nts 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.

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 th