How to use a digital multimeter to determine the breakpoint of wires and cables?

Everyone should be familiar with digital multimeters. In addition to measuring basic parameters such as voltage, current, resistance, capacitance and transistors, it can also be used in various ways to further expand its functions and achieve the purpose of a multi-purpose meter. But did you know that a digital multimeter can also be used to determine the breakpoints of wires and cables?

When a cable or cable breaks, the exact location of the break is difficult to determine due to the outer insulation. This problem can be easily solved with a digital multimeter.

specific method:

Connect one end of the wire (cable) with a breakpoint to the live wire of the 220V mains power supply, and leave the other end floating. Pull the digital multimeter to AC2V, start from the live wire access end of the wire (cable), hold the tip 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 The voltage value displayed on the screen is approximately 0.445V (measured by DT890D meter). When the red test lead moves somewhere, the voltage displayed on the display suddenly drops to 0.0 volts (about one-tenth of the original voltage). About 15cm from this position forward (the live wire access end) is the wire ( Cable) break point.

When using this method to check the shielded wire, if only the core wire is broken but the shielding layer is not broken, this method is powerless.

The core broken fault point can be detected by the following methods:

induction method  

Inductive pens and digital multimeters are available;

Suitable for cables without metal armor and steel tape shielding;

Pay attention to prevent electric shock at the testing place and the terminal connection point when rewinding the equipment.

specific methods:

1. Hang the conductor core of the cable in the air to ensure that it will not cause road accidents or 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 intact insulated core in the cable and connect the 220VAC phase wire (live wire) without grounding the wire;

3. If an induction pen is used, touch the sensing contact on the pen with your finger, and test whether the pen is normal outside the insulating layer of the charged body. If you use a digital multimeter, set the multimeter to 20 or 200mV, put a thin plastic insulating sleeve on the red test lead, hold the black test lead with your hand, test outside the insulation layer of the clearly charged body, and take a reading; then move it away Place the charged body far away and take a reading; compare the difference between the two readings. Usually there should be a higher reading on the charged body, such as 0.4mV, and a lower reading far away from the charged body, such as 0.15mV. Remember this feature, you can Start testing.

4. Test closely along the cable. When you find that the indicator light of the induction pen dims or the multimeter reading drops significantly, the change 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. In this case, the capacitance method is used; it is suitable for all cables; when using the capacitance method, first understand the principle of capacitance testing - when testing the capacitance, The AC/pulse signal is used in the test circuit, that is, the AC partial voltage is measured or the accumulated electricity on the capacitor body (two mutually insulated metal poles) is charged and discharged, and then converted into a capacitance reading.

The accuracy of the capacitance method may be affected by the inductance caused by the cable being wound in a circle and the insulating cores being wound together, the resistance of bad 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. There is only a small difference in the capacitance between connecting the conductor to the steel strip and not connecting it, so it can be ignored; but the stray capacitance has a greater impact, so we did an experiment : The capacitance between the intact core and the steel strip is 117nF. When other cores are connected to the steel strip, the measurement 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 steps are as follows:

1. Suspend all insulated core conductors, armor layers, etc. at both ends of the cable;

2. Measure the capacitance values ​​of the intact insulated core and the broken core insulated core to the steel strip (or the third core intact insulated core) at both ends, and record the values; at this time, the values ​​measured at both ends of the corresponding intact insulated core The capacitance value 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 insulated core at the same position, which means 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 insulated core at the same position, it means that there are at least two breakpoints;

Note: Theoretically, if there is only one breakpoint or multiple breakpoints but they are very close, the sum of the capacitance values ​​at both ends should be greater than the capacitance value of the intact insulated core at the same location. The amount varies depending on the cable. See the theoretical analysis below.

3. Compare and calculate the capacitance values ​​of the broken core insulated core and the intact insulated core to obtain the lengths of both ends. At this time, the length may be different from the actual length. Recalibrate it in the next step; but if the two-core cable is not armored, it cannot Make corrections. 

4. If the sum of the calculated lengths is greater than the actual length, the excess length value is negative, and if it is less than the actual length, it is positive; then use the capacitance value of the broken core insulated core, distribute the difference, and correct the long segment by the long segment. , short segment corrects the short segment, and the actual location of the breakpoint is obtained.