How to detect cable broken core
Inductive electric pen and digital multimeter can be used; adapt to cable without metal armor and steel strip shield; pay attention to prevent electric shock, test station and terminal connection when rewinding equipment.
1. Hang the conductor core of the cable and ensure that it will not cause short circuit and electric shock accidents; at the same time ensure that the cable is far away from the grounding body (such as the ground, equipment, etc.);
2. Select the intact insulation core in the cable, connect the 220VAC phase line (fire line), and not ground the line;
3. If an inductive electric pen is used, the finger touches the inductive contact on the electric pen, and the electric pen is tested for normality outside the insulating layer of the charged body. If using a digital multimeter and placing the multimeter in a 20 or 200 mV position, place a thin plastic insulating sleeve on the red test pen and hold the black test lead with the hand; test outside the clear charged body insulation layer and read; then remove The charged body is far away and reads; comparing the difference between the two readings, usually there should be a higher reading on the charged body, such as 0.4mV, and lower from the charged body, such as 0.15mV; remember this feature, you can I started testing.
4. Test the cable along the cable. When the indicator light of the induction pen is darkened, or the reading of the multimeter is significantly reduced, the change point is the break point.
5, the test is completed, pay attention to discharge.
When there is a metal armor layer such as a copper strip or a steel strip outside the cable, the induction method cannot be detected. In this case, the capacitance method is used; it is suitable for all cables; when using the capacitance method, the principle of the capacitance test is first understood - when testing the capacitance, The AC/pulse signal is used in the test loop, that is, the AC voltage divider is measured or the capacitor body (two mutually insulated metal poles) is charged and discharged, and the accumulated power on the capacitor body is tested and converted into a capacitance reading.
Capacitance method, which may be caused by the winding of the cable, the inductance of the insulated cores, the resistance of the non-conductor (such as steel strip), the stray capacitance between the conductors, etc., affecting the accuracy; The inductance is small, it can be ignored; the resistance has little effect on the measurement capacitance, but the difference between the conductor and the steel strip connection and the unconnected capacitance change is not large, but the stray capacitance has a great influence. : The capacitance between the intact core and the steel strip is 117nF. The other cores are connected to the steel strip. The result is still 117nF, and there is 72nF between the two cores.
For convenience of explanation, it is assumed that the cable is a 2-core steel tape armor, and one of the cores has a cable with one break point;
The specific practices are as follows:
1. Hang all the insulated core conductors, armor layers, etc. at both ends of the cable;
2. Measure the capacitance values of the insulated core and the broken core insulating core on the steel strip (or the third core intact insulating core) at both ends, and record the values; at this time, the corresponding ends of the intact insulating core are measured. The capacitance value should be very close; the capacitance value at both ends of the same core should be slightly larger than the insulation core capacitance at the same position, indicating that there is only one breakpoint, or multiple breakpoints but the distance is very close; if both ends of the same core If the capacitance values are added less than the insulation core capacitance value at the same position, it means that there are at least two breakpoints;
Note: In theory, if there is only one breakpoint or multiple breakpoints but very close, the sum of the capacitance values at both ends should be greater than the capacitance value of the insulated core at the same position, and the number varies depending on the cable. See the theoretical analysis below.
3. According to the calculation of the capacitance values of the broken core insulation core and the intact insulation core, the lengths of the two ends are respectively obtained. At this time, the length and the actual length may be different, and the next step is to re-correct; but the two cores do not have the cable, it cannot be Make corrections.
4. If the sum of the 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; the value is determined by the capacitance value of the broken core insulating core, and the difference is assigned, and the long segment is determined by the long segment. The short segment is corrected for the short segment, and the actual position of the breakpoint is obtained.
This method is used for the core of the cable core, which requires that the core is not covered with a metal layer and there is no extrusion coating. In the search, first use the capacitor to find the approximate position of the cable break, and then use the induced voltage method to accurately find the disconnection point. This method is very simple and fast, and it is also the most used method. Inductive voltage method is to connect 650V AC voltage to one end of the broken core of the cable, ground the other end and other cores, and then test with an inductive pen that can send optical signals. When the sensor pen slides from the broken core during the test, The signal changes so that the cable break point can be accurately found.
This method is for a completed cable or a cable core that has been coated with a metal layer. The method of searching is to first burn the insulation at the broken core of the flame-retardant cable with a constant current source, and then accurately locate the fault point by using the bridge method fault locator, which is also a method frequently used.