, which soon destroys the surfaces. Possibly currents are induced in the closed path of which the ball bearings and the spindle forms a part. The top plate or base plate carrying the main or fixed contacts by an oil switch is liable to have eddy currents let up in it. If made of cast iron, it should be slotted, so as to place reluctances in the paths of the magnetic fluxes. For the larger switches the Union Electric Company employs pressed steel, which, of course, cuts down the cross section. Messrs. Johnson and Phillips use non-magnetic materials, such as brass or aluminium. It is a somewhat curious fact that wood still continues to be used in switchgear. It is used for the rods carry- ing the movable contacts of oil break switches. English ash and American hickory are frequently employed. It is important that the wood should be properly seasoned rod XI___ J II—J r. u* Fig. 1.—Steel Rod and Porcelain Insulator for Oil-break Switch. and treated, otherwise it is a bad insulator. There is also danger of over-treatment when the wood becomes brittle. The saccharine method of treating these wooden rods has been adopted in the United States of America. For their heavy mining switchgear, Messrs. Johnson and Phillips use steel rods babbited into porcelain insu- lators, as shown in fig. 1. Aluminium is coming largely into use for solenoid coils, blow out coils, etc. It has the advantages of not requiring any cotton covered insulation, because the oxide on the surface is sufficient to insulate the adjacent turns from each other. An aluminium coil keeps cooler than a copper coil of the same size and wattage loss, because it is easier to dissipate the heat better from bare aluminium wire. As a matter of fact, it is usual to work aluminium at a relatively higher current density, and so the coil takes up less space. Starters for small alternating current motors usually have the arm held in the full on position by a hook or detent. Beneath the hook is a solenoid with a movable core, and the core is held down when the current is on. Should the current fail, the core is driven smartly upwards by a spring, and the switch arm is released. Pot shaped electro magnets are sometimes used instead of the ordinary straight magnets, the advantage being that the coil is enclosed and protected. When alternating current is used, the core must be laminated or partly split so as to avoid eddy currents. The question of the appropriate position of the over- load release coils for a three phase motor is a debatable one, and is affected by the fact that when such a motor is running, it will continue to do so even if the supply of one phase fails. The current in the rotor circuit will increase about 25 per cent, more than in the stator circuit. This explains why more burns out occur in the rotors than in the stators of multiphase motors. Seeing that in the case of a partial failure of supply, heavier currents flow through the rotor windings, the overload protecting devices should naturally be in the rotor circuit, and they are so placed in the case of the Ellison motor starters. It may be contended that a no volt release would trip if one of the phases to the stator should fail, but this is not the case, because the motor windings supply sufficient voltage to the release coil to prevent operation in case of failure of one phase only. Two overload releases in the rotor circuit provide efficient protection against overloading the motor, whilst a no volt release across one of the line phases will put the starter to the “ off ” position, in the event of a total failure of supply. Of course, if brush lifting and short circuiting gear is used, the trips must then be in the stator circuits. On medium tension three phase circuits many engineers provide an overload release trip in the neutral earth circuit. This trip coil is adjustable so as to pull out the circuit breaker when the earth leakage current attains Fig. 2.—Slow-motion Gear applied to Ellison Oil-immersed Motor-starter. about two amperes. If this leakage coil is provided, one overload release trip is sufficient for the protection of the circuit. With regard to trip gear for main switches, it is of interest to note that for complete protection the number varies. A three phase three wire system entirely insu- lated can be protected with two trips, but a three phase four wire system entirely insulated must have three trips. A three phase three wire system with the neutral point earthed must have three trips. It is safe to say that about 90 per cent, of the main switches supplied to mines have only twTo trips. Slow Motion Gear. Slow motion switchgear is preferable for mines, because the apparatus gets into the hands of men who have not the patience or knowledge that is required for the safe manual operation of such switches. In the Ellison switches the movement is made in a series of jerks, by means of the worm wheel and excentric worm shown in fig. 2. Only a third of the thread at each revolution is inclined, so that for two-thirds of the revolution of the worm the switch arm remains stationary while for the following third it moves from one contact to the next. Oil for Switches. For big alternating currents at high voltages, the oil switch has survived all others, because the interruption of current is effected at about the point where the