ster 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.

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 curre