Any electrical product is characterized by a number of parameters. For cables, one of the main ones is insulation resistance. There are certain standards that must be taken into account during design and installation, as well as during operation and maintenance of communication routes.
What are the standards for cable insulation resistance? The fact is that there are often discrepancies on this issue. This is caused, according to the author, by several factors.
Firstly, cable is a general concept. This group of products includes samples used for laying power, signal and telephone lines. Cables can be coaxial (radio frequency), control, distribution and general purpose. That is, there are many options for the design of protective shells, differing, among other things, in thickness.
Secondly, a variety of materials are used to make insulation - rubber, plastics, even paper impregnated in a special way. Although in more modern cables the protection is usually complex, that is, combining various dielectric layers.
Thirdly, what kind of insulation resistance are we talking about - the outer shell or the surface coating of the cores?
Fourthly, the specifics of installation and further operation of a particular cable should be taken into account. For example, the route laying method is open or closed. Where it is laid - in the ground, in trays (there are plenty of options). What characterizes the environment - the maximum value and changes in temperature, humidity, aggressiveness, and so on.
All values are in MOhm.
This is a fairly large group of products. This includes cables installed for control circuits, automation, power supply of electric drives, connection of protective and distribution devices, and so on. For them, it is considered normal if the insulation resistance is not lower than 1. But this is a generally accepted indicator. The exact meaning, depending on, should be found in its accompanying documentation.
For communication cables, the resistance standards are somewhat different, more “strict”. For city low-speed lines – at least 5, trunk lines – 10 (MOhm/km).
If the cable has an outer sheath made of aluminum coated with PVC, then the resistance standard is higher and equal to 20.
Note. The PUE stipulates that the measurement of insulation resistance is carried out with a megohmmeter with the inductor voltage:
Specialists do not need to explain that all requirements for insulation resistance are specified in technical specifications, GOST and SNiP for a certain type of work. Its value can be easily found out from the cable passport, and if it is necessary to monitor the condition of the product, make the appropriate measurement. The specifics of this operation are specified in clause 1.8.7. PUE (7th edition).
In everyday life, to assess the degree of wear of the power cable insulation, you can use the following table, which reflects the approximate average standards.
Since a non-professional is not able to take into account all the nuances of the design of the product and its use, this, as a rule, is quite enough to understand whether a given sample is worth laying down or whether it is no longer suitable for use. That is, reject it. Well, if there are certain doubts, then it is a good idea to consult with a specialized specialist.
2016-08-221 area of use.
1. This document has been developed for electrical laboratory applications during acceptance testing of consumer electrical installations.
2. This document defines the methodology for measuring insulation resistance and determining the insulation state of power, lighting wiring and cable lines with voltages up to 1 kV and testing the insulation of secondary circuits and devices with increased voltage of industrial frequency.
3. Tests are carried out to determine the presence of the required safety margin of the insulation of electrical conductors, the absence of general and local defects after installation work.
4. The purpose of the inspection is to check the compliance of the condition of porcelain insulators with the requirements of the PUE.
2. Test object.
Secondary electrical wiring circuits and power cable lines up to 1000 V are subject to testing.
3. Determined characteristics.
When checking power cable lines up to 1000 V, tests are carried out in the following scope:
4. Test conditions.
Tests of power cable lines up to 1000 V are carried out at an ambient temperature of not lower than +5 ° C and a relative humidity of not more than 90%.
5. Measuring tools.
Technical data of measuring instruments used in testing power cable lines up to 1000 V:
6. Procedure for carrying out measurements.
4. Checking the functionality of measuring instruments in accordance with the operating instructions.
7. Insulation resistance measurement.
Before use, it is recommended to subject the megohmmeter to a control test, which consists of measuring the readings on the scale with open and short-circuited wires of the megohmmeter itself. In the case of open wires, the megohmmeter needle should be at the “infinite” scale mark, and in the case of short-circuited wires, at the “0” scale mark. Familiarize yourself with the electrical diagram of the facility. Measure insulation resistance with an open external circuit. Turn on the switches that directly supply the lighting groups. The light bulbs must be turned out. Protective grounding from the object is allowed to be removed only after the device is connected to it.
When measuring insulation resistance, the following operations must be performed:
— measure the insulation resistance of wiring and cables in the direction from the supply feeders and further as the circuit branches. Measurements should be taken between the conductors and between each conductor and the “ground” in turn.
— to develop the specified voltage on the megohmmeter generator, rotate the handle at a speed of 120 rpm. The generator is equipped with a centrifugal regulator that limits the rotation speed, so that the output voltage remains constant.
— measurements should be made with a stable position of the instrument needle. Readings should be taken 1 minute after the start of measurements.
— after completion of measurements, the test object must be discharged by short-term grounding.
— measure the insulation resistance of the cables in each ShR distribution cabinet, going from the fuse group to the ShchO lighting panels.
— the readings of all measurements are recorded in the work log and analyzed. Insulation is considered unsuitable for use if its resistance is below the minimum permissible value. In this case, the insulation temperature must not be lower than +5°C.
- when determining high insulation resistances, in damp weather (at high humidity), so that the megohmmeter readings are not influenced by leakage currents along the insulation surface, connect the megohmmeter to the test object using the “screen” clamp (E). Connect the “screen” terminal to a current-carrying electrode located on the insulated cable winding near the funnel, or to the grounded cable sheath.
— before carrying out measurements, it is necessary to reduce, if possible, the number of factors causing additional error.
8. High voltage insulation tests
industrial frequency.
Preparation for testing:
— before testing, all ground connections are removed and all equipment, the use of which does not allow high voltage testing, is disconnected.
— Temporary jumpers, which must be installed according to the condition of combining sections of the electrical circuit, must differ from the wires that make up the wiring diagrams.
— before applying voltage to the test installation, it is necessary to:
a) check whether all team members are in place and whether there are any unauthorized persons;
b) warn the crew with the words “Applying voltage”, after which remove the ground from the input of the testing installation and turn on the installation.
— at the end of the tests, the manufacturer removes the voltage from the test installation to zero, turns off the device, grounds the terminal of the test installation and informs the team workers with the words: “The voltage has been removed.” Only then can the wires be reconnected from the test setup or disconnected at the end of the test.
Performing tests:
— insulation testing with high frequency voltage is carried out according to the diagram in Fig. 1
— with a large number of branched circuits, to prevent overloading of the test installation with capacitive currents, tests should be carried out in sections.
— insulation is considered to have passed the high voltage test if there was no breakdown, partial discharges, gas or smoke release, a sharp decrease in voltage and increase in current, or local heating of the insulation.
Fig.1. Scheme for testing the insulation of secondary circuits with increased voltage.
10. Data processing and registration of measurement results.
1. Based on the data received, a protocol of the established form is drawn up.
The lowest of the obtained insulation resistance values of the measured circuit is entered into the protocol. The protocol is drawn up in the form of a table.
11. Safety and environmental requirements.
1. When performing tests, it is necessary to be guided by the requirements of the “Inter-industry rules for labor protection during the operation of electrical installations”.
2. Tests of busbars and connecting busbars do not pose any danger to the environment.
Testing of power cables 0.4-6-10 kV with increased voltage
Device for testing power cables up to 10 kV (AID-70M)
During its operation, the cable is constantly exposed to certain external unfavorable factors: temperature changes, pressure and soil displacement, and other loads that in one way or another affect the condition of the cable insulation. And since insulation cannot last forever, testing power cables is an absolutely necessary activity. In any case, it will at least give you an idea of the condition of the power cable.
Cable testing with increased voltage is carried out in accordance with GOST. The voltage used in testing is also set according to technical specifications or GOST for specific cables.
Testing of power cables with impregnated paper insulation
When performing high-voltage tests on a cable with a metal sheath and screen, the screen and sheath are connected and - if the test takes a long time - an initial voltage equal to about 40% of the full test voltage is applied. Then testing the cables with 10 kV voltage is continued, gradually increasing it to the level of the established test voltage. The increase should not be faster than 1 kV per second. When adjusting stepwise, the voltage at each step should not exceed 5% of the main value of the full test voltage.
In case of improper operation, storage or poor-quality connection of electrical conductors, the insulating qualities of the coating may be impaired. These violations can lead to insulation breakdown and short circuits between conductors. To eliminate or prevent these problems, one of the means is to measure the insulation resistance of the electrical wiring.
Before carrying out electrical installation work, and during the operation of cables and wires, various measurements must be made. These measurements also include testing for insulation resistance.
Factors taken into account when measuring the resistance of electrical wiring:
It is worth noting that under the name “cable” there are a huge number of products. These include wires and cables that are used for laying various power lines, when installing signal or telephone communications. The cables themselves can be coaxial, distribution, control or general purpose. It follows from this that the variability in insulation design is quite wide, since the insulation may vary in thickness.
In the manufacture of insulating covers of conductors, various materials that are radically different from each other are used. The insulation is made of rubber, PVC plastic (polyvinyl chloride) or paper, which is impregnated with a special compound. Depending on the purpose of the cable, the insulation can be complex, which combines several types of insulating coatings.
Note! All characteristics are specified in GOST rules and are indicators of product quality.
When measuring resistance, the type of insulation must also be taken into account. Since the insulation can be an outer shell, or a layer that provides insulation for each core.
The installation features and operational characteristics of the conductor must also be taken into account. These features include the type of route laying (open or closed), laying is carried out in the ground or trays. Environmental features, temperature changes and humidity are also important.
To ensure the safety of using electrical wiring, SNiP and GOST Rules have established regulations according to which tests for insulation resistance are carried out.
Types of postings:
In this case, closed-type wiring refers to conductors located indoors (private houses, apartments, offices). The main condition for carrying out measuring work is the absence of high humidity in the room.
In order to measure resistance on open sections of conductors (located outdoors), the following factors must be taken into account. There should be no high humidity outside, and the air temperature should be positive.
Note! In winter, at subzero temperatures, it is impossible to accurately measure resistance.
The quality of the insulating coating for closed-type wiring of private houses and apartments must be measured once every three years. The best option to check the insulation would be to do it in the summer.
It is worth noting that in some cases, the quality of insulation of open wiring is checked once a year, and subject to the following conditions:
For control measurements of insulation resistance, a megger is used. Insulation resistance testing in apartments is carried out at a voltage of 1000 V, cables are tested at a voltage of 2500 V.
Since there are quite a lot of different wires and cables, the rules have established standards that determine the normal value of insulation resistance for a particular conductor.
Conductors are divided into:
High-voltage cables include overhead cable power lines whose voltage is higher than 1000 Volts. For these lines, there are no specific standards for insulation resistance values, but when carrying out measuring work, resistance values should not be less than 10 megaohms.
Low-voltage power networks include electrical wiring in houses and apartments and secondary electrical circuits used in various electrical installations. The minimum insulation resistance value for conductors of these systems should be from 0.5 megaohm.
The list of control conductors includes various types that are used to connect the control circuit, various automation systems; these wires connect electrical drives, distribution and protective devices. For these conductors, resistance values are set to 1 megaohm.
Note! Before measuring work, each cable is classified.
Measuring work to determine insulation resistance for low-voltage and high-voltage cables and wires is carried out with a voltage of 2500 Volts. Control cables, depending on their characteristics, are tested with voltages from 500 to 2500 Volts.
Table of resistance standards:
Measuring work to determine the insulation resistance of current-carrying conductors is carried out both individually and on the scale of electrical measuring laboratories. This work is performed with a megger.
What types of megohmmeters are there:
Mechanical devices are made on the basis of an electric current generator and a measuring device. Electronic models can be connected to a computer using software.
First of all, the device is checked. If the wires of the device are open, then when checking, the arrow should tend to the infinity sign; if the wires are closed, the arrow of the device should be in the zero position.
Note! If measurements are made on the home electrical network, be sure to disconnect all electrical devices.
After that, the probes of the device are fixed on the conductor, and measuring work is carried out. Measurement data is entered into the protocol.
Operating electrical networks pose a danger. Therefore, it is possible to ensure the normal operation of devices and conductors not only by the quality of their manufacture, but also by carrying out various tests.
The quality of the cable insulation layer greatly affects the reliability of the electrical installation as a whole. It can change both during production at the factory and during storage, transportation, installation of the circuit, and, especially, during its operation.
For example, moisture that gets inside the insulation at subzero temperatures will freeze and change its electrically conductive properties. Determining its presence in this situation is very problematic.
Types of checks
Constant attention is paid to the quality of insulation, which is comprehensively implemented:
periodic mandatory inspections by trained personnel;
automatic tracking by special monitoring devices during the execution of a constant technological cycle.
During cable assessment, personnel determine its mechanical condition and check its electrical characteristics.
During an external inspection, which is mandatory during any inspection, quite often you can see only the ends of the cable brought out for connection, and the rest of it is hidden from view. But even with full access, it is impossible to determine the quality of the insulating layer.
Electrical tests make it possible to identify all insulation defects, which allows us to draw a conclusion about the suitability of the cable for further use and provide guarantees for its use. They are divided according to the degree of complexity into:
1. measurements;
2. tests.
The first method is used to assess quality in the following cases:
after purchase, before installation in the electrical circuit begins, so as not to waste time on laying and subsequent dismantling of a faulty cable;
after completing installation work to assess their quality;
when the tests are completed. This allows you to assess the serviceability of insulation exposed to increased voltage;
periodically during operation to monitor the safety of technical characteristics under the influence of operating current loads or environmental factors.
Cable insulation tests are carried out after its installation before connection to operation or periodically during operation as necessary.
How the cable works
To explain the principle of electrical testing, let’s consider the structure of a simple, commonly found cable of the VVGng brand.
Each of its current-carrying conductors is equipped with its own layer of dielectric coating, which insulates it from neighboring conductors and leakage to ground. Current-carrying wires are placed in the core and protected by a sheath.
In other words, any electrical cable consists of metal wires, most often based on copper or aluminum, and an insulation layer that protects the conductors from leakage currents and short circuits between all phases and the ground.
Each cable is designed to transmit a specific type of energy under different operating conditions. It is subject to certain, specific requirements. They must be familiarized with before carrying out electrical measurements.
Instruments for testing
Sometimes novice electricians, to measure the insulation of a cable or electrical wiring, use testers or multimeters, which have a scale for measuring resistance in kiloohms and megaohms. This is a grave mistake. Such devices are designed to evaluate the parameters of radio components and operate from low-power batteries. They are not able to create the necessary load on the insulation of cable lines.
Special devices serve these purposes - megohmmeters, called “megohmmeters” in electricians’ jargon. They have many designs and modifications.
Before you start using any device, you must check its serviceability each time:
external inspection;
assessment of the timing of inspections by the metrological laboratory based on the state of its mark on the body. Safety rules do not allow the use of a measuring device with a broken mark, even when there is a passport about the inspection carried out before its expiration;
checking the timing of periodic insulation tests of the high-voltage part of the device by an electrical laboratory. A faulty megohmmeter or damaged connecting wires can cause electric shock to personnel.
control measurement of known resistance.
Attention! All work with a megohmmeter is classified as dangerous! They can only be performed by trained, tested and approved by the commission personnel with electrical safety group III and higher.
Technical issues of preparing cables for insulation measurements and testing
Please note that the organizational part is discussed here very briefly and not completely. This is a big, important topic for another article.
1. All measurement work must be carried out on the cable with the voltage removed from it and, as a rule, the surrounding equipment. The effect of induced electric fields on the measurement circuit must be excluded.
This is dictated not only by safety, but also by the operating principle of the device, which is based on supplying a calibrated voltage to the circuit from its own generator and measuring the currents generated in it. The scale divisions of analog instruments and the readings of digital models in Ohms are proportional to the magnitude of the resulting leakage currents.
2. The cable connected to the equipment must be disconnected from all sides.
Otherwise, the insulation resistance of not only its cores, but the entire remaining connected circuit will be measured. Sometimes this technique is used to speed up work. But, in any case, to obtain reliable information, the equipment connection diagram must be taken into account.
To disconnect a cable, its ends are separated or the switching devices to which it is connected are disconnected.
In the latter case, if negative results are obtained, it is necessary to check the insulation of the circuits of these devices.
3. The cable length can reach a large value on the order of a kilometer. At the remote end, at the most unexpected moment, people may appear and, through their actions, influence the measurement result or suffer from high voltage applied to the cable from the megohmmeter. This must be prevented by running .
Features of safe use of a megohmmeter and measurement technology
Long cables laid in electrical networks near a working one may be under induced voltage, and when disconnected from the ground loop, have a residual charge, the energy of which can harm the human body. The megohmmeter generates increased voltage, which is applied to the cable cores insulated from the ground. In this case, a capacitive charge is also created: each core acts as the plate of a capacitor.
Both of these factors together impose a safety condition - to use portable grounding when measuring the resistance of each core, both individually and in combination. Without it, touching the metal parts of the cable without using protective electrical equipment is strictly prohibited.
How to measure the insulation resistance of conductors relative to ground
Consider, as an example, checking the insulation resistance of one core relative to ground.
The first end of the portable grounding is initially securely attached to the ground loop and is no longer removed until the electrical checks are completed. One of the two wires of the megohmmeter is also connected here.
The second end of the grounding, equipped with an insulated tip with a safety ring and a clip for quick connection of the “Crocodile” type, in compliance with safety rules, is connected to the metal core of the cable to remove the capacitive charge from it. Then, without removing the grounding, the output of the second wire from the megohmmeter is switched here.
Only after this is the grounding “crocodile” allowed to be removed for measurements by applying voltage to the prepared electrical circuit. The measurement time must be at least one minute. This is necessary to stabilize transient processes in the circuit and obtain accurate results.
When the megohmmeter generator is stopped, it is impossible to disconnect the device from the circuit due to the capacitive charge present on it. To remove it, it is necessary to reuse the second end of the portable grounding and apply it to the core being tested.
The conductor coming from the megohmmeter is removed from the core after connecting a portable ground to it. Thus, the circuits of the measuring device are always connected to the test circuit only when grounding is installed, which is removed at the time of measurement.
The described check of the cable insulation condition with a megohmmeter for phase C is demonstrated by a sequence of drawings.
In the given example, to simplify the understanding of the technology, the actions with other conductors that remain under induced voltage, which must be removed by installing a short circuit with additional portable grounding, are not described, which significantly complicates the circuit and measurements.
In practice, in order to speed up the work of checking the phase insulation relative to the ground, all cable cores are connected to a short circuit. This operation must be performed by personnel authorized to work under voltage. She's dangerous.
In the example under consideration, these are phases PE, N, A, B, C. Next, measurements are carried out using the above technology for all parallel-connected chains at once.
Usually the cables are in good condition, then such a check is sufficient. If you get an unsatisfactory result, you will have to carry out all measurements in phases.
How to measure insulation resistance between cable cores
In order to improve understanding of the process, we will simplify that the cable is not under the influence of induced voltage and has a short length, which does not create significant capacitive charges. This will allow us not to describe actions with portable grounding that must be performed using the technology already discussed.
Before taking measurements, it is necessary to inspect the assembled circuit and check it using a voltage absence indicator on the cores. They must be moved apart without touching each other or any surrounding objects. The megohmmeter is connected at one end to the phase relative to which the measurement will be performed, and the remaining phases are alternately switched with the second wire to carry out measurements.
In our example, the insulation of all cores is measured one by one relative to the PE phase. When it ends, we select the next phase, for example, N as the common one. In the same way, we take measurements relative to it, but we no longer work with the previous phase. Its insulation between all cores has been checked.
Then we select the next phase as a common one and continue measurements with the remaining cores. In this way, we go through all possible combinations of connecting the cores to each other to analyze the state of their insulation.
Once again, I would like to draw your attention to the fact that this test is described for a cable that is not subject to induced voltage and does not have a large capacitive charge. It is impossible to blindly copy it for all possible cases.
How to document measurement results
The date and scope of the inspection, information about the composition of the team, the measuring instruments used, the connection diagram, temperature conditions, work conditions, all obtained electrical characteristics must be kept in a record. They may be required in the future for a working cable and serve as evidence of a fault for a rejected product.
Therefore, a protocol is drawn up for the measurements taken, certified by the signature of the work manufacturer. To draw it up, you can use an ordinary notepad, but it is more convenient to use a pre-prepared form containing information about the sequence of operations, reminders on safety measures, basic technical standards and tables prepared for filling out.
It is convenient to create such a document once using a computer, and then simply print it on a printer. This method saves time on preparation, registration of measurement results, and gives the document an official appearance.
Features of insulation testing
This work is carried out using special stands containing extraneous high voltage sources with measuring instruments, which are classified as dangerous. It is performed by specially trained and authorized personnel, who are organizationally part of a separate laboratory or service at enterprises.
The testing technology is much like the insulation measurement process, but uses more powerful energy sources and high-precision measuring instruments.
The test results, as with measurements, are documented in a protocol.
Operation of insulation monitoring devices
A lot of attention is paid to automatic checking of the insulation condition of electrical equipment in the energy sector. It can significantly increase the reliability of power supply to consumers. However, this is a separate large topic that requires additional disclosure in another article.
