CHAPTER XVIII

ELECTRICAL HAZARDS

All telephone systems are exposed to certain electrical hazards.  When these hazards become actively operative as causes, harmful results ensue.  The harmful results are of two kinds:  those causing damage to property and those causing damage to persons.  The damage to persons may be so serious as to result in death.  Damage to property may destroy the usefulness of a piece of apparatus or of some portion of the wire plant.  Or the property damage may initiate itself as a harm to apparatus or wiring and may result in greater and extending damage by starting a fire.

Electrical currents which endanger life and property may be furnished by natural or artificial causes.  Natural electricity which does such damage usually displays itself as lightning.  In rare cases, currents tending to flow over grounded lines because of extraordinary differences of potential between sections of the earth’s surface have damaged apparatus in such lines, or only have been prevented from causing such damage by the operation of protective devices.

Telegraph and telephone systems have been threatened 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.