nduction. The
lightning hazard is least where the most lines are exposed. In a small
city with all of the lines formed of exposed wires and all of them
used as grounded circuits, a single lightning discharge may damage
many switchboard signals and telephone ringers if there be but 100 or
200 lines, while the damage might have been nothing had there been 800
to 1,000 lines in the same area.

Means of protecting lines and apparatus against damage by lightning
are little more elaborate than in the earliest days of telegraph
working. They are adequate for the almost entire protection of life
and of apparatus.

Power circuits are classified by the rules of various governing bodies
as high-potential and low-potential circuits. The classification of
the National Board of Fire Underwriters in the United States defines
low-potential circuits as having pressures below 550 volts;
high-potential circuits as having pressures from 550 to 3,500 volts,
and extra high-potential circuits as having pressures above 3,500
volts. Pressures of 100,000 volts are becoming more common. Where
power is valuable and the distance over which it is to be transmitted
is great, such high voltages are justified by the economics of the
power problem. They are a great hazard to telephone systems, however.
An unprotected telephone system meeting such a hazard by contact will
endanger life and property with great certainty. A very common form of
distribution for lighting and power purposes is the three-wire system
having a grounded neutral wire, the maximum potential above the earth
being about 115 volts.

Telephone lines and apparatus are subject to damage by any power
circuit whether of high or low potential. The cause of property damage
in all cases is the flow of current. Personal damage, if it be death
from shock, ordinarily is the result of a high potential between two
parts of the body. The best knowledge indicates that death uniformly
results from shock to the heart. It is believed that death has
occurred from shock due to pressure as low as 100 volts. The critical
minimum voltage which can not cause death is not known. A good rule is
never willingly to subject another person to personal contact with any
electrical pressure whatever.

Electricity can produce actions of four principal kinds:
physiological, thermal, chemical, and magnetic. Viewing electricity as
establishing hazards, the physiological action may injure or kill
living things; the