I. induction method
It can be used as induction pencil and digital multimeter; it is suitable for cables without metal armouring and steel belt shielding; it is necessary to pay attention to prevent electric shock, test place and end connection when rewinding equipment, etc.
1) hang the conductor core of the cable in the air, and make sure that it will not cause road and electric shock accidents; together, make sure that the cable is as far away from the grounding body (such as ground, equipment, etc.) as possible;
2) select the intact insulation core in the cable, connect the phase line (front) of 220VAC without grounding wire;
3) if the inductive pen is used, touch the inductive contact on the pen with fingers, and test whether the pen is normal outside the insulating layer of the clear electrified body. If a digital multimeter is used, put a thin plastic insulating sleeve on the red probe and hold the black probe with your hand; test outside the clear insulating layer of the electrified body and read; then move away from the electrified body in a far direction than the difference between the two readings. Generally, there should be a higher reading on the electrified body, such as 0.4mv, and a lower reading away from the electrified body, such as 0.15mv; remember this feature, it can Enough to start the test.
4) close to the cable and test along the cable. When the indicator light of the inductive pen is dimmed or the reading of the multimeter drops obviously, the change point is the breakpoint.
5) after the test, pay attention to the discharge!
II. Capacitance method
When there is metal armor layer such as copper strip or steel strip outside the cable, the induction method cannot be checked. At this moment, the capacitance method is used; it is suitable for all cables; when the capacitance method is used, the communication / pulse signal is used in the test circuit, that is, the communication partial voltage is measured or the capacitance (two insulated metal poles) is charged and discharged to test the capacitance volume. Convert to capacitance reading.
Capacitance method, maybe the inductance formed by the winding of cables and insulation cores together, the resistance of non-good conductor (such as steel belt), stray capacitance between conductors, etc., have little influence on the inductance, which can be ignored; the resistance has little influence on the capacitance measurement, but the change of connecting and non connecting capacitance between conductor and steel belt is not the same, which can also be ignored; but stray capacitance has a shadow. Experiment: the capacitance between the core free and the steel strip is 117nf. When the other cores are connected with the steel strip, the measurement result is still 117nf, while the capacitance between the two cores is 72nf.
For the convenience of illustration, it is assumed that the cable is 2-core steel tape armored cable, in which 1 core has 1 breakpoint. The specific method is as follows:
1) suspend all insulated core conductors, armor layers, etc. at both ends of the cable;
2) measure the capacitance value of the steel strip (or the third core without missing insulating core) between the non missing insulating core and the broken insulating core at both ends, and record the value; at this moment, the capacitance value of the corresponding non missing insulating core shall be very close; the sum of the capacitance values of the two ends of the same broken core shall be slightly greater than the capacitance value of the non missing insulating core in the same direction, so as long as there is one or more broken points, but the interval is very broken. If the sum of the two capacitance values is less than the capacitance value of the non defective insulating core in the same direction, it is clarified that there are at least two breakpoints; note: theoretically, if only one or more breakpoints are needed, but the sum of the capacitance values should be greater than the electric value of the non defective insulating core in the same direction.
How much depends on the cable, see the theoretical analysis behind.
3) according to the capacitance value of the broken core insulation core and the non missing core insulation core, the length of the two ends can be obtained separately. At this moment, the length may be different from the actual length. In the next step, the cable with non Armored Core cannot be calibrated.
4) if the sum of accounting length is greater than the practical length, the extra length value is negative, and if it is less than the practical length, it is positive; moreover, if the capacitance value of the broken core insulation core is used, the difference value is distributed, divided into long sections, and the short section is corrected, then the practical direction of the breakpoint is obtained.