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.

[Illustration:  Fig. 226.  Sneak-Current and Air-Gap Arrester]