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 associated 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.

Assume 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 passage 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.

[Illustration:  Fig. 225.  Complete Line Protection]

Assume, 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.