A new method to judge the breaking point of wires

A new method to judge the breaking point of wires and cables

everyone should be familiar with digital multimeter. In addition to measuring basic parameters such as voltage, current, resistance, capacitance and transistor, it can also be used flexibly to further expand its function and achieve the purpose of multi-purpose. But do you know that the digital multimeter can also be used to judge the breakpoint of wires and cables

when there is a broken wire fault in the cable or the inside of the cable, the exact location of the broken wire is difficult to determine due to the wrapping of the external insulating skin. This problem can be easily solved with a digital multimeter

specific method: connect one end of the wire (cable) with the first breakpoint to the live wire of 220V mains, and the other end is suspended. Pull the digital multimeter to the ac2v gear, then start from the hot wire access end of the wire (cable), hold the pen tip of the black probe with one hand, and slowly move the red probe along the insulation of the wire with the other hand. At this time, the voltage value displayed on the display is about 0.445v (measured by dt890d meter). When the red probe moves to a certain place, the voltage displayed on the display screen suddenly drops to a few volts (about one tenth of the original voltage), and the breakpoint of the wire (cable) is about 15cm forward (hot wire access end) from this position

when checking the shielding wire with this method, if only the core wire is broken and the shielding layer is not broken, this method is powerless

the broken core fault point can be detected by the following methods:

induction method can be used with induction pen and digital multimeter

suitable for cables without metal armor and steel tape shielding

pay attention to the prevention of electric shock, the test place and the end connection when rewinding with equipment, etc

specific methods:

1. Hang the conductor core of the cable in the air and ensure that it will not lead to road and electric shock accidents; At the same time, ensure that the cable is as far away from the grounding body (such as the ground, equipment, etc.)

2. Select a good insulating core in the cable, connect the 220VAC phase wire (live wire), and do not connect the ground wire

3. If an inductive pen is used, touch the inductive contact on the pen with your fingers, and test whether the pen is normal outside the insulating layer of the electrified body. If you use a digital multimeter, put the multimeter in the 20 or 200mV gear, put a thin plastic insulating sleeve on the red probe, and hold the black probe with your hand; Test and read outside the insulating layer of the clear electrified body; Then move away from the charged body and read; Compare the difference between the two readings. Usually, there should be a higher reading on the charged body, such as 0.4mv, and a lower reading away from the charged body, such as 0.15mv; With this feature in mind, you can start the test

4. Close to the cable and test along the cable. When it is found that the induction pen indicator has a lot of scientific and technological content, it becomes dark, or the reading of the multimeter decreases significantly, the change is the breakpoint

5. Pay attention to discharge after the test is completed

capacitance method

when there is a metal armor layer such as copper strip or steel strip outside the cable, the induction method cannot detect, and the capacitance method is used at this time; Suitable for all cables; When using the capacitance method, first understand the principle of capacitance test - when testing capacitance, the AC/pulse signal is used in the test circuit, that is, measuring the AC partial voltage or charging and discharging the capacitor (two mutually insulated metal poles). The test electricity requires that the automotive interior materials have the characteristics of low volatility, low odor, low gloss, high wear resistance, soft touch, pollution resistance, etc. the accumulated electricity on the capacitor is converted into the reading of capacitance

capacitance method may affect the accuracy due to the inductance formed by the cable winding in a ring shape, the insulation cores winding together, the resistance of non good conductors (such as steel strip), the stray capacitance between conductors, etc; The inductance is small and can be ignored; The resistance has little effect on the capacitance measurement, but the capacitance change between connecting and unconnecting the conductor and the steel strip is not different, which can also be ignored; However, the stray capacitance has a great impact. A test was carried out: the capacitance between the intact core and the steel strip is 117nf. When other cores are connected with the steel strip, the test result is still 117nf, and there is 72nf between the two cores

for convenience, assume that the cable is 2-core steel tape armored, of which 1 core has 1 breakpoint

the specific methods are as follows:

1. Hang all insulated core conductors and armor layers at both ends of the cable in the air

2. Measure the capacitance of the intact insulating core and the broken insulating core to the steel strip (or the third intact insulating core) at both ends, and record the value; At this time, the capacitance values measured at both ends of the corresponding intact insulating core should be very close; The sum of the capacitance values at both ends of the same broken core should be slightly greater than the capacitance value of the intact insulating core at the same position, which means that there is only one breakpoint, or multiple breakpoints, but the distance is very close; If the sum of the capacitance values at both ends of the same broken core is less than the capacitance value of the intact insulating core at the same position, it indicates that there are at least two breakpoints; Note: theoretically, if there is only one or more breakpoints but they are very close, the sum of the capacitance values at both ends should be greater than the capacitance value of the intact insulating core at the same position, and the amount varies with different cables. See the following theoretical analysis

3. Compare and calculate the capacitance values of the broken insulating core and the intact insulating core, and get the length of both ends respectively. At this time, the length may be different from the actual length, and then correct it again in the next step; However, two core unarmoured cables cannot be corrected

4. If the sum of the calculated length is greater than the actual length, the extra length value is negative, and if it is less than the actual length, it is positive; Then, the capacitance value of the broken insulating core is used to distribute the difference. The long segment is corrected for the long segment, and the short segment is corrected for the short segment, and the actual position of the breakpoint is obtained