【正文】 ults 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 tow 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 wing, sleet, trees, cranes, airplanes, or damage to supporting structures. Overvoltages 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 overvoltages due to lightning or switching transients. Short circuits due to insulation failure between adjacent turns of the same winding may result from suddenly applied overvoltages. Major insulation may fail, permitting arcs to be established between primary and secondary windings or between a winding and grounded metal parts such as the core or tank.Generators may fail due to breakdown of the insulation between adjacent turns in the same slot, resulting in a short circuit in a single turn of the generator. Insulation breakdown may also occur between one of the windings and the grounded steel structure in which the coils are embedded. Breakdown between different windings lying in the same slot results in shortcircuiting extensive sections of machine.Balanced threephase faults, like balanced threephase loads, may be handled on a linetoneutral basis or on an equivalent singlephase basis. Problems may be solved either in terms of volts, amperes, and ohms. The handling of faults on singlephase lines is of course identical to the method of handling threephase faults on an equivalent singlephase basis. Voltage transformersVoltage transformers are used with voltmeters, wattmeters, watthour meters, powerfactor meters, frequency meters, synchroscopes and synchronizing apparatus, protective and regulating relays, and the novoltage and overvoltage trip coils of automatic circuit breakers. One transformer can be used for a number of instruments at the same time if the total current taken by the instruments does not exceed that for which the transformer is designed and pensated.Voltage transformers are generally designed for a capacity of about 200 voltamp. There are two causes of errors in voltage transformers, namely, ratio error and phaseangle error. The part of these errors due to the exciting current is constant for any particular voltage. It can be reduced to a minimum by choosing the best quality of iron and working it at a low magnetic density. The part of the errors due to the load current varies directly with the load and can be minimized by making the resistance of the windings very slow.Voltage transformers are pensated for their iron losses at rated voltage. When used on some other voltage, either higher or lower, an error is introduced. In general this error will not be more than percent of rated voltage. A voltage transformer should never be used on a circuit whose voltage is more than 10 percent above the rated voltage of the transformer.The secondary terminals of a voltage transformer should never be shortcircuited, a heavy current will flow which, if continued, will burn out the windings. In order to protect the system against sustained short circuits in the transformer circuit, it is generally recognized as good practice to introduce into the primary circuit a resister and fuse, these been connected in series. The resistors are designed to limit the current to about 20 to 40 amp., while the fuses are designed to break such current. In normal operation the current which the resistor carries is only the very small primary current of the voltage transformer, and the drop in voltage that they cause is inappreciable.Current transformersCurrent transformers are used with ammeters, wattmeters, powerfactor meters, watthour meters, pensators, protective and regulating relays, and the trip coil of circuit breakers. One current transformer can be used to operate not to exceed that for which the transformer is designed and pensated.The current transformer is connected directly in series with the line, and usually has a fixed number of instruments in the secondary. A rise or fall in the line current requires a corresponding rise or fall in the secondary voltage to force the secondary current through the impedance of the meter load. the magnetic flux in the iron, which supplies the voltage, thus follows the rise and fall of the primary or line current.The instruments connected in the secondary circuit of the transformer are placed in series, so that the secondary current will pass through each instrument. As the instrument are added, higher voltage is required to force the current through the instruments. This requires a high magnetic density in the iron. A higher magnetic density increases both the iron loss and the magnetizing cur