【正文】 nection is through an arc. Currents of abnormally high magnitude flow through the work to the point of fault. These shortcircuit currents will usually be much greater than the designed thermal ability of the conductors in the lines or machines feeding the fault. The resultant rise in temperature may cause damage by the annealing of conductors and by the charring of insulation. In the period during which the fault is permitted to exist, the voltage on the system in the near vicinity of the fault will be so low that utilization equipment will be inoperative. It is apparent that the power system designer must anticipate points at which faults may occur, be able to calculate conditions that exist during a fault, and provide equipment properly adjusted to open the switches necessary to disconnect the faulted equipment from the remainder of the system1. Ordinarily it is desirable that no other switches on the system are opened, as such behavior would result in unnecessary modification of the system circuits. Overload A distinction must be made between a fault and an overload. An overload implies only that loads greater than the designed values have been imposed on system. Under such a circumstance the voltage at the overload point may be low, but not zero. This under voltage condition may extend for some distance beyond the overload point into the remainder of the system. The currents in the overloaded equipment are high and may exceed the thermal design limits. Nevertheless, such currents are substantially lower than in the case of a fault. Service frequently may be maintained, but at belowstandard voltage. Overloads are rather mon occurrences in homes. For example, a housewife might plug five waffle irons into the kitchen circuit during a neighborhood party. Such an overload, if permitted to continue, would cause heating of the wires from the power center and might eventually start a fire. To prevent such trouble, residential circuits are protected by fuses or circuit breakers which open quickly when currents above specified values persist. Distribution transformers are sometimes overloaded as customers install more and more appliances. The continuous monitoring of distribution circuits is necessary to be certain that transformer sizes are increased as load grows. Various Faults Faults of many types and causes may appear on electric power systems. Many of us in our homes have seen frayed lamp cords which permitted the two conductors of the cord to e in contact with each other. When this occurs, there is a resulting flash, and if breaker or fuse equipment functions properly, the circuit is opened. Overhead lines, for the most part, are constructed of bare conductors. These are sometimes accidentally brought together by action of wind, sleet, trees, cranes, airplanes, or damage to supporting structures. Over voltages due to lightning or switching may cause flashover of supporting or from conductor to conductor. Contamination on insulators sometimes results in flashover even during normal voltage conditions. The conductors of underground cables are separated from each other and from ground by solid insulation, which may be oilimpregnated paper or a plastic such as polyethylene. These materials undergo some deterioration with age, particularly if overloads on the cables have resulted in their operation at elevated temperature. Any small void present in the body of the insulating material will result in ionization of the gas contained therein, the products of which react unfavorably with the insulation, deterioration of the insulation may result in failure of the material to retain its insulating properties, and short circuits will develop between the cable conductors. The possibility of cable failure is increased if lightning or switching produces transient voltage of abnormally high values between the conductors. Transformer failures may be the result of insulation deterioration bined with over voltages due to lightning or switching transients. Short circuits due to insulation failure between adjacent turns of the same winding may result from suddenly applied over voltages. Major insulation may fail, permitting arcs to be established between primary and secondary windings or between a winding and grounded met