_Heat Coil._ Fig. 224 shows a practical way of bringing the heating and to-be-heated elements together. A copper spool is wound with resistance wire. A metal pin is soldered in the bore of the spool by an easily melting alloy. When current heats the spool enough, the pin may slide or turn in the spool. It may slide or turn in many ways and this happily enables many types of arresters to result. For example, the pin may pull out, or push in, or push through, or rotate like a shaft in a bearing, or the spool may turn on it like a hub on an axle. Messrs.
Hayes, Rolfe, Cook, McBerty, Kaisling, and many other inventors have utilized these combinations and motions in the production of sneak-current arresters. All of them depend on one action: the softening of a low-melting alloy by heat generated in a resistance.
When a heat coil is a.s.sociated with the proper switching springs, it becomes a sneak-current arrester. The switching springs always are arranged to ground the line wire. In some arresters, the line wire is cut off from the wire leading toward the apparatus by the same movement which grounds it. In others, the line is not broken at all, but merely grounded. Each method has its advantages.
Complete Line Protection. Fig. 225 shows the entire scheme of protectors in an exposed line and their relation to apparatus in the central-office equipment and at the subscriber"s telephone. The central-office equipment contains heat coils, springs, and carbon arresters. At some point between the central office and the subscriber"s premises, each wire contains a fuse. At the subscriber"s premises each wire contains other fuses and these are a.s.sociated with carbon arresters. The figure shows a central battery equipment, in which the ringer of the telephone is in series with a condenser. A sneak-current arrester is not required at the subscriber"s station with such equipment.
a.s.sume the line to meet an electrical hazard at the point _X_. If this be lightning, it will discharge to ground at the central office or at the subscriber"s instrument or at both through the carbon arresters connected to that side of the line. If it be a high potential from a power circuit and of more than 350 volts, it will strike an arc at the carbon arrester connected to that wire of the line in the central office or at the subscriber"s telephone or at both, if the separation of the carbons in those arresters is .005 inch or less. If the carbon arresters are separated by celluloid, it will burn away and allow the carbons to come together, extinguishing the arc. If they are separated by mica and one of the carbons is equipped with a globule of low-melting alloy, the heat of the arc will melt this, short-circuiting the gap and extinguishing the arc. The pa.s.sage of current to ground at the arrester, however, will be over a path containing nothing but wire and the arrester. The resulting current, therefore, may be very large. The voltage at the arrester having been 350 volts or more, in order to establish the arc, short-circuiting the gap will make the current 7 amperes or more, unless the applied voltage miraculously falls to 50 volts or less. The current through the fuse being more than 7 amperes, it will blow promptly, opening the line and isolating the apparatus. It will be noted that this explanation applies to equipment at either end of the line, as the fuse lies between the point of contact and the carbon arrester.
[Ill.u.s.tration: Fig. 225. Complete Line Protection]
a.s.sume, on the other hand, that the contact is made at the point _Y_.
The central-office carbon arrester will operate, grounding the line and increasing the amount of current flowing. There being no fuse to blow, a worse thing will befall, in the overheating of the line wire and the probable starting of a fire in the central office. It is obvious, therefore, that a fuse must be located between the carbon arrester and any part of the line which is subject to contact with a potential which can give an abnormal current when the carbon arrester acts.
a.s.sume, as a third case, that the contact at the point _X_ either is with a low foreign potential or is so poor a contact that the difference of potential across the gap of the carbon arrester is lower than its arcing point. Current will tend to flow by the carbon arrester without operating it, but such a current must pa.s.s through the winding of the heat coil if it is to enter the apparatus. The sneak current may be large enough to overheat the apparatus if allowed to flow long enough, but before it has flowed long enough it will have warmed the heat-coil winding enough to soften its fusible alloy and to release springs which ground the line, just as did the carbon arrester in the case last a.s.sumed. Again the current will become large and will blow the fuse which lies between the sneak-current arrester and the point of contact with the source of foreign current. In this case, also, contact at the point _Y_ would have operated mechanism to ground the line at the central office, and, no fuse interposing, the wiring would have been overheated.
_Exposed and Unexposed Wiring._ Underground cables, cables formed of rubber insulated wires, and interior wiring which is properly done, all may be considered to be wiring which is unexposed, that is, not exposed to foreign high potentials, discharges, sneak, or abnormal currents. _All other wiring_, such as bare wires, aerial cables, etc., should be considered as _exposed_ to such hazards and a fuse should exist in each wire between its exposed portion and the central office or subscriber"s instrument. The rule of action, therefore, becomes:
_The proper position of the fuse is between exposed and unexposed wiring._
It may appear to the student that wires in an aerial cable with a lead sheath--that sheath being either grounded or ungrounded--are not exposed to electrical hazards; in the case of the grounded sheath, this would presume that a contact between the cable and a high potential wire would result merely in the foreign currents going to ground through the cable sheath, the arc burning off the high-potential wire and allowing the contact to clear itself by the falling of the wire. If the a.s.sumption be that the sheath is not grounded, then the student may say that no current at all would flow from the high-potential wire.
Both a.s.sumptions are wrong. In the case of the grounded sheath, the current flows to it at the contact with the high-potential wire; the lead sheath is melted, arcs strike to the wires within, and currents are led directly to the central office and to subscribers" premises.
In the case of the ungrounded sheath, the latter charges at once through all its length to the voltage of the high-potential wire; at some point, a wire within the cable is close enough to the sheath for an arc to strike across, and the trouble begins. All the wires in the cable are endangered if the cross be with a wire of the primary circuit of a high-tension transmission line. Any series arc-light circuit is a high-potential menace. Even a 450-volt trolley wire or feeder can burn a lead-covered cable entirely in two in a few seconds.
The authors have seen this done by the wayward trolley pole of a street car, one side of the pole touching the trolley wire and the extreme end just touching the telephone cable.
The answer lies in the foregoing rule. Place the fuse between the wires which _can_ and the wires which _can not_ get into contact with high potentials. In application, the rule has some flexibility. In the case of a cable which is aerial as soon as it leaves the central office, place the fuses in the central office; in a cable wholly underground, from central office to subscriber--as, for example, the feed for an office building--use no fuses at all; in a cable which leaves the central office underground and becomes aerial, fuse the wires just where they change from underground to aerial. The several branches of an underground cable into aerial ones should be fused as they branch.
Wires properly installed in subscribers" premises are considered unexposed. The position of the fuse thus is at or near the point of entrance of the wires into that building if the wires of the subscriber"s line outside the premises are exposed, as determined by the definitions given. If the line is unexposed, by those definitions, no protector is required. If one is indicated, it should be used, as compliance with the best-known practice is a clear duty. Less than what is known to be best is not honest practice in a matter which involves life, limb, and indefinite degrees of property values.
Protectors in central-battery subscribers" equipments need no sneak-current arresters, as the condenser reduces that hazard to a negligible amount. Magneto subscribers" equipments usually lack condensers in ringer circuits, though they may have them in talking circuits on party lines. The ringer circuit is the only path through the telephone set for about 98 per cent of the time. Sneak-current arresters, therefore, should be a part of subscribers" station protectors in magneto equipment, except in such rural districts as may have no lighting or power wires. When sneak-current arresters are so used the arrangement of the parts then is the same as in the central-office portion of Fig. 225.
Types of Central-Office Protectors. A form of combined heat coil and air-gap arrester, widely used by Bell companies for central-office protection, is shown in Fig. 226. The two inner springs form the terminals for the two limbs of the metallic-circuit line, while the two outside springs are terminals for the continuation of the line leading to the switchboard. The heat coils, one on each side, are supported between the inner and outer springs. High-tension currents jump to ground through the air-gap arrester, while sneak currents permit the pin of the heat coil to slide within the sleeve, thus grounding the outside line and the line to the switchboard.
[Ill.u.s.tration: Fig. 226. Sneak-Current and Air-Gap Arrester]
_Self-Soldering Heat Coils._ Another form designed by Kaisling and manufactured by the American Electric Fuse Company is shown in Fig.
227. In this the pin in the heat coil projects unequally from the ends of the coil, and under the action of a sneak current the melting of the solder which holds it allows the outer spring to push the pin through the coil until it presses the line spring against the ground plate and at the same time opens the path to the switchboard. When the heat-coil pin a.s.sumes this new position it cools off, due to the cessation of the current, and _resolders_ itself, and need only be turned end for end by the attendant to be reset. Many are the variations that have been made on this self-soldering idea, and there has been much controversy as to its desirability. It is certainly a feature of convenience.
[Ill.u.s.tration: Fig. 227. Self-Soldering Heat-Coil Arrester]
Instead of using a wire-wound resistance element in heat-coil construction some manufacturers employ a ma.s.s of high-resistance material, interposed in the path of the current. The Kellogg Company has long employed for its sneak-current arrester a short graphite rod, which forms the resistance element. The ends of this rod are electroplated with copper to which the bra.s.s terminal heads are soldered. These heads afford means for making the connection with the proper retaining springs.
[Ill.u.s.tration: Fig. 228. Cook Arrester]
Another central-office protector, which uses a ma.s.s of special metal composition for its heat producing element is that designed by Frank B.
Cook and shown in Fig. 228. In this the carbon blocks are cylindrical in form and specially treated to make them "self-cleaning." Instead of employing a self-soldering feature in the sneak-current arrester of this device, Cook provides for electrically resoldering them after operation, a clip being designed for holding the elements in proper position and pa.s.sing a battery current through them to remelt the solder.
In small magneto exchanges it is not uncommon to employ combined fuse and air-gap arresters for central-office line protection, the fuses being of the mica-mounted type already referred to. A group of such arresters, as manufactured by the Dean Electric Company, is shown in Fig. 229.
[Ill.u.s.tration: Fig. 229. Mica Fuse and Air-Gap Arresters]
Types of Subscribers" Station Protectors. Figs. 230 and 231 show types of subscribers" station protectors adapted to the requirements of central-battery and magneto systems. These, as has been said, should be mounted at or near the point of entrance of the subscriber"s line into the premises, if the line is exposed outside of the premises. It is possible to arrange the fuses so that they will be safe and suitable for their purposes if they are mounted out-of-doors near the point of entrance to the premises. The sneak-current arrester, if one exists, and the carbon arrester also, must be mounted inside of the premises or in a protecting case, if outside, on account of the necessity of shielding both of these devices from the weather. Speaking generally, the wider practice is to put all the elements of the subscriber"s station protector inside of the house. It is nearer to the ideal arrangement of conditions if the protector be placed immediately at the point of entrance of the outside wires into the building.
[Ill.u.s.tration: Fig. 230. Western Electric Station Arrester]
[Ill.u.s.tration: Fig. 231. Cook Arrester for Magneto Stations]
_Ribbon Fuses_. A point of interest with relation to tubular fuses is that in some of the best types of such fuses, the resistance material is not in the form of a round wire but in the form of a flat ribbon.
This arrangement disposes the necessary amount of fusible metal in a form to give the greatest amount of surface, while a round wire offers the least surface for a given weight of metal--a circle encloses its area with less periphery than any other figure. The reason for giving the fuse the largest possible surface area is to decrease the likelihood of the fuse being ruptured by lightning. The fact that such fuses do withstand lightning discharges much more thoroughly than round fuses of the same rating is an interesting proof of the oscillating nature of lightning discharges, for the density of the current of those discharges is greater on and near the surface of the conductor than within the metal and, therefore, flattening the fuse increases its carrying capacity for high-frequency currents, without appreciably changing its carrying capacity for direct currents. The reason its capacity for direct currents is increased at all by flattening it, is that the surface for the radiation of heat is increased. However, when enclosed in a tube, radiation of heat is limited, so that for direct currents the carrying capacity of fuses varies closely with the area of cross-section.
City-Exchange Requirements. The foregoing has set down the requirements of good practice in an average city-exchange system.
Nothing short of the general arrangement shown in Fig. 225 meets the usual a.s.sortment of hazards of such an exchange. It is good modern practice to distribute lines by means of cables, supplemented in part by short insulated drop wires twisted in pairs. Absence of bare wires reduces electrical hazards enormously. Nevertheless, hazards remain.
Though no less than the spirit of this plan of protection should be followed, additional hazards may exist, which may require additional elements of protection. At the end of a cable, either aerial or underground, long open wires may extend into the open country as rural or long-distance circuits. If these be longer than a mile or two, in most regions they will be subjected to lightning discharges. These may be subjected to high-potential contacts as well.
If a specific case of such exposure indicates that the cables may be in danger, the long open lines then are equipped with additional air-gap arresters at the point of junction of those open lines with the cable. Practice varies as to the type. Maintenance charges are increased if carbon arresters separated .005 inch are used, because of the cost of sending to the end of the long cable to clear the blocks from carbon dust after each slight discharge. Roughened metal blocks do not become grounded as readily as do carbon blocks. The occasions of visit to the arresters, therefore, usually follow actual heavy discharges through them.
The recommendations and the practice of the American Telephone and Telegraph Company differ on this point, while the practice of other companies varies with the temperaments of the engineers. The American Company specifies copper-block arresters where long country lines enter cables, if those lines are exposed to lightning discharges only.
The exposed line is called _long_ if more than one-half mile in length. If it is exposed to high-potential hazards, carbon blocks are specified instead of copper. Other specifications of that company have called for the use of copper-block arresters on lines exposed to hazards above 2,500 volts.
[Ill.u.s.tration: ONE OF THE FOUR WINGS OF THE OLD KELLOGG DIVIDED MULTIPLE BOARD OF THE CUYAHOGA TELEPHONE COMPANY, CLEVELAND, OHIO Ultimate Capacity, 24,000 Lines. One of the Two Examples in the United States of a Multiple Switchboard Having an Ultimate Capacity over 18,000 Lines. Replaced Recently by a Kellogg Straight Multiple Board Having an Ultimate Capacity of 18,000 Lines and a Present Capacity of 10,000 Lines.]
The freedom of metal-block arresters from dust troubles gives them a large economical advantage over carbon. For similar separations, the ratio of striking voltages between carbon blocks and metal blocks respectively is as 7 to 16. In certain regions of the Pacific Coast where the lightning hazard is negligible and the high tension hazard is great, metal-block arresters at the outer ends of cables give acceptable protection.
High winds which drive snow or dust against bare wires of a long line, create upon or place upon those wires a charge of static electricity which makes its way from the line in such ways as it can. Usually it discharges across arresters and when this discharge takes place, the line is disturbed in its balance and loud noises are heard in the telephones upon it.
[Fig. 232. Drainage Coils]
A telephone line which for a long distance is near a high-tension transmission line may have electrostatic or electromagnetic potentials, or both, induced upon it. If the line be balanced in its properties, including balance by transposition of its wires, the electrostatic induction may neutralize itself. The electromagnetic induction still may disturb it.
_Drainage Coils_. The device shown in Fig. 232, which amounts merely to an inductive leak to earth, is intended to cure both the snowstorm and electromagnetic induction difficulties. It is required that its impedance be high enough to keep voice-current losses low, while being low enough to drain the line effectively of the disturbing charges.
Such devices are termed "drainage coils."
Electrolysis. The means of protection against the danger due to chemical action, set forth in the preceding chapter, form such a distinct phase of the subject of guarding property against electrical hazards as to warrant treatment in a separate chapter devoted to the subject of electrolysis.
[Ill.u.s.tration: MAIN EXCHANGE, CLEVELAND, OHIO.
Largest Four-Party Selective Ringing Switchboard in the World. Kellogg Switchboard and Supply Co.]
CHAPTER XX
GENERAL FEATURES OF THE TELEPHONE EXCHANGE
Up to this point only those cla.s.ses of telephone service which could be given between two or more stations on a single line have been considered. Very soon after the practical conception of the telephone, came the conception of the telephone exchange; that is, the conception of centering a number of lines at a common point and there terminating them in apparatus to facilitate their interconnection, so that any subscriber on any line could talk with any subscriber on any other line.