Four practical methods for determining cable fault points

Four practical methods for locating cable fault points

1.  Types and judgments of cable faults Whether it is high-voltage or low-voltage cables, faults are often caused by short circuits, overload operation, insulation aging or external force damage during construction, installation and operation. Cable faults are divided into three categories: grounding, short circuit and disconnection. The main types of three-core cable faults are as follows: one or two cores are in contact; two-phase core wires are short-circuited; three-phase core wires are completely short-circuited; one-phase core wire is disconnected or multiple phases are disconnected. For direct short circuit or disconnection faults, a multimeter can be used to directly measure and judge. For indirect short circuit and pool connection faults, a megohmmeter is used to remotely measure the insulation resistance between core wires or the insulation resistance of the core wire to the ground. The type of fault can be determined based on its resistance value.

2.  Methods for finding cable fault points

1.  Sound measurement method The so-called sound measurement method is to find according to the sound of the faulty cable discharge. This method is more effective for the flashover discharge of the high-voltage cable core wire to the insulation layer. The equipment used in this method is a DC withstand voltage tester. The circuit wiring is shown in Figure 1, where SYB is a high-voltage test transformer, C is a high-voltage capacitor, ZL is a high-voltage rectifier silicon stack, R is a current limiting resistor, Q is a discharge ball gap, and L is a cable core. When the capacitor C is charged to a certain voltage value, the ball gap discharges to the cable fault core, and the cable core discharges to the insulation layer at the fault to produce a "sizzle, sizzle" spark discharge sound. When the noise is the smallest, use an audio amplifier such as a hearing aid or a medical stethoscope to search. When searching, place the pickup close to the ground and move slowly along the direction of the cable. When the "sizzle, sizzle" discharge sound is the largest, the place is the fault point. Be sure to pay attention to safety when using this method, and a dedicated person should be set up to monitor at the test equipment end and the cable end.
2.  2. Bridge method The bridge method is to use a double-arm bridge to measure the DC resistance value of the cable core, and then accurately measure the actual length of the cable, and calculate the fault point according to the positive proportional relationship between the cable length and the resistance. For the fault of direct short circuit between cable cores or contact resistance of short circuit point less than 1Ω, the judgment error of this method is generally not more than 3m. For the fault of contact resistance of fault point greater than 1Ω, the method of burning through with high voltage can be used to reduce the resistance to less than 1Ω, and then measure it according to this method. The measurement circuit is shown in Figure 2. First, measure the resistance R1 between cores a and b, then R1＝2RX＋R, where R is the single-phase resistance value from phase a or phase b to the fault point, and R is the contact resistance of the short-circuit point. Then measure the DC resistance value R2 between cores a' and b' at the other end of the cable, then R2＝2R（L－X）＋R, where R（L－X） is the single-phase resistance value from phase a' and phase b' to the fault point. After measuring R1 and R2, short-circuit b' and C' according to the circuit shown in Figure 3, and measure the DC resistance value between the cores of phases b and c. Then 1/2 of this resistance value is the resistance value of each phase core, represented by RL. RL＝RX＋R（L－X）, from which the contact resistance value of the fault point can be obtained: R＝R1＋R2－2RL. Therefore, the resistance value of the core wires on both sides of the fault point can be expressed by the following formula: RX＝（R1－R）/2，R（L－X）＝（R2－R）/2. After the three values ​​of RX, R（L－X） and RL are determined, the distance X or (L－X) between the fault point and the cable end can be calculated according to the proportional formula: X＝（RX/RL）L，（L－X）＝（R（L－X）/RL）L, where L is the total length of the cable. When using the bridge method, the measurement accuracy should be guaranteed. The bridge connection wire should be as short as possible and have a large enough diameter. The connection with the cable core wire should be crimped or welded, and all decimal places should be retained during the calculation process.

3.  3. Capacitance and current measurement method When the cable is in operation, there is capacitance between the core wires and between the core wires and the ground. The capacitance is evenly distributed, and the capacitance is linearly proportional to the cable length. The capacitance and current measurement method is based on this principle and is very accurate for the measurement of cable core wire break faults. The measurement circuit is shown in Figure 4. The equipment used is a 1-2kVA single-phase voltage regulator, a 0-30V, 0.5-level AC voltmeter, and a 0-100mA, 0.5-level AC milliammeter. Measurement steps: (1) First, measure the capacitance current Ia, Ib, and Ic of each core wire at the beginning of the cable (the applied voltage should be kept equal). (2) Measure the capacitance current Ia', Ib', and Ic' of each phase core wire at the end of the cable to verify the ratio of the capacitance between the intact core wire and the broken core wire, and preliminarily determine the approximate point of the broken wire distance. (3) According to the capacitance calculation formula C = 1/2πfU, it can be known that when the voltage U and frequency f remain unchanged, C is proportional to I; because the power frequency voltage f (frequency) remains unchanged, as long as the applied voltage remains unchanged during measurement, the ratio of capacitance to current is the ratio of capacitance. Assuming the total length of the cable is L and the distance between the core wire break point is x, then Ia/Ic = L/x, x = (Ic/Ia) L. During the measurement process, as long as the voltage remains unchanged, the ammeter reading is accurate, and the total cable length measurement is accurate, the measurement error is relatively small.

4.  Zero potential method The zero potential method is also the potential comparison method. It is suitable for short cable core wire to ground faults. This method is simple and accurate to measure, and does not require precision instruments and complex calculations. Its wiring is shown in Figure 5. The measurement principle is as follows: Connect the cable fault core wire in parallel with the comparison wire of equal length. When voltage E is applied at both ends, it is equivalent to connecting the power supply at both ends of the two parallel uniform resistance wires. At this time, the potential difference between any point on one resistance wire and the corresponding point on the other resistance wire must be zero. Conversely, two points with zero potential difference must be corresponding points. Because the negative pole of the microvoltmeter is grounded, it is at the same potential as the cable fault point. Therefore, when the positive pole of the microvoltmeter moves to the point where the indication is zero on the comparative conductor, it is at the same potential as the fault point, that is, the corresponding point of the fault point. In Figure 5, K is a single-phase knife switch, E is a 6V storage battery or 4 No. 1 dry batteries, and G is a DC microvoltmeter. The measurement steps are as follows: (1) First connect the battery E to the b and c phase core wires, and then lay a comparison wire S on the ground with the same length as the fault cable. The wire should be made of bare copper wire or bare aluminum wire, and its cross-section should be equal and there should be no intermediate joints. (2) Ground the negative pole of the microvoltmeter and connect the positive pole to a longer soft wire. The other end of the wire is required to be fully in contact when sliding on the laid comparison wire. (3) Close the knife switch K and slide the broken end of the soft wire on the comparison wire. The position when the microvoltmeter indicates zero is the position of the cable fault point.