Volume I Part 27 (1/2)
_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, extinguis.h.i.+ng 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 extinguis.h.i.+ng 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.