How to judge the breakpoint of wire and cable by multimeter
Judging when the core wire is broken and the shielding layer is broken
Our commonly used digital multimeter, in addition to measuring basic parameters such as voltage, current, resistance, capacitance and transistors, can also be used flexibly to further expand its functions and achieve the purpose of multi-purpose.
So how do we use a digital multimeter to judge the breakpoint of a wire and cable?
When there is a disconnection fault inside the cable or the cable, the exact location of the disconnection is not easy to determine due to the wrapping of the outer insulation skin. This problem can be easily solved with a digital multimeter.
One method is disconnection resistance measurement, which is the most commonly used, but troublesome. It is necessary to constantly cut the cable for testing.
In fact, there is another method: connect one end of the wire (cable) with a breakpoint to the live wire of the 220V mains, and leave the other end in the air (pay attention to safety is the key point). Pull the digital multimeter to the AC2V gear, start from the live wire access end of the wire (cable), hold the nib of the black test lead with one hand, and slowly move the red test lead along the insulation of the wire with the other hand, at this time the display shows The voltage value displayed on the screen is about 0.445V (measured by the DT890D meter). When the red test pen moves to a certain place, the voltage displayed on the display suddenly drops to 0.0 volts (about one tenth of the original voltage), and about 15cm forward from this position (the live wire access end) is the wire ( cable) where the breakpoint is located.
But when using this method to check the shielded wire, if only the core wire is broken but the shielding layer is not broken, then this method is powerless.
The following methods are mainly applicable to the testing of cables.
The broken core fault point can be detected by the following methods:
induction method
Available induction pen and digital multimeter;
Suitable for cables without metal armor and steel tape shielding;
Pay attention to prevent electric shock, the test place and the terminal where the terminal is connected to electricity when rewinding with equipment, etc.
specific methods:
1. Suspend 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 as far away from the grounding body (such as the ground, equipment, etc.) as possible;
2. Select a good insulating core in the cable, connect the 220VAC phase line (live line), and not ground the line;
3. If you use an induction electric pen, touch the inductive contacts on the electric pen with your fingers, and test whether the electric pen is normal outside the insulating layer of the charged body. If you use a digital multimeter, put the multimeter in the 20 or 200mV range, put a thin plastic insulating sleeve on the red test lead, and hold the black test lead with your hand;
Test outside the insulating layer of the charged body, and read; 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 far away from the charged body Body is lower, such as 0.15mV; remember this feature, you can start testing.
4. Test along the cable close to the cable. When the indicator light of the induction pen is dimmed or the reading of the multimeter drops obviously, the change point is the breakpoint.
5. After the test is completed, pay attention to discharge.
capacitance method
When there is a metal armor layer such as copper tape or steel tape outside the cable, the induction method cannot be used for detection, and the capacitance method is used at this time;
Adapt to all cables;
When using the capacitance method, first understand the principle of capacitance testing - 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 body (two mutually insulated metal poles) , to test the accumulated electricity on the capacitor body, and convert it into a reading of the capacitance.
Capacitance method, the accuracy may be affected by the inductance formed by winding the cable and the insulation cores together, the resistance of non-good conductors (such as steel strips), and the stray capacitance between conductors; among them The inductance is very small and can be ignored; the resistance has little effect on the measured capacitance, but the difference between the connection of the conductor and the steel strip and the unconnected capacitance is not large, and it can be ignored; but the stray capacitance has a greater influence, and an experiment was done. : The capacitance between the intact core and the steel strip is 117nF, when the other cores are connected to the steel strip, the measured result is still 117nF, while there is 72nF between the two cores.
For the convenience of explanation, it is assumed that the cable is a 2-core steel tape armored cable, one of which has a breakpoint;
The specific method is as follows:
1. Suspend all insulating core conductors and armor layers at both ends of the cable;
2. Measure the capacitance value 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 corresponding two ends of the intact insulating core are measured The capacitance values 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, indicating that there is only one breakpoint, or multiple breakpoints but very close to each other; if the two ends of the same broken core If the sum of the capacitance values is less than the capacitance value of the intact insulating core 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 intact insulating core at the same position, and the amount varies with different cables, see the theoretical analysis later.
3. According to the comparison and calculation of the capacitance value of the broken insulating core and the intact insulating core, the lengths of both ends are obtained respectively. At this time, the length may be different from the actual length, and the next step is to re-calibrate; but the two-core unarmoured cable cannot be used. Make corrections.
4. If the sum of the calculated lengths is greater than the actual length, the value of the excess length is negative, and if it is less than the actual length, it is positive; then use the capacitance value of the broken core insulation core to distribute the difference, and the long segment obtained is corrected for the long segment , the short segment corrects the short segment, and the actual position of the breakpoint is obtained.
