hreatened by natural
electrical hazards since the beginning of the arts and by artificial
electrical hazards since the development of electric light and power
systems. At the present time, contrary to the general supposition, it
is in the artificial, and not in the natural electrical hazards that
the greater variety and degree of danger lies.

Of the ways in which artificial electricity may injure a telephone
system, the entrance of current from an external electrical power
system is a greater menace than an abnormal flow of current from a
source belonging to the telephone system itself. Yet modern practice
provides opportunities for a telephone system to inflict damage upon
itself in that way. Telephone engineering designs need to provide
means for protecting _all_ parts of a system against damage, from
external ("foreign") as well as internal ("domestic") hazards, and to
cause this protection to be inclusive enough to protect persons
against injury and property from damage by any form of overheating or
electrolytic action.

A part of a telephone system for which there is even a remote
possibility of contact with an external source of electrical power,
whether natural or artificial, is said to be _exposed_ to electrical
hazard. The degree or character of possible contact or other
interference often is referred to in relative terms of _exposure_. The
same terms are used concerning inductive relations between circuits.
The whole tendency of design, particularly of wire plants, is to
arrange the circuits in such a way as to limit the exposure as greatly
as possible, the intent being to produce a condition in which all
parts of the system will be _unexposed_ to hazards.

Methods of design are not yet sufficiently advanced for any plant to
be formed of circuits wholly unexposed, so that protective means are
required to safeguard apparatus and circuits in case the hazard,
however remote, becomes operative.

Lightning discharges between the clouds and earth frequently charge
open wires to potentials sufficiently high to damage apparatus; and
less frequently, to destroy the wires of the lines themselves.
Lightning discharges between clouds frequently induce charges in lines
sufficient to damage apparatus connected with the lines. Heavy rushes
of current in lines, from lightning causes, occasionally induce
damaging currents in adjacent lines not sufficiently exposed to the
original cause to have been injured without this i