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 induction.  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.