Cyclopedia of Telephony and Telegraphy Volume I Part 26

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.

There are two reasons for this: One is that lightning discharges act very quickly and may have destroyed apparatus before heating the fuse enough to melt it; the other reason is that when a fuse is operated with enough current even to vaporize it, the vapor serves as a conducting path for an instant after being formed. This conducting path may be of high resistance and still allow currents to flow through it, because of the extremely high pressure of the lightning discharge. A comprehensive protective system may include fuses, but it is not to be expected that they always will arrest lightning or even a.s.sist other things in arresting lightning. They should be considered as of no value for that purpose. Furthermore, fuses are best adapted to be a part of a general protective system when they do all that they must do in stopping abnormal currents and yet withstand lightning discharges which may pa.s.s through them. Other things being equal, that system of protection is best in which all lightning discharges are arrested by gap arresters and in which no fuses ever are operated by lightning discharges.

Mica Fuse:--A convenient and widely used form of fuse is that shown in Fig. 216. A mica slip has metal terminals at its ends and a fuse wire joins these terminals. The fuse is inserted in the circuit by clamping the terminals under screws or sliding them between clips as in Figs.

217 and 218. Advantages of this method of fuse mounting for protecting circuits needing small currents are that the fuse wire can be seen, the fuses are readily replaced when blown, and their mountings may be made compact. As elements of a comprehensive protective system, however, the ordinary types of mica-slip fuses are objectionable because too short, and because they have no means of their own for extinguis.h.i.+ng an arc which may follow the blowing of the fuses. As protectors for use in distributing low potential currents from central-office power plants they are admirable. By simple means, they may be made to announce audibly or visibly that they have operated.

[Ill.u.s.tration: Fig. 216. Mica Slip Fuse]

[Ill.u.s.tration: Fig. 217. Postal Type Mica Fuse]

[Ill.u.s.tration: Fig. 218. Western Union Type Mica Fuse]

Enclosed Fuses:--If a fuse wire within an insulating tube be made to connect metal caps on that tube and the s.p.a.ce around the tube be filled with a non-conducting powder, the gases of the vaporized fuse metal will be absorbed more quickly than when formed without such imbedding in a powder. The filling of such a tubular fuse also m.u.f.fles the explosion which occurs when the fuse is vaporized.

[Ill.u.s.tration: Fig. 219. Pair of Enclosed Fuses]

Fuses of the enclosed type, with or without filling, are widely used in power circuits generally and are recommended by fire insurance bodies. Fig. 219 ill.u.s.trates an arrester having a fuse of the enclosed type, this example being that of the H. W. Johns-Manville Company.

[Ill.u.s.tration Fig. 220. Bank of Enclosed Fuses]

In telephony it is frequently necessary to mount a large number of fuses or other protective devices together in a restricted s.p.a.ce. In Fig. 220 a group of Western Electric tubular fuses, so mounted, is shown. These fuses have ordinarily a carrying capacity of 6 or 7 amperes. It is not expected that this arrester will blow because 6 or 7 amperes of abnormal currents are flowing through it and the apparatus to be protected. What is intended is that the fuse shall withstand lightning discharges and when a foreign current pa.s.ses through it, other apparatus will increase that current enough to blow the fuse. It will be noticed that the fuses of Fig. 220 are open at the upper end, which is the end connected to the exposed wire of the line The fuses are closed at the lower end, which is the end connected to the apparatus. When the fuse blows, its discharge is somewhat m.u.f.fled by the lining of the tube, but enough explosion remains so that the heated gases, in driving outward, tend to break the arc which is established through the vaporized metal.

A pair of Cook tubular fuses in an individual mounting is shown in Fig. 221. Fuses of this type are not open at one end like a gun, but opportunity for the heated gases to escape exists at the caps. The tubes are made of wood, of lava, or of porcelain.

Fig. 222 is another tubular fuse, the section showing the arrangement of asbestos lining which serves the two purposes of m.u.f.fling the sound of the discharge and absorbing and cooling the resulting gases.

[Ill.u.s.tration: Fig. 221. Pair of Wooden Tube Fuses]

_Air-Gap vs. Fuse Arresters._ It is hoped that the student grasps clearly the distinction between the purposes of air-gap and fuse arresters. The air-gap arrester acts in response to high voltages, either of lightning or of high-tension power circuits. The fuse acts in response to a certain current value flowing through it and this minimum current in well-designed protectors for telephone lines is not very small. Usually it is several times larger than the maximum current apparatus in the line can safely carry. Fuses _can_ be made so delicate as to operate on the very smallest current which could injure apparatus and the earlier protective systems depended on such an arrangement. The difficulty with such delicate fuses is that they are not robust enough to be reliable, and, worse still, they change their carrying capacity with age and are not uniform in operation in different surroundings and at different temperatures. They are also sensitive to lightning discharges, which they have no power to stop or to divert.

Protection Against Sneak Currents. For these reasons, a system containing fuses and air-gap arresters only, does not protect against abnormal currents which are continuous and small, though large enough to injure apparatus _because_ continuous. These currents have come to be known as sneak currents, a term more descriptive than elegant.

Sneak currents though small, may, when allowed to flow for a long time through the winding of an electromagnet for instance, develop enough heat to char or injure the insulation. They are the more dangerous because insidious.

[Ill.u.s.tration: Fig. 222. Tubular Fuse with Asbestos Filling]

_Sneak-Current Arresters._ As typical of sneak-current arresters, Fig. 223 shows the principle, though not the exact form, of an arrester once widely used in telephone and signal lines. The normal path from the line to the apparatus is through a small coil of fine wire imbedded in sealing wax. A spring forms a branch path from the line and has a tension which would cause it to bear against the ground contact if it were allowed to do so. It is prevented from touching that contact normally by a string between itself and a rigid support.

The string is cut at its middle and the knotted ends as thus cut are imbedded in the sealing wax which contains the coil.

[Ill.u.s.tration: Fig. 223. Principle of Sneak-Current Arrester]

A small current through the little coil will warm the wax enough to allow the string to part. The spring then will ground the line. Even so simple an apparatus as this operates with considerable accuracy.

All currents below a certain critical amount may flow through the heating coil indefinitely, the heat being radiated rapidly enough to keep the wax from softening and the string from parting. All currents above this critical amount will operate the arrester; the larger the current, the shorter the time of operating. It will be remembered that the law of these heating effects is that the heat generated = _C^{2}Rt_, so that if a certain current operates the arrester in, say 40 seconds, twice as great a current should operate the arrester in 10 seconds. In other words, the time of operation varies inversely as the square of the current and inversely as the resistance. To make the arrester more sensitive for a given current--_i.e._, to operate in a shorter time--one would increase the resistance of the coil in the wax either by using more turns or finer wire, or by making the wire of a metal having higher specific resistance.

The present standard sneak-current arrester embodies the two elements of the devices of Fig. 223: a _resistance_ material to transform the dangerous sneak current into localized heat; and a _fusible_ material softened by this heat to release some switching mechanism.

The resistance material is either a resistance wire or a bit of carbon, the latter being the better material, although both are good.

The fusible material is some alloy melting at a low temperature. Lead, tin, bis.m.u.th, and cadmium can be combined in such proportions as will enable the alloy to melt at temperatures from 140 to 180 F. Such an alloy is a solder which, at ordinary temperatures, is firm enough to resist the force of powerful springs; yet it will melt so as to be entirely fluid at a temperature much less than that of boiling water.

[Ill.u.s.tration: Fig. 224. Heat Coil]