The types and judgment of cable fault
Whether it is a high voltage or low voltage cable, in the construction and installation, operation process often due to short circuit, overload operation, insulation aging or external damage and other causes of failure. Cable fault is divided into three categories of grounding, short circuit and disconnection. Three core cable fault types are mainly in the following aspects: a core or two core contact; two phase core wire short circuit; three-phase core line completely short circuit; a phase core line or multiphase disconnection.
For direct short circuit or disconnection fault with a multimeter direct measurement can judge for non direct short circuit and connecting the battery fault, megohm table telemetry between core line insulation resistance or cored wire insulation resistance to earth, the resistance is used to the fault type judges.
Two, cable fault point search method
1, sound detection method
The so-called sound detection method is based on the sound of the breakdown of the cable to find the sound, the method for the high voltage cable core line on the insulation layer flashover discharge is more effective. The equipment used in this method is DC voltage withstand test machine. Circuit wiring as shown in Figure 1, where the SYB for the high voltage test transformer, C for the high voltage capacitor, ZL for the high voltage silicon rectifier stack, R limit flow resistance, Q for the discharge ball gap, L for the cable core line.
When the capacitor C is charged to a voltage value, the ball gap of cable fault core wire discharge. In fault cable wire of insulating layer discharge produced "AIDS, AIDS, the spark of acoustic, again in the miscellaneous noise when the minimum, with the aid of the hearing aid or medical stethoscope and other audio equipment in your search. Find the pickup close to the ground, along the cable to move slowly, when heard "AIDS, AIDS, put electroacoustic largest, where is the point of failure. Use this method must pay attention to safety, in the test equipment and cable ends should be set up to monitor.
2, bridge method
Bridge method is to measure the DC resistance of the cable core wire, and then accurately measure the actual length of the cable, in accordance with the length of the cable and the positive ratio of resistance, the calculation of the fault point. According to this method for the measurement of the method for cable core wire short circuit or short circuit point contact resistance less than 1 ohm fault judgment error is generally less than 3m, for the contact resistance of the fault point greater than 1 ohm fault, using high voltage burn through the method the resistance is reduced to less than 1 ohm. The measuring circuit is shown in Figure 2. First, the resistance R1 between core wire a and B is measured, then R1 = 2RX + R, where R is a phase resistance value of the a phase or B phase to the fault point, and R is the contact resistance of short contact point. Then on the other end of the cable to measure the a 'and B' between the DC resistance value R2, then R2 = 2R (X - L) + R, R (X - L) for the a 'and B' 'core line to the fault point of a phase resistance value. Finished measuring R1 and R2, and according to figure 3 shows circuit will b 'and C' 'short circuit, measured between B and C two-phase core line DC resistance value is 1 / 2 of the resistance of the resistance of the core line of each phase value, RL said. RL = RXR (LX), the contact resistance of the fault point value: R = R1R22RL. Therefore, the resistance value of the fault point on both sides can be expressed by the following formula: RX = (R - /2) R1, R (X - L) = (R - /2) R2. Rx, R (L x), RL three numerical determined, according to the proportion of formula can for fault location from the cable end of the distance X or (L x): x = (RX/RL) l (L - x) = (R (L x) /RL) l, type L is the total length of the cable.
When the bridge method is used to ensure the accuracy of the measurement, the bridge connecting line should be as short as possible, the diameter should be large enough, and the cable core wire connection to the use of compression or welding arm, the calculation process of small and medium-sized to all retain.
3, capacitive current measurement method
Cable in operation, between the core wire and core wire to ground there is a capacitance and the capacitance is uniformly distributed, the capacitance and the length of cable is linearly proportional to, the capacitance current determination method is based on the principle of determination, for the cable core wire disconnection fault determination is very accurate. The measuring circuit as shown in Figure 4, the use of equipment for 1 ~ 2kVA single phase voltage regulator, 0 ~ 30V, 0.5 AC voltage meter, a 0 ~ 100mA, 0.5 AC Ma Table 1.
(1) first in the first end of the cable to measure the capacitance of each line of the current (should maintain the voltage to maintain the same voltage) Ia, Ib, Ic value.
(2) at the end of the cable is measured per phase core wire capacitance current ia '', IB ', IC' value, to check the intact core wire and the wire core volume ratio, initially determine the approximate point break distance.
(3) according to the capacity calculation formula C = 1 / 2 pi Fu shows, in U voltage and frequency f constant C and I is proportional to, because the voltage frequency f (frequency) unchanged, when measured as long as the guarantee applied voltage is kept constant, capacitive current ratio as the ratio of capacitance. The length of the cable is L, the distance between the core and the line break point is Ia/Ic, then L/x = x, x = (Ic/Ia) L.
In the process of measurement, as long as the voltage is guaranteed, the current meter readings are accurate, the total length of the cable is accurate, and the measurement error is relatively small.
4, zero potential method
The zero potential method is also the potential comparison method, which adapts to the short cable core line to ground fault, the application of this method is simple and accurate measurement, does not require precision instruments and complex calculations, the wiring is shown in figure 5. The measuring principle is as follows: the cable fault core wire is parallel to the length of the equal length, and is equal to the power supply at both ends of the two parallel resistors when the E is pressed at both ends. At this point, the electric potential difference between any point on the resistance wire and the corresponding point on the other resistance wire is bound to be zero. On the other hand, the potential difference between the zero points must corresponding points, because the anode grounding microvoltmeter