Assume, 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 pass 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 assumed.  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 assumption 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 assumptions 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.