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Whether it is a high-voltage cable or a low-voltage cable, faults often occur due to lines, overloaded operation, insulation aging, or external force damage during construction, installation, and operation. Zhili Electric is one of the professional manufacturers of cable fault testers. This article introduces you to several practical measurement methods for cable fault points so that you can easily find the fault points of cables.
The cable fault is divided into three types: grounding, line, and disconnection. Three-core cable fault types mainly include the following aspects: one-core or two-core contact; two-phase core wire; three-phase core wire complete line; one-phase core wire breakage or polyphase breakage. For a direct line or disconnection fault multimeter can be directly measured and judged, for indirect lines and ground faults, using megohmmeter remote measurement of insulation resistance between cores or core to ground insulation resistance, according to its resistance value can be determined type of failure.
Four ways to find cable faults
1. Sound measurement method: The so-called sound measurement method is based on the sound of faulty cable discharge. This method is more effective for high-voltage cable cores for insulation layer flashover discharge. The equipment used in this method is a DC voltage transformer.
When the capacitor is charged to a certain voltage value, the ball gap discharges the cable fault core wire. At the fault, the cable core wire discharges the insulating layer to produce a sparking sound of “nozzle and nourishmentâ€. In order to bury the cable, you must first determine and mark the direction of the cable. Then when the noise noise is minimal, use an audiophile or medical stethoscope to search for it. When looking up, place the pickup close to the ground and slowly move along the direction of the cable. When you hear the sound of the “zigand and sizzle†discharge, it is the point of failure. Use this method must pay attention to safety, in the end of the test equipment and the end of the cable should be set up to monitor.
2. Bridge method: The bridge method is to measure the DC resistance value of the cable core by two-arm bridge, then accurately measure the actual length of the cable, and calculate the fault point according to the proportional relationship between the cable length and the resistance. This method determines that the error is generally not more than 3m for faults where the direct line or line point contact resistance between cable cores is less than 1Ω. For faults where the contact resistance at the fault point is greater than 1Ω, the resistance can be reduced to 1Ω by increasing the voltage through burning. Below, measure again according to this method.
First, the resistance R1 between the cores a and b is measured, and R1=2Rx+R, where Rx is a phase resistance value of a phase or b phase to the failure point, and R is the contact resistance of the line contact. At the other end of the cable, the DC resistance R2 between the a' and b' cores is measured. Then R2 = 2R(LX) + R, where R(LX) is the a' phase or the b' phase core is faulty. The resistance value of one phase of the point, after measuring R1 and R2, connect the b′ and c′ lines, and measure the DC resistance between the two phases of the cores of b and c. Then, 1/2 of the resistance is each phase. The resistance value of the core wire is represented by RL, RL=Rx+R(LX), and the contact resistance value of the fault point can be obtained: R=R1+R2-2RL, therefore, the resistance value of the core wire on both sides of the fault point The following formula can be used: Rx = (R1-R) / 2, R (LX) = (R2-R) / 2. After the three values ​​of Rx, R(LX), and RL are determined, the distance X from the fault point to the end of the cable or (LX) can be obtained by a proportional formula: X=(RX/RL)L, (LX)=(R (LX)/RL)L, where L is the total length of the cable.
When using the bridge method, the accuracy of measurement shall be ensured. The bridge connection line shall be as far as possible, and the wire diameter shall be large enough. The connection with the cable core shall be crimped or welded. The decimal places shall be retained during the calculation.
3. Capacitance amperometry: When the cable is in operation, capacitance exists between the cores and the core to the ground. The capacitance is evenly distributed.
The capacitance is linearly proportional to the length of the cable. Capacitance amperometry is based on this principle. It is very accurate in the determination of cable breakage.
Measurement steps:
First, measure the value of the capacitance current of each phase of the core wire (which should be equal to the applied voltage) Ia, Ib, and Ic at the head end of the cable.
At the end of the cable, the values ​​of the capacitance currents Ia', Ib', and Ic' of the core of each phase are measured to check the ratio of the capacitance of the perfect core to the disconnected core, and the approximate point of the disconnection distance can be preliminarily determined.
According to the capacitance calculation formula C=1/2Ï€fU, C and I are proportional to voltage U and frequency f. Since the f-frequency of the power-frequency voltage does not change, the applied voltage must remain constant during the measurement, and the ratio of the capacitance current is the ratio of the capacitance. Let the total length of the cable be L, and the distance of the broken point of the core line be X, then Ia/Ic=L/X, X=(Ic/Ia)L.
During the measurement process, as long as the voltage is kept constant, the ammeter reading is accurate and the total length of the cable is measured accurately. The measurement error is relatively small.
4, zero potential method: zero potential method is the potential comparison method, it is adapted to the length of the cable core to the ground fault, using this method is simple and accurate measurement, does not require precision instruments and complex calculations. The principle of measurement is as follows: Connect the cable fault core in parallel with the equal length of the comparison conductor. When applying the voltage E at both ends, it is equivalent to connecting the power at both ends of the two parallel uniform resistance wires. At this time, any resistance wire The potential difference between one point and the corresponding point on the other resistance wire must be zero. Conversely, the two points where the potential difference is zero must be the corresponding point. Because the negative terminal of the microvoltmeter is grounded, and the point of failure of the cable is equalized, therefore, when the positive terminal of the microvoltmeter moves on the comparative lead to the point where the indicated value is zero and the fault point is equal, that is, the corresponding point of the fault point.
Measurement steps:
Firstly connect the battery E to the core wires of phase b and c, and then lay a comparative wire S equal to the length of the faulty cable on the ground. The bare wire or bare aluminum wire should be used. Intermediate connector.
Ground the negative pole of the microvoltmeter and the positive pole to a long, flexible wire. The other end of the wire requires full contact when sliding on the comparative wire. Close the knife switch K and slide the end of the soft wire on the comparative wire. When the microvoltmeter indicates zero, it is the position of the cable fault point.
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Thank you for your attention to our products. In addition to this product introduction, the company also has insulation ladders, high-voltage insulation mats, cable fault testers, high-voltage measuring instruments, insulation boots gloves pressure test equipment, power safety equipment cabinets, etc. Introduction, if you are interested in our products, welcome to inquire. Thank you!
Cable fault types and methods