It's not easy to get a monitoring project, the line is finished and the front camera is installed.A power supply, the image is not coming out.By the way, a look of confusion.

Do how?It's no use saying anything else.Find out why: power, cable, setting, and so on.

In the circuit part, sometimes the engineering treasure is helpless, why?The power supply circuit didn't know which one was cut off.Actually in the process of construction, for the quality of the power supply we are generally use multimeter test first on and off, it can effectively eliminate the early stage of the wiring cable fault.

But what if there is a real break in the power supply?In line?It's a little unrealistic.So today we're learning how to check the line's breakpoints through the common digital multimeter.

We commonly used digital multimeter, in addition it can be voltage, current, resistance, capacitance and the transistor measuring basic parameters, such as, can also through the flexible use, to further expand its function, achieve the goal of one table is multi-purpose.

So how do we use the digital multimeter to determine the breakpoints of wire cables?

When cable or cable bolt failure occurs, because of the external insulation skin package, make the exact location of bolt are difficult to determine, using a digital multimeter can be easily fix this problem.

One way to do this is to measure the line resistance, which is the most common, but it's a problem.Constant intercepting cables are required for testing.

There is another way of doing this: one end of a wire (cable) that has a break point is connected to the fire line of 220V, and the other is dangling (safety is the key).The digital multimeter to pull AC2V block, since the wire access end of the wire (cable), with one hand hold black pens and pen, another hand move the red pens and slowly along the wire insulation skin, at this time display shows the voltage value is about 0.445 V (DT890D table measured).Red pens and move to somewhere, the display shows the voltage suddenly dropped to 0.0 a few volts (about one over ten of the original voltage), from the position forward about 15 cm (wire) access to the place where is wire (cable) breakpoint.

But this method is impotent if it is only when the core line is broken and the shield is not broken.

The following methods are mainly applicable to the testing of cables.

Fault-core failure points have the following methods for testing:

A, induction method,

Available induction pen and digital multimeter;Used for cables without metal armoring and steel band shielding;Be careful to prevent electrical shocks, test points, and the end of the device. 

The specific approach:

Suspending the conductor's core of the cable and ensuring that no short circuit or electric shocks occur;Also ensure that the cable is as far away from ground as possible, such as ground, equipment, etc.

Select the perfect insulated core in the cable and connect the 220VAC phase line (fire line).

3, if you use the induction pen, the finger contacts the contact point on the pen, and it is normal to test the electric pen outside the insulating layer of the electrically charged body.If you use the digital multimeter, put the multimeter in the 20 or 200mV, you will put the thinner plastic insulation on the red pen and hold the black marker in your hand.Testing and reading in a clear, electrically charged insulator.And then move away from the charged body and take a reading;The difference between the two readings is usually high on the charged body, such as 0.4 mV, while away from the charged body, such as 0.15 mV.With this feature in mind, you can start testing.

To test the cable along the cable, the change is the break point when the sensing electric pen is dimmed, or the multimeter reading is clearly decreasing.

Test completion, attention discharge.

Second, the capacitance method

When the metal armoring layer, such as a copper belt or steel belt, is outside the cable, the induction method cannot be tested, and the capacitor method is applied at this time.Adapted to all cables;When using capacitance method to understand the principle of capacitance test - test capacitance, the test circuit is used in the communication/pulse signal, namely measuring ac partial pressure or by means of capacitance body (two mutual insulated metal) for charging and discharging, test capacitance on the accumulation of power and converted into electricity capacity of the readings.

Capacitance method, which may be due to cable around the circle shape, the insulation core around together to form the inductor, a good conductor, such as steel belt resistance and stray capacitance between the conductive body and affect the accuracy;The inductance in this case is very small.The effect of resistance on the capacitance is small, but the difference between the conductor and the steel band connection and the unconnected capacitance varies, and is not negligible;But is greatly influenced by stray capacitance, made a test: intact core with steel belt between capacitance is 117 nf, connect the other core with steel belt, or 117 nf measure results, and 72 nf between two cores.

For the sake of explanation, suppose the cable is 2 steel tape armoring, one of which has 1 breakpoint cable;

Here's how to do this:

1, suspension of all insulation core conductors, armor layer, etc.

The capacitance of the steel band (or the third core intact insulation) of the steel band (or the third core) of the insulated core, which is measured in both ends, and the numerical value;At this point, the capacitive value of the corresponding insulation core should be very close;The same on both ends of the broken core capacitance values should be slightly bigger than the sound insulation core capacitance value in a similar position, then only a breakpoint, or multiple breakpoints but very close;If the capacitive value of the two ends of the same core is less than that of the same location, it is indicated that there are at least two breakpoints.

Note: in theory, if only a breakpoint or multiple breakpoints but very close to both ends of capacitance value should be greater than the sum of the same position of sound insulation core capacitance value, how much is different because of different cables, see the back of the theoretical analysis.

3, according to the broken core insulation core and sound insulation core calculation, comparing the capacitance respectively on both ends of the length, the length and the actual length may differ, the next step in correction;But two unarmored cables cannot be corrected.

If the length of the calculation is greater than the actual length, the length is negative, if less than the actual length is positive.Revalue the capacitor value of the broken core insulation core, the difference value is allocated, the long section of the length of the section is fixed, the short segment is modified short, and the actual position of the break point is obtained.