How to find the fault point of the rubber sheath cable After the fault type is determined, finding the fault point is not an easy task. Here are some methods to find the fault point.
(1) Zero potential method Zero potential method is the potential comparison method. It adapts to the short cable core ground fault. This method is simple and accurate, does not require precision instruments and complex calculations, and its grounding is shown in Figure 1. Show. The principle of measurement is as follows: Connect the cable fault core in parallel with the equal length of the comparison conductor. When applying voltage VE across b, c, it is equivalent to connecting the power supply at both ends of the two parallel uniform resistance wires. At this time, a resistance wire The potential difference between any point on the resistance wire and the corresponding point on the other resistance wire must be zero. Otherwise, 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 point of failure is equal, that is, the corresponding point of the fault point.
S is a single-phase switch, E is a 6E battery or 4 AA batteries, G is a DC microvoltmeter, and the measurement procedure is as follows:
1) Connect the batteries E to the cores of phase b and c first, 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 have the same cross-section. There can be no intermediate connector.
2) Connect the negative pole of the microvoltmeter to ground and the positive pole to a long soft wire. The other end of the wire requires full contact when sliding on the comparative wire.
3) Close the knife switch S, and slide the end of the soft wire on the comparative wire. When the microvoltmeter indicates zero, the position is the location of the cable fault point.
(2) Bridge method The bridge method uses the double-arm bridge to measure the DC resistance of the cable core, and then accurately measures the actual length of the cable. According to the proportional relationship between the cable length and the resistance, the fault point is calculated. In this method, the error for the direct short-circuit or short-circuit contact resistance between cable cores is less than 1Ω, and the error is generally not more than 3m. For faults with contact resistance greater than 1Ω at the fault point, the method of increasing the voltage to burn-through can be used to reduce the resistance to 1Ω. Below, measure again according to this method.
Measurement circuit shown in Figure 2, first measured between the core wire a and b between the resistance R1, R1 = 2RX + R where RX is a phase or b phase to the failure point of a phase resistance value, only the short contact resistance. Then the bridge is moved to the other end of the cable and the direct current resistance R2 between the a1 and b1 cores is measured. Then R2 = 2R(LX) + R, R(LX) is the phase a1 or b1 core to the point of failure A phase resistance value. After measuring R1 and R2, and then short circuit b1 and c1 according to the circuit shown in Figure 3, to measure the DC resistance between the two phases of the b, c core, then the 1/2 of the organization for the resistance of each phase of the core With RL, RL = RX + R (LX), which can be found at the fault contact resistance value: R = R1 + R2-2RL table, so the fault point on both sides of the core resistance value can be expressed by : 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 found by a proportional formula: X=(RX/RL)L, (LX)=(R (LX)/RL)L, where L is the total length of the cable.
When the bridge method is adopted, the accuracy of measurement shall be ensured. The bridge connection line shall be as short 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 cable In operation, there is a capacitance 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. The principle of measuring, for the determination of broken cable core fault is very accurate. The measurement circuit is shown in Fig. 4. The device uses a 1-2kVA single-phase voltage regulator 2S, 1 to 100mA, and 0.5 AC milliammeter.
