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 thermal action may produce heat enough to melt metals, to char things which can be burned, or to cause them actually to burn, perhaps with a fire which can spread; the chemical action may destroy property values by changing the state of metals, as by dissolving them from a solid state where they are needed into a state of solution where they are not needed; the magnetic action introduces no direct hazard. The greatest hazard to which property values are exposed is the electro-thermal action; that is, the same useful properties by which electric lighting and electric heating thrive may produce heat where it is not wanted and in an amount greater than can safely be borne.

The tendency of design is to make all apparatus capable of carrying without overheating any current to which voltage within the telephone system may subject it, and to provide the system so designed with specific devices adapted to isolate it from currents originating without. Apparatus which is designed in this way, adapted not only to carry its own normal working currents but to carry the current which would result if a given piece of apparatus were connected directly across the maximum pressure within the telephone system itself, is said to be self-protecting. Apparatus amply able to carry its maximum working current but likely to be overheated, to be injured, or perhaps to destroy itself and set fire to other things if subjected to the maximum pressure within the system, is not self-protecting apparatus.

To make all electrical devices self-protecting by surrounding them with special arrangements for warding off abnormal currents from external sources, is not as simple as might appear. A lamp, for example, which can bear the entire pressure of a central-office battery, is not suitable for direct use in a line several miles long because it would not give a practical signal in series with that line and with the telephone set, as it is required to do. A lamp suitable for use in series with such a line and a telephone set would burn out by current from its own normal source if the line should become short-circuited in or near the central office. The ballast referred to in the chapter on "Signals" was designed for the very purpose of providing rapidly-rising resistance to offset the tendency toward rapidly-rising current which could burn out the lamp.

As another example, a very small direct-current electric motor can be turned on at a snap switch and will gain speed quickly enough so that its armature winding will not be overheated. A larger motor of that kind can not be started safely without introducing resistance into the armature circuit on starting, and cutting it out gradually as the armature gains speed. Such a motor could be made self-protecting by having the armature winding of much larger wire than really is required for mere running, choosing its size great enough to carry the large starting current without overheating itself and its insulation. It is better, and for long has been standard practice, to use starting boxes, frankly admitting that such motors are not self-protecting until started, though they are self-protecting while running at normal speeds. Such a motor, once started, may be overloaded so as to be slowed down. So much more current now can pass through the armature that its winding is again in danger. Overload circuit-breakers are provided for the very purpose of taking motors out of circuit in cases where, once up to speed, they are mechanically brought down again and into danger. Such a circuit-breaker is a device for protecting against an internal hazard; that is, internal to the power system of which the motor is a part.

Another example: In certain situations, apparatus intended to operate under impulses of large current may be capable of carrying its normal impulses successfully but incapable of carrying currents from the same pressure continuously. Protective means may be provided for detaching such apparatus from the circuit whenever the period in which the current acts is not short enough to insure safety. This is cited as a case wherein a current, normal in amount but abnormal in duration, becomes a hazard.

The last mentioned example of damage from internal hazards brings us to the law of the electrical generation of heat. The greater the current or the greater the resistance of the conductor heated or the longer the time, the greater will he the heat generated in that conductor. But this generated heat varies directly as the resistance and as the time and as the square of the current, that is, the law is

Heat generated = C2Rt