Power cable is an underground equipment. Generally, the soil layer plays a protective role for the cable. However, once a fault occurs, it is also very difficult to find it. It takes a lot of time, which is a great waste of human and material resources. In this case, it is necessary to use cable fault detection equipment, such as power cable fault tester. The initial measurement method of common fault cable is divided into control circuit bridge balance method, bottom pressure single pulse reflector method and flash detector method. In the initial measurement method of cable fault, the appropriate cable fault detector is adopted according to the different characteristics of common faults, The distance between the common fault point and the test case and the relative path of the common fault point are accurately measured and calculated by using the cable route technical document. There are three key methods of initial measurement: control circuit bridge balance method, bottom pressure single pulse reflector method and flash detector method. The method to improve the sensitivity of the galvanometer can refit the DC amplifier in front of the galvanometer, but the gain value of the amplifier is too high, resulting in serious zero drift and the bridge can not be balanced. When the working voltage of the switching power supply of the bridge rises, the compressive strength of the insulating layer of the bridge body to the ground must be relatively increased and the safety precautions for the maintenance and actual operation personnel shall be taken. When the working voltage of the switching power supply rises to a certain value, the common fault resistor is generally unstable. If the resistance of the grounding wire suddenly decreases to zero, High voltage will be applied to the bridge and the galvanometer will be damaged. Common use methods of power cable fault tester. The characteristics and provisions of the bridge method are: it is only applicable to accurately measure the server downtime in the cable line. If there are many other common fault points in the line, it can not be used. The less the jumper wire at the other end of the cable route, the higher the jumper wire. Its cross section shall be close to the cross section of the cable conductor, and the connection shall be tightened to make its contact resistance close to zero. When raising the experimental working voltage for fault test of high resistor grounding device, be sure to give a very safe prompt. After the common fault burn through equipment is used to convert the common fault of flashover into the common fault of grounding device, the power bridge method shall be used for accurate measurement immediately, so as to prevent the insulation layer from being repaired and the common fault point from being closed again. In addition, it is not necessary to burn the resistance of the grounding wire of common faults too low, so as to prevent the difficulty of producing too little sound when using the precise measuring point method. The bottom pressure single pulse reflector method, commonly known as the single pulse method, uses the basic principle of the radio wave reflector caused by the mismatch of wave impedance when the differential signal is dispersed in the cable line. The time of the single pulse wave reflector and the wave speed of the cable are measured on the digital oscilloscope to clarify the distance between the cable fault points. In general, the characteristic impedance in the cable line is not matched with the right point. In addition to the disconnection (lead) of the electrical conductor, short circuit fault and common faults of the grounding device, there are all points with uneven characteristic impedance at the cable joint and where the cable crosses the metal material pipeline, which will also cause the reflection surface of the wave. Be sure to identify them carefully during detection. Especially when the resistance of the grounding wire exceeds about 2 3 times of the wave impedance of the cable, the fluctuation amplitude of the reflecting surface is small, and it is impossible to identify common fault points. The single pulse method is most suitable for measuring the common faults of disconnection. In addition, it is also suitable for measuring the resistance of grounding wire less than 100 & omega; Faulty cable.