Content:
- Methods for determining cable damage in the ground
- Pulse method
- Loop method
- Acoustic method
- Step voltage method
- Induction method
- Finding a break in a hidden wiring in a concrete wall
Methods for determining cable damage in the ground
To find the place of damage to the cable line, it is necessary to understand the specifics and methods of searching. The process must be divided into two stages:
- Search for problem areas along the entire length of the line.
- Search for the accident site on the established section of the route.
In view of the differences between these two stages, the search methods themselves differ and are:
- relative (distance) - these include the impulse and loopback method;
- absolute (topographic) - acoustic, induction and step voltage method.
Well, let's look at all the methods in order.
Pulse method
This method involves searching for damage using an OTDR. Work can be carried out, for example, with the REIS-305 device, which is shown in the photo below.
The operation of the device is based on sending probing pulses of a certain frequency, which, meeting an obstacle in their path, are reflected and returned back to the device. That is, the device is located at one end of the power cable, which is very convenient and practical. To calculate the exact distance to the damage site, you need to use the following formula:
Where, according to the formula, L - cable length from the point of connection of the device to damage, tx - a variable value of the amount of time spent for the impulse to reach the break point and back. υ - the speed with which the pulse follows the cable (for cable lines from 0.4 kV to 10 kV it is 160 m / μs).
In this way, you can detect not only an open circuit in the power cable, but also a short circuit between the cores. To understand what happened, refer to the screen image during the test. The pictures will be like this (short circuit on the left, open circuit on the right):
The tests should be carried out on a completely disconnected line. An example video clearly demonstrates how to use the short circuit location finder:
Instructions for using the ISKRA-3M reflectometer
Loop method
This method is applicable provided that at least one wire in the cable remains intact, or another conductor with intact conductors runs nearby. To find out the distance to the place of damage by the loop method, you need to measure the resistance of the conductors to direct current with the P333 device. This is a DC measuring bridge that looks like this:
Before starting measurements, we connect the end of the whole and damaged core with a short circuit, connect the other two ends according to the scheme:
You can calculate the distance to the point at which the breakage occurred using the following formula:
- R1 - resistance that connects to the whole core;
- R2 - resistance, which is connected to the conductor with a break;
- L is the length of the cable to the point of damage;
- LTo - the length of the entire conductor.
This is perhaps one of the first invented methods used to find the location of a fault, and it is used exclusively for single-phase and two-phase closures. Gradually, they stop using it, due to its laboriousness and large measurement errors.
Acoustic method
It is possible to find a break in the cable using the acoustic method by creating a discharge at the place of damage using a high-voltage pulse generator (in the picture below). At the point of breakage or short circuit, sound vibrations of a certain frequency will appear. Listening quality depends on the type of ground, the distance from the surface to the cable line, and the type of damage. A prerequisite for the operation of the method is the excess of the value of the contact resistance of 40 ohms.
An example of finding a damaged line using an acoustic method is shown in the video:
Application of an acoustic device
Step voltage method
The method is based on the transmission of the current generated by the generator through the cable. It creates a potential difference between two points located in the ground, which can be judged by leakage current at the scene of the accident. To find a point with a reduced insulation resistance, the contact pins are installed as follows - the first one exactly above the running conductor, the second at an angle of 900 a meter from the first.
The point at which the cable is damaged is under the first pin, assuming the signal is at its maximum. Learn more about step voltage you can find out from our article!
Induction method
The method very accurately determines the breakage points, however, its use is associated with cable burn-through. With a large contact resistance, it is necessary to reduce its value by burning, using special devices, for example, installing a VUPK-03-25 burning cable:
The method is based on the transmission of a high-frequency current through the conductor, which forms an electromagnetic field over the cable line. In places of mechanical damage to the track, passing the receiving frame, the sound will change. Thus, the absence of sound indicates a wire break.
The video below clearly demonstrates the location of the emergency site by burning:
Burn cable line
Finding a break in a hidden wiring in a concrete wall
The place of a wire break in a concrete wall will help you find a special device - a locator. It is a combination of a receiver and a generator. This method can be associated with the induction method in the search for cable faults under the ground.
So, it is not difficult to determine the place of the cliff by the locator. The end of the wire, in which there is a break, is connected to a generator, which sends pulses of a certain frequency to it. Passing the frame over the wiring location, the sound generated by the impulses will be clearly heard in the headphones. As soon as the sound disappears, mark this spot on the wall - this will be the point of damage to the wire.
To find an open circuit in the phase wire will also help a contactless voltage indicator. Everything is simple here. We lead the device along the wall until the voltage presence indicator stops lighting. We carry the device several times in a circle in this area of the wall to make sure that we have not left the route of the wires. The absence of a luminescence of the indication will indicate the approximate location of the break.
In conclusion, I would like to note that the locator and non-contact voltage indicator can be used to search for damage to the wiring under plaster or under drywall.
Finally, we recommend watching a useful video on finding a short circuit in the wiring:
Determining the location of the short circuit in the wall
So we examined the most famous methods for finding the location of cable damage. We hope the information was useful and interesting for you!
We also recommend reading:
- How to find a junction box in the wall
- How to identify a short circuit in the network
- How to lay the cable underground
