Protection Against High Potentials

. Lightning is the most nearly universal hazard. All open wires are exposed to it in some degree. Damaging currents from lightning are caused by extraordinarily high potentials. Furthermore, a lightning discharge is oscillatory; that is, alternating, and of very high frequency. Drops, ringers, receivers, and other devices subject to lightning damage suffer by having their windings burned by the discharge. The impedance these windings offer to the high frequency of lightning oscillations is great. The impedance of a few turns of heavy wire may be negligible to alternating currents of ordinary frequencies because the resistance of the wire is low, its inductance small, and the frequency finite. On the other hand, the impedance of such a coil to a lightning discharge is much higher, due to the very high frequency of the discharge. Were it not for the extremely high pre ure of lightning discharges, their high frequency of oscillation would enable ordinary coils to be self-protecting against them. But a discharge of electricity can take place through the air or other insulating medium if its pre ure be high enough. A pre ure of 70,000 volts can strike acro a gap in air of one inch, and lower pre ures can strike acro smaller distances. When lightning encounters an impedance, the discharge seldom takes place through the entire winding, as an ordinary current would flow, usually striking acro whatever short paths may exist. Very often these paths are acro the insulation between the outer turns of a coil. It is not unusual for a lightning discharge to plow its way acro the outer layer of a wound spool, melting the copper of the turns as it goes. Often the discharge will take place from inner turns directly to the core of the magnet. This is more likely when the core is grounded. Air-Gap Arrester . The tendency of a winding to oppose lightning discharges and the ease with which such discharge may strike acro insulating gaps, points the way to protection against them. Such devices consist of two conductors separated by an air space or other insulator and are variously known as lightning arresters, spark gaps, open-space cutouts, or air-gap arresters. The conductors between which the gap exists may be both of metal, may be one of metal and one of carbon, or both of carbon. One combination consists of carbon and mercury, a liquid metal. The space between the conductors may be filled with either air or solid matter, or it may be a vacuum. Speaking generally, the conductors are separated by some insulator. Two conductors separated by an insulator form a condenser. The insulator of an open-space arrester often is called the dielectric. IMG:384091999325875926_fig203_t.gif.png:Illustration_ Fig. 203. Saw Tooth Arrester Fig. 203. Saw Tooth Arrester View full size illustration. Discharge Acro Gaps:—Electrical discharges acro a given distance occur at lower potentials if the discharge be between points than if between smooth surfaces. Arresters, therefore, are provided with points. Fig. 203 shows a device known as a "saw-tooth" arrester because of its metal plates being provided with teeth. Such an arrester brings a ground connection close to plates connected with the line and is adapted to protect apparatus either connected acro a metallic circuit or in series with a single wire circuit. Fig. 201 shows another form of metal plate air-gap arrester having the further po ibility of a discharge taking place from one line wire to the other. Inserting a plug in the hole between the two line plates connects the line wires directly together at the arrester. This practice was designed for use with series lines, the plug short-circuiting the telephone set when in place. A defect of most ordinary types of metal air-gap lightning arresters is that heavy discharges tend to melt the teeth or edges of the plates and often to weld them together, requiring special attention to re-establish the nece ary gap. Advantages of Carbon:—Solid carbon is found to be a much better material than metal for the reasons that a discharge will not melt it and that its surface is composed of multitudes of points from which discharges take place more readily than from metals. IMG:384091999325875926_fig204_t.gif.png:Illustration_ Fig. 204. Saw-Tooth Arrester Fig. 204. Saw-Tooth Arrester View full size illustration. IMG:384091999325875926_fig205_t.gif.png:Illustration_ Fig. 205. Carbon Block Arrester? Fig. 205. Carbon Block Arrester View full size illustration. Carbon arresters now are widely used in the general form shown in Fig. 205. A carbon block connected with a wire of the line is separated from a carbon block connected to ground by some form of insulating separator. Mica is widely used as such a separator, and holes of some form in a mica slip enable the discharge to strike freely from block to block, while preventing the blocks from touching each other. Celluloid with many holes is used as a separator between carbon blocks. Silk and various special compositions also have their uses. IMG:384091999325875926_fig206_t.gif.png:Illustration_ Fig. 206. Arrester Separators Fig. 206. Arrester Separators View full size illustration. Dust Between Carbons:—Discharges between the carbon blocks tend to throw off particles of carbon from them. The separation between the blocks being small—from.005 to.015 inch—the carbon particles may lodge in the air-gap, on the edges of the separator, or otherwise, so as to leave a conducting path between the two blocks. Slight moisture on the separator may help to collect this dust, thus placing a ground on that wire of the line. This ground may be of very high resistance, but is probably one of many such—one at each arrester connected to the line. In special forms of carbon arresters an attempt has been made to limit this danger of grounding by the deposit of carbon dust. The object of the U-shaped separator of Fig. 206 is to enable the arrester to be mounted so that this opening in the separator is downward, in the hope that loosened carbon particles may fall out of the space between the blocks. The deposit of carbon on the inside edges of the U-shaped separator often is so fine and clings so tightly as not to fall out. The separator projects beyond the blocks so as to avoid the collection of carbon on the outer edges. Commercial Types:—Fig. 207 is a commercial form of the arrangement shown in Fig. 205 and is one of the many forms made by the American Electric Fuse Company. Line wires are attached to outside binding posts shown in the figure and the ground wire to the metal binding post at the front. The carbon blocks with their separator slide between clips and a ground plate. The air-gap is determined by the thickne of the separator between the carbon blocks. IMG:384091999325875926_fig207_t.gif.png:Illustration_ Fig. 207. Carbon Block Arrester Fig. 207. Carbon Block Arrester View full size illustration. IMG:384091999325875926_fig208_t.gif.png:Illustration_ Fig. 208 Roberts "Self-Cleaning" Arrester Fig. 208 Roberts "Self-Cleaning" Arrester View full size illustration. The Roberts carbon arrester is designed with particular reference to the disposal of carbon dust and is termed self-cleaning for that reason. The arrangement of carbons and dielectric in this device is shown in Fig. 208; mica is cemented to the line carbon and is large enough to provide a projecting margin all around. The spark gap is not uniform over the entire surface of the block but is made wedge-shaped by grinding away the line carbon as shown. It is claimed that a continuous arcing fills the wedge-shaped chamber with heated air or gas, converting the whole of the space into a field of low resistance to ground, and that this gas in expanding drives out every particle of carbon that may be thrown off. It seems obvious that the wedge-shaped space offers greater freedom for carbon dust to fall out than in the case of the parallel arrangement of the block faces. An outdoor arrester for metallic circuits, designed by F.B. Cook, is shown in Fig. 209. The device is adapted to mount on a pole or elsewhere and to be covered by a protecting cap. The carbons are large and are separated by a special compound intended to a ist the self-cleaning feature. The three carbons being grouped together as a unit, the device has the ability to care for discharges from one terminal to either of the others direct, without having to pa through two gaps. In this particular, the arrangement is the same as that of Fig. 204. IMG:384091999325875926_fig209_t.gif.png:Illustration_ Fig. 209. Cook Air-Gap Arrester Fig. 209. Cook Air-Gap Arrester View full size illustration. A form of Western Electric arrester particularly adapted for outside use on railway lines is shown with its cover in Fig. 210. IMG:384091999325875926_fig210_t.gif.png:Illustration_ Fig. 210. Western Electric Air-Gap Arrester Fig. 210. Western Electric Air-Gap Arrester View full size illustration. The Kellogg Company regularly equips its magneto telephones with air-gap arresters of the type shown in Fig. 211. The two line plates are semicircular and of metal. The ground plate is of carbon, circular in form, covering both line plates with a mica separator. This is mounted on the back board of the telephone and permanently wired to the line and ground binding posts. IMG:384091999325875926_fig211_t.gif.png:Illustration_ Fig. 211. Kellogg Air-Gap Arrester Fig. 211. Kellogg Air-Gap Arrester View full size illustration. Vacuum Arresters:—All of the carbon arresters so far mentioned depend on the discharge taking place through air. A given pre ure will discharge further in a fairly good vacuum than in air. The National Electric Specialty Company mounts three conductors in a vacuum of the incandescent lamp type, Fig. 212. A greater separation and le likelihood of short-circuiting can be provided in this way. Either carbon or metal plates are adapted for use in such vacuum devices. The plates may be further apart for a given discharge pre ure if the surfaces are of carbon. IMG:384091999325875926_fig212_t.gif.png:Illustration_ Fig. 212. Vacuum Arrester Fig. 212. Vacuum Arrester View full size illustration. Introduction of Impedance:—It has been noted that the existence of impedance tends to choke back the pa age of lightning discharge through a coil. Fig. 213 suggests the relation between such an impedance and air-gap arrester. If the coil shown therein be considered an arrangement of conductors having inductance, it will be seen that a favorable place for an air-gap arrester is between that impedance and the line. This fact is made known in practice by frequent damage to aërial cables by electricity brought into them over long open wires, the discharge taking place at the first turn or bend in the aërial cable; this discharge often damages both core and sheath. It is well to have such bends as near the end of the cable as po ible, and turns or goosenecks at entrances to terminals have that advantage. IMG:384091999325875926_fig213_t.gif.png:Illustration_ Fig. 213. Impedance and Air-Gap Fig. 213. Impedance and Air-Gap View full size illustration. This same principle is utilized in some forms of arresters, such as the one shown in Fig. 214, which provides an impedance of its own directly in the arrester element. In this device an insulating base carries a grounded carbon rod and two impedance coils. The impedance coils are wound on insulating rods, which hold them near, but not touching, the ground carbon. The coils are arranged so that they may be turned when discharges roughen the surfaces of the wires. IMG:384091999325875926_fig214_t.gif.png:Illustration_ Fig. 214. Holtzer-Cabot Arrester Fig. 214. Holtzer-Cabot Arrester View full size illustration. Metallic Electrodes:—Copper or other metal blocks with roughened surfaces separated by an insulating slip may be substituted for the carbon blocks of most of the arresters previously described. Metal blocks lack the advantage of carbon in that the latter allows discharges at lower potentials for a given separation, but they have the advantage that a conducting dust is not thrown off from them. IMG:384091999325875926_fig215_t.gif.png:Illustration_ Fig. 215. Carbon Air-Gap Arrester Fig. 215. Carbon Air-Gap Arrester View full size illustration. Provision Against Continuous Arc:—For the purpose of short-circuiting an arc, a globule of low-melting alloy may be placed in one carbon block of an arrester. This feature is not e ential in an arrester intended solely to divert lightning discharges. Its purpose is to provide an immediate path to ground if an arc arising from artificial electricity has been maintained between the blocks long enough to melt the globule. Fig. 215 is a plan and section of the Western Electric Company's arrester used as the high potential element in conjunction with others for abnormal currents and sneak currents; the latter are currents too small to operate air-gap arresters or substantial fuses.