How to quickly judge wire and cable breakpoints?

Apr 13,2019
 Everyone should be familiar with the digital multimeter. In addition to the measurement of basic parameters such as voltage, current, resistance, capacitance and transistor, it can also be modified to make its function further expanded to achieve the purpose of multi-use. But do you know that digital multimeters can also be used to judge wire and cable breakpoints?
When there is a wire breakage inside the cable or cable, the exact position of the wire break is difficult to determine due to the wrapping of the outer insulation skin. This problem can be easily solved with a digital multimeter.
A new method for judging breakpoints in wire and cable
  specific method:
Connect one end of the wire (cable) with a breakpoint to the 220V mains line and the other end to the left. Pull the digital multimeter to the AC2V gear, start from the live wire end of the wire (cable), pinch the tip of the black test pen with one hand, and slowly move the red test pen along the insulation of the wire with the other hand. The voltage displayed on the screen is approximately 0.445V (measured by the DT890D meter). When the red test pen moves to somewhere, the voltage displayed on the display suddenly drops to 0.0 volts (approximately one tenth of the original voltage), from which the wire is about 15 cm forward (the firewire access end). Cable) The breakpoint is located.
When the shield wire is inspected by this method, if the core wire is broken and the shield layer is not broken, this method is powerless.
The core break point has the following methods to detect:
Induction method
Induction pens and digital multimeters are available;
Adapted to cables without metal armor and steel strip shielding;
digital multimeter
Pay attention to prevent electric shock, the test station and the terminal connection when the device is rewinded.
  specific methods:
1. Suspend the conductor core of the cable and ensure that it will not cause road and electric shock accidents; at the same time, ensure that the cable is farther 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 lead 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.
Capacitance method
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, can not be counted; 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, and a test is done. : 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.