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          Two methods for determining the fault point of cable core breakage

          2022-05-17 管理員 Read 83

          1、 Induction method

          Can be used as an induction pen and a digital multimeter; Suitable for cables without metal armor and steel tape shielding; When rewinding equipment, attention should be paid to preventing electric shock, testing locations, and end connections.

          Specific methods:

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

          2) Select intact insulated core wires for cables, and connect the 220VAC phase line (front) without grounding wire;

          3) When using an inductive pen, touch the inductive contacts on the pen with your fingers and test whether the pen is functioning properly outside the insulation layer of the transparent charged body. If using a digital multimeter, place a thin plastic insulation sleeve on the red probe, hold the black probe with your hand, test and read outside the transparent insulation layer of the charged body, and then move away from the charged body in a direction that is much greater than the difference between the two readings. Generally speaking, there should be a higher reading on the charged body, such as 0.4mv, and a lower reading, such as 0.15mv, far away from the charged body; Keeping this in mind is enough to start testing.

          4) When the indicator light of the inductive pen dims or the reading of the multimeter drops significantly, the change point is the breakpoint when testing close to the cable and along the cable.

          5) After the experiment, pay attention to discharge!

          Two capacitance method

          When there is a metal armor layer such as copper or steel tape outside the cable, the induction method cannot be checked. At this point, the capacitance method is used, which is applicable to all cables; When using the capacitance method, the testing circuit uses communication/pulse signals, which measure the voltage of the communication part or charge and discharge the capacitors (two insulated metal electrodes), and convert the test capacitance into capacitance readings.

          The capacitance method may be the inductance formed by the winding of cables and insulated core wires, the resistance of non good conductors (such as steel strips), the stray capacitance between conductors, etc., which has a small impact on the inductance and can be ignored; The influence of resistance on capacitance measurement is minimal, but the changes in connecting capacitance and non connecting capacitance between conductors and steel strips are different and can be ignored; But stray capacitors have shadows. Experiment: The capacitance between the coreless wire and the steel strip is 117nf. When other core wires are connected to the steel strip, the measurement result is still 117nf, while the capacitance between the two core wires is 72nf.

          For ease of explanation, it is assumed that the cable is a 2-core steel strip armored cable, with one core having one breakpoint. The specific methods are as follows:

          1) Hang all insulated core wires, armor layers, etc. at both ends of the cable;

          2) Measure the capacitance value of the steel strip (or the third core without missing core insulation wire) between the non missing core insulation wire at both ends and the broken core insulation wire, and record it. At this time, the capacitance value of the corresponding non missing core insulation wire 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 non defective insulation core in the same direction. As long as there are one or more breakpoints, but the interval is very short. If the sum of the two capacitance values is less than the capacitance value of the non defective insulation core in the same direction, it indicates that there are at least two breakpoints; Note: In theory, if only one or more breakpoints are needed, but the sum of capacitance values should be greater than the electrical value of the defect free insulation core, the insulation core in the same direction has defects.

          How much depends on the cable, see theoretical analysis later.

          3) Calculate the lengths of both ends based on the capacitance values of the broken core insulation wire and the non missing core insulation wire. At this point, the length may differ from the actual length. Next, non armored core cables cannot be calibrated.

          4) If the sum of calculated lengths is greater than the actual length, the excess length value is negative; if it is less than the actual length, it is positive; If the capacitance value of the broken core insulation core is used, the difference distribution is divided into long segments, and the short segments are corrected to obtain the actual direction of the break point.



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