Volume I Part 26 (1/2)
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.
[Ill.u.s.tration Fig. 206. Arrester Separators]
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 s.p.a.ce 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 thickness of the separator between the carbon blocks.
[Ill.u.s.tration: Fig. 207. Carbon Block Arrester]
[Ill.u.s.tration: Fig. 208 Roberts ”Self-Cleaning” Arrester]
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 s.p.a.ce 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 s.p.a.ce 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.s.sist 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.s.s through two gaps. In this particular, the arrangement is the same as that of Fig. 204.
[Ill.u.s.tration: Fig. 209. Cook Air-Gap Arrester]
A form of Western Electric arrester particularly adapted for outside use on railway lines is shown with its cover in Fig. 210.
[Ill.u.s.tration: Fig. 210. Western Electric Air-Gap Arrester]
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.
[Ill.u.s.tration: OLD SWITCHBOARD OF BELL EXCHANGE SERVING CHINATOWN, SAN FRANCISCO, CALIFORNIA]
[Ill.u.s.tration: Fig. 211. Kellogg Air-Gap Arrester]
Vacuum Arresters:--All of the carbon arresters so far mentioned depend on the discharge taking place through air. A given pressure 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 less 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 pressure if the surfaces are of carbon.
[Ill.u.s.tration: Fig. 212. Vacuum Arrester]
Introduction of Impedance:--It has been noted that the existence of impedance tends to choke back the pa.s.sage 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 aerial cables by electricity brought into them over long open wires, the discharge taking place at the first turn or bend in the aerial cable; this discharge often damages both core and sheath. It is well to have such bends as near the end of the cable as possible, and turns or goosenecks at entrances to terminals have that advantage.
[Ill.u.s.tration: Fig. 213. Impedance and Air-Gap]
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.
[Ill.u.s.tration: Fig. 214. Holtzer-Cabot Arrester]
Metallic Electrodes:--Copper or other metal blocks with roughened surfaces separated by an insulating slip may be subst.i.tuted 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.
[Ill.u.s.tration: Fig. 215. Carbon Air-Gap Arrester]
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 essential 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.
Protection Against Strong Currents. _Fuses._ A fuse is a metal conductor of lower carrying capacity than the circuit with which it is in series at the time it is required to operate. Fuses in use in electrical circuits generally are composed of some alloy of lead, which melts at a reasonably low temperature. Alloys of lead have lower conductivity than copper. A small copper wire, however, may fuse at the same volume of current as a larger lead alloy wire.
Proper Functions:--A fuse is not a good lightning arrester. As lightning damage is caused by current and as it is current which destroys a fuse, a lightning discharge _can_ open a circuit over which it pa.s.ses by melting the fuse metal. But lightning may destroy a fuse and at the same discharge destroy apparatus in series with the fuse.