In electric power distribution systems, a wide variety of cable faults can occur. The problem may be in a communication circuit or in a power circuit, either in the low- or medium-voltage class.
Regardless of the class of equipment involved or the type of fault, the one common problem is to determine the location of the fault so that repairs can be made.
The vast majority of cable faults encountered in an electric power distribution system occur between conductor and ground. Most fault-locating techniques are made with the circuit deenergized.
In ungrounded or high-resistance grounded, low-voltage systems, however, the occurrence of a single line-to-ground fault will not result in automatic circuit interruption; therefore, the process of locating the fault may be carried out by special procedures with the circuit energized.
Once a line-to-ground fault has occurred, the resistance of the fault path can range from almost zero up to millions of ohms. The fault resistance has a bearing on the method used to locate the failure.
In general, a low-resistance fault can be located more readily than one of high resistance. In some cases, the fault resistance can be reduced by the application of voltage sufficiently high to cause the fault to break down as the excessive current causes the insulation to carbonize.
A wide variety of commercially available equipment and a number of different approaches can be used to locate cable faults. The method used to locate a cable fault depends on the following:
a) Nature of fault
b) Type and voltage rating of cable
c) Value of rapid location of faults
d) Frequency of faults
e) Experience and capability of personnel
Megohmmeter Instrument Test
When the fault resistance is sufÞciently low that it can be detected with a megohmmeter, the cable can be sectionalized and each section tested to determine which contains the fault. This procedure may require that the cable be opened in a number of locations before the fault is isolated to one replaceable section. This could, therefore, involve considerable time and expense, and might result in additional splices. Since splices are often the weakest part of a cable circuit, this method of fault locating may introduce additional failures at a subsequent time.
Conductor Resistance Measurement
This method consists of measuring the resistance of the conductor from the test location to the point of fault by using either the Varley loop or the Murray loop test. Once the resistance of the conductor to the point of fault has been measured, it can be translated into distance by using handbook values of resistance per unit length for the size and conductor material involved, correcting for temperature as required.
This method consists of applying a high-voltage and high-current impulse to the faulted cable. A high-voltage capacitor is charged by a relatively low-current capacity source such as that used for high-potential testing. The capacitor is then discharged across an air gap or by a timed closing contact into the cable. The repeated discharging of the capacitor provides a periodic pulsing of the faulted cable. The maximum impulse voltage should not exceed 50% of the allowable dc cable test voltage since voltage doubling can occur at open circuit ends. Where the cable is accessible, or the fault is located at an accessible position, the fault may be located simply by sound.
A tone signal may be used on energized circuits. A Þxed-frequency signal, generally in the audio frequency range, is imposed on the faulted cable. The cable route is then traced by means of a detector, which consists of a pickup coil, receiver, and a head set or visual display, to the point where the signal leaves the conductor and enters the ground return path.
This class of equipment has its primary application in the low-voltage Þeld and is frequently used for fault location on energized ungrounded circuits. On systems over 600 V, the use of a tone signal for fault location is generally unsatisfactory because of the relatively large capacitance of the cable circuit.
A short-duration, low-energy pulse is imposed on the faulted cable and the time required for propagation to and return from the point of fault is monitored on an oscilloscope. The time is then translated into distance to locate the fault. Although this equipment has been available for a number of years, its major application in the power field has been on long-distance, high-voltage lines.