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How to determine the break point of wire and cable through a multimeter
It's hard to get a monitoring project, the line is finished, and the front camera is also installed. A power supply, the image did not come out. Rely on and look aggressive.
What about? It is useless to say anything else. Quickly find the reason: power, cables, settings, etc., one by one.
For the circuit part, sometimes engineering treasure is powerless. Why? The power supply circuit does not know which cut off. In fact, in the construction process, for the power supply part, we generally use the multimeter to test the continuity. This can effectively eliminate the early wiring cable faults.
But what about the real disconnection in the power supply? Change line? It's a bit unrealistic. Ever since then, we have learned from each other how to use the commonly used digital multimeter to detect the breakpoint of the cable.
Our commonly used digital multimeter, in addition to measuring voltage, current, resistance, capacitance, and transistor parameters, can also be used through modifications to further expand its functionality to achieve a multi-purpose purpose.
So how do we use digital multimeter to determine the wire and cable breakpoints?
When there is a broken wire inside the cable or cable, the exact location of the broken wire is not easily determined due to the wrapping of the external insulation. A digital multimeter can easily solve this problem.
One method is to measure the resistance of broken wires. This is the most common, but it is more troublesome. The need to constantly cut the cable for testing.
In fact, there is another way: Connect one end of a wire (cable) with a breakpoint to the live line of 220V mains, and the other end to the air (note that safety is the key). Pull the digital multimeter to AC2V block, starting from the hot wire access end of the wire (cable), pinch the nib of the black meter pen with one hand, and slowly move the red lead along the insulation of the wire. The voltage displayed on the screen is approximately 0.445V (measured on the DT890D meter). When the red pen moves to somewhere, the voltage displayed on the display suddenly drops to a few tenths of a volt (approximately one-tenth of the original voltage), and from that point (approximately 15 cm from the live line) is the wire ( Cable) The breakpoint is located.
However, when using this method to inspect the shielded wire, if only the core wire is broken and the shield layer is not broken, this method is powerless.
The following methods are mainly suitable for cable testing.
Broken core fault point has the following methods to detect:
First, induction method
Inductive pens and digital multimeters are available. They are suitable for cables without metal armouring and steel tape shielding; care should be taken to prevent electric shocks, test points, and end connections where the equipment is rewinded.
specific methods:
1. Hang the conductor core of the cable and ensure that it will not cause short circuit and electric shock accident; at the same time, ensure that the cable is far away from the grounding body (such as the ground, equipment, etc.);
2. Select the good insulation core in the cable, connect the 220VAC phase wire (FireWire), not the ground wire;
3. If the induction pen is used, the finger touches the sensing contact on the pen, and the pen is tested to determine whether the pen is normal outside the insulating layer of the live body. If using a digital multimeter and placing the multimeter in the 20 or 200mV range, place a thin plastic insulating sleeve over the red table pen and hold the black meter pen with your hand. Test outside the insulating layer of the live body and take a reading; then remove. The charged body is located far away and reads the reading. When comparing the difference between the two readings, there should be a higher reading on the live body, such as 0.4mV, but far away from the charged body, such as 0.15mV; remember this feature, you can Start the test.
4, close to the cable along the cable test, when the induction lamp is found to be dimmed, or when the multimeter reading decreased significantly, the change is the breakpoint.
5, the test is completed, pay attention to discharge.
Second, the capacitance method
When the cable is covered with a metal armored layer such as a copper tape or a steel tape, the induction method cannot be detected. At this time, the method of capacitance is used; it is applicable to all cables; when the capacitance method is used, 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 to measure the AC partial voltage or to charge and discharge the capacitor body (two mutually insulated metal electrodes), and to test the accumulated electric quantity on the capacitor body and convert it into a capacitance reading.
The capacitance method may affect the accuracy due to the inductance of the cable wound into a loop and the insulating cores are wound together, the resistance of the non-good conductor (such as a steel strip), and the stray capacitance between the electrical conductors; The inductance is very small and can be counted; the resistance has little effect on the measuring capacitance, but the difference between the conductor and the steel strip and the connection of the capacitance is not significant, and it can be ignored; however, the stray capacitance has a greater influence and a test is done. : The capacitance between the intact core and the strip is 117nF, and the other cores are connected to the steel strip, the measurement result is still 117nF, and there is 72nF between the two cores.
For the convenience of explanation, assume that the cable is a 2-core steel tape armor, in which 1 core has a breakpoint cable;
The specific approach is as follows:
1. Hang all insulated core conductors, armored layers, etc. at both ends of the cable;
2. Measure the capacitance values ​​of the insulation core and the core insulation core to the steel strip (or the third core intact insulation core) at both ends and record the values; at this time, the corresponding ends of the intact insulation core are measured. Capacitance values ​​should be very close; the sum of the capacitances at the ends of the same breakage core should be slightly larger than the intact insulation core capacitances at the same position, indicating that there is only one breakpoint, or multiple breakpoints but very close distances; if both ends of the same breakage core If the capacitance value is less than the intact insulating core capacitance value at the same position, there are at least two breakpoints;
Note: Theoretically, 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 location, and how many will vary from cable to cable, see later theoretical analysis.
3. Comparing the capacitance values ​​of the broken core insulation core with the intact insulation core, and obtaining the lengths of the two ends respectively, the length and the actual length may be different at this time, and the next step will be corrected; but the two cores cannot be armored and cannot be cabled. Make corrections.
4. If the sum of the calculated lengths is greater than the actual length, the multi-out length value is negative, if it is less than the actual length, it is positive; and then the value of the capacitance value of the core insulation core is used to distribute the difference, and the long length of the long section is divided. , Short paragraph to correct short section, then get the actual position of the breakpoint.
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