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Once the insulation of the cable is damaged, causing a fault and power interruption, testing personnel generally need to choose appropriate testing methods and instruments, and search for the fault point according to certain methods. Today, we will talk about the fault location method.  

1. Acoustic measurement method

This method is to identify the fault point by listening to the sound of the discharge when applying high voltage pulses to the faulty cable to discharge the fault point. This method is relatively easy to understand, but due to the noisy external environment and significant interference, it is sometimes difficult to distinguish the true sound of the fault point.  

2. Acoustic magnetic synchronization method

This method also requires applying high-voltage pulses to the faulty cable to discharge the fault point. When a high-voltage pulse signal is applied to a faulty cable, a pulse magnetic field signal is generated around the cable, and at the same time, a discharge sound signal is generated due to the discharge of the fault point. Due to the fast propagation speed of the pulse magnetic field signal, the speed of the sound signal propagation is relatively slow. When they reach the ground, there will be a time difference. Use the probe of the instrument to simultaneously receive the sound and magnetic field signal generated by the discharge of the fault point on the ground, measure this time difference, and move the position of the probe on the ground to find the place with the smallest time difference. The position of the probe is directly below the location of the fault point. The biggest advantage of using this method to locate points is that when discharging at the fault point, the instrument has a clear and intuitive indication, making it easy to eliminate environmental interference; At the same time, this method has high accuracy in fixed-point positioning (<0.1m), and the signal is easy to understand and distinguish.

3. Audio signal method

This method is mainly used to detect the path direction of cables. By adding an audio current signal between two phases of the cable or between the cable and the metal sheath (in the case of a short circuit at the opposite end), and using an audio signal receiver to receive the audio magnetic field signal generated by this audio current, the cable laying path can be identified; When there is a metallic short circuit fault in the middle of the cable, the opposite end does not need to be short circuited. After adding an audio current signal between the two parties where the metallic short circuit occurs, the signal received by the audio signal receiver directly above the fault point will suddenly increase. After passing the fault point, the audio signal will significantly weaken or disappear. This method can be used to find the fault point. This method is mainly used to locate the fault point of metallic short circuit faults or open circuit faults that are relatively close (the presence of distributed capacitance in the line and capacitance at the fault point can enable the transmission of high-frequency audio signals). This method is no longer applicable for short-circuit faults with a fault resistance greater than tens of ohms or open circuit faults with a relatively long distance.  

4. Step voltage method

The method of finding the fault point is to add a DC high-voltage pulse signal between the fault phase and the ground, and use a voltmeter to monitor the magnitude and direction of the step voltage jump between the two points during discharge near the fault point. The advantage of this method is that it can indicate the direction of the fault point, providing strong guidance for testers; But this method can only search for open faults with damaged outer sheaths of buried cables, and is not suitable for searching for closed faults or faults of non buried cables; At the same time, for open faults in buried cables, if the insulation layer outside the metal protective layer is damaged at non fault points, causing multiple discharge channels between the metal protective layer and the ground, using the step voltage method may find many points where the step voltage suddenly changes. This situation is more common in cables of 10KV and below.  

In the process of precise positioning, it is necessary to choose a suitable positioning method reasonably. The positioning method naturally varies depending on the type of fault.