Grounding or short circuit fault, line break fault, line break and grounding fault and flashover fault, etc. Fault judgment method The method to determine the cable fault type is to use an insulation resistance meter to measure the insulation resistance of each phase at one end of the line. The fault type is generally determined according to the following situations: (1) When the insulation resistance of one or more cores of the cable to the ground, or the insulation resistance between cores is lower than 100 kilohms, it is a low resistance grounding or short circuit fault. (2) When the insulation resistance of one or more cores of the cable to the ground, or the insulation resistance between cores is much lower than the normal value but higher than 100 kilohms, it is a high resistance grounding fault. (3) When the insulation resistance of one or more cores of the cable to the ground is high or normal, a conductor continuity test should be carried out to check whether there is a broken wire. If there is, it is a broken wire fault. (4) When one or more cores of the cable are discontinuous and grounded through a resistor, it is a broken wire and grounding fault.
(5) Flashover faults often occur during preventive withstand voltage tests, and the locations of occurrence are mostly at the cable terminals and intermediate joints. Flashovers sometimes occur repeatedly, with intervals of several seconds to minutes between each occurrence.
Fault testing methods
In the past, the instruments and equipment used included the QF1-A cable detector, the DLG-1 flash tester, the cable path meter, and the fault location meter. Currently, the most popular testing method is the flash test method, which includes impulse flash and direct flash, with the impulse flash method being the most commonly used. The impulse test has high accuracy, is simple to operate, and is safe and reliable. The equipment mainly consists of two parts, namely a high-voltage generator and a current pulse meter. The high-voltage generator is used to generate DC high voltage or impulse high voltage, which is applied to the faulty cable, forcing the fault point to discharge and generate a reflected signal. The current pulse meter is used to pick up the reflected signal to measure the fault distance or directly use a low-voltage pulse to measure open circuit, short circuit or low-resistance faults. The following is a brief description of the test method based on the resistance of the fault point:
(1) When the resistance of the fault point is equal to infinity, it is easy to find the open circuit fault by measuring with the low-voltage pulse method. Generally speaking, pure open circuit faults are not common. Usually, the open circuit fault is a combination of high-resistance faults relative to ground or between phases and low-resistance faults relative to ground or between phases.
(2) When the resistance of the fault point is equal to zero, it is easy to find the short circuit fault by measuring with the low-voltage pulse method, but such faults are rarely encountered in actual work.
(3) When the resistance of the fault point is greater than zero and less than 100 kΩ, it is easy to find the low-resistance fault by measuring with the low-voltage pulse method.
(4) Flashover faults can be measured by the direct flash method. This type of fault generally exists inside the connector. The resistance of the fault point is greater than 100 kΩ, but the value varies greatly and is uncertain each time it is measured.
(5) High-resistance faults can be measured by the flash method. The resistance of the fault point is greater than 100 kΩ and the value is certain. Generally, when the test current is greater than 15 mA, the test waveforms are repeatable and can overlap, and a waveform has one transmission and three reflections with gradually decreasing pulse amplitude, the measured distance is the distance from the fault point to the cable test end. Otherwise, it is the distance from the fault point to the other end of the cable test.
To improve cable fault testing technology, different methods should be adopted for different fault properties. New technologies and equipment should be continuously introduced, while also leveraging experience and developing new functions with new equipment. For example, the current testing technology uses audio signals to transmit to the cable and receive them at the fault point, and the SDC series of highly intelligent cable fault detectors, such as the T16/910 cable fault tester, can accurately locate the fault point. These devices can control measurement errors to within tens of centimeters, directly identifying the fault point for treatment and improving fault location efficiency.
