Cable fault detector is mainly a testing instrument that promptly identifies cable fault points, quickly eliminates faults, and restores electrical equipment in the line to normal working condition.  

Generally speaking, a cable is a rope-like conductor made of several or several groups of wires, each group twisted together with at least two wires, with each group insulated from each other, often twisted around a central core, and the whole covered with a highly insulating layer. Common cables in power systems include power cables and control cables, among which power cables are used to transmit and distribute high-power electrical energy. The following introduces several methods for testing cable fault detectors.  

I. Basic testing methods for cable faults with cable fault detectors  

After a cable fault occurs, first use a megger above 1500V or a high-resistance meter to determine the fault type, then use different instruments and methods for preliminary fault location, and finally use the pinpointing method to accurately determine the fault point. There are two precise measurement methods for the fault point.  

One is the acoustic method, whose principle is to use high-voltage pulses to cause discharge at the fault point, producing discharge sounds. A sensor on the ground receives these discharge sounds to determine the precise location of the fault point.  

The other is the induction method, whose principle is that when audio-frequency current passes through the cable core, electromagnetic waves exist around the cable. Therefore, carrying an electromagnetic induction receiver and walking along the line, the audio of the electromagnetic waves can be heard. When the audio-frequency current reaches the fault point, the current changes abruptly, and the audio frequency of the electromagnetic waves changes abruptly. This method is very convenient for finding low-resistance short-circuit faults between phases of broken wires, but it is not suitable for finding high-resistance short-circuit and single-phase grounding faults.  

II. Specific fault types need to be tested according to the following methods.  

1. High-resistance grounding faults: A high-resistance grounding fault in a cable refers to the insulation resistance between the conductor and the aluminum sheath or between conductors being much lower than normal, but greater than 100kΩ, while the core continuity is good.  

Testing methods for high-resistance grounding faults:  

(1) High-voltage bridge method: Due to the high resistance at the fault point, a high-voltage DC power supply is necessary to ensure that the current through the fault point is not too small. The bridge arm resistance is a slide wire resistance of about 3.5Ω divided into 100 equal parts. The voltage applied to the bridge is 10 to 200 kV, and the microammeter indicates 100 to 20 μA. The distance from the measuring end to the fault point can be calculated using the following formula.  

(2) One-shot oscilloscope (Type 711) method: The so-called one-shot oscilloscope method uses a high-voltage one-shot oscilloscope to record the discharge oscillation waveform at the fault point and determine the fault point. The distance to the fault point on the oscilloscope screen can be calculated using the following formula: where V——wave velocity, m/μs; T——oscillation period, μs.  

There are four points to note during measurement: In the same cable, the unmeasured core wires must also be reliably grounded to prevent dangerous induced voltages; during measurement, the voltage should be increased gradually. If the ammeter pointer fluctuates or a flashover fault occurs, stop measurement immediately to avoid burning the instrument; since measurement is carried out under high voltage, reliable insulation from the ground is necessary. Operators should wear insulating gloves, use insulating rods for operation, and maintain a certain distance from the high-voltage leads; when changing connections after measurement with the positive connection method, the voltage must be reduced, the power supply cut off, and only after the residual charge in the circuit is completely discharged can the connections be changed for measurement with the reverse connection method.  

2. Low-resistance grounding faults: Low-resistance grounding faults are divided into single-phase low-resistance grounding faults, two-phase short-circuit faults, and three-phase short-circuit faults.  

(1) Testing three-phase short-circuit fault points:  

When a three-phase short-circuit fault occurs, measurement must use other parallel lines or install temporary lines as a return circuit. When installing temporary lines, the resistance of the line must be accurately measured. It can be calculated using the following formula: where R is the single-wire resistance of the temporary line, and the meanings of other symbols are the same as in formula (2).  

(2) Testing two-phase short-circuit fault points:  

When a two-phase short-circuit fault point occurs, during measurement, any faulty core wire can be used as a grounding wire, and the other faulty core wire is connected to the bridge. The calculation formula and measurement method are the same as for single-phase low-resistance grounding fault points.  

(3) Testing method for single-phase low-resistance grounding fault points:  

A single-phase low-resistance grounding fault in a cable refers to the insulation resistance of one core wire to ground being lower than 100kΩ, while the core continuity is good. This type of fault has strong concealment, so we can use the loop pinpointing method principle for testing. Form a measurement loop with the faulty core wire and another intact core wire, use a bridge for measurement, connect one end with a jumper wire, and connect the other end to the power supply, bridge, or galvanometer. Adjust the bridge resistance to balance the bridge. When the cable core material and cross-section are the same, it can be calculated using the following formula:  

If the positions of the damaged core wire and the good core wire on the bridge are swapped, then there is: where Z——distance from the measuring end to the fault point in m; L——total cable length in m; R1, R2——resistance arms of the bridge.  

Under normal circumstances, the measurement results of these two connections should be the same, with an error generally of 0.1% to 0.2%. If it exceeds this range or X>L/2, the measuring instrument can be moved to the other end of the line for measurement.  

Precautions during measurement: The cross-section of the jumper wire should be close to that of the cable core wire, and the jumper wire should be as short as possible and maintain good contact. The measurement loop should avoid branch boxes or substations as much as possible, and the shorter the better. The DC power supply voltage should not be lower than 1500V. The negative pole of the DC power supply should be connected to the cable conductor through the bridge, and the positive pole should be connected to the inner sheath of the cable and grounded. Operators should stand on insulating pads and place the bridge arm resistance, galvanometer, shunt, etc., on insulating pads.