Insulation resistance measurements and regulatory documents: characteristics of dielectrics, testing

Operation of the electrical network is impossible without the use of cables. The wires used are characterized by various parameters, one of which is insulation. Over time, this parameter deteriorates due to the influence of the external environment and heating with the flow of current. Therefore, in order to avoid the occurrence of a breakdown, it is periodically necessary to carry out, in accordance with regulatory documents, insulation resistance measurements.

The essence of isolation

In electrical engineering, the term "insulation" refers to the part of a structure that resists the passage of electric current. It is used to prevent the occurrence of a short circuit between conductive materials and to protect a living organism from electric shock. There are different types of insulation, selected depending on the place of use of the cable and technical conditions. The main requirements for it are resistance to mechanical damage, resistance to moisture and temperature extremes.

Electric current passing through the wire loses its power. This is due to the structure of the conductor, namely the type of its crystal lattice, the presence of impurities, defects. The charge carriers, colliding with inhomogeneities, release energy, which is transformed into heat, leading to heating of the insulation. If the dissipated power turns out to be large enough, then the parameters of the protective layer change, and thermal and then electrical breakdown can occur. This is also facilitated by the environment, which does not allow sufficient heat to be removed from the wires or even contributes to heating outside.

Dielectric breakdown usually results in a short circuit (short circuit), accompanied by the generation and release of the maximum power that the power supply can provide.

The enormous current generated during a short circuit not only disables electrical appliances and equipment in a matter of fractions of a second, but also causes a fire. Therefore, heat resistance and resistance is, perhaps, the main characteristic of a dielectric. It is also important that the insulation does not support combustion in the event of abnormal temperatures.

In nature absolute dielectrics do not exist. Any physical body is capable of conducting electric current, as it consists of atoms and molecules. Therefore, depending on the power of the electric current, insulators with the required internal resistance are used to prevent the occurrence of conduction. During operation, this value should not go beyond the established standards. These values ​​for various conditions are determined by regulatory documents, while they also regulate the timing of measurements of insulation resistance.

Types of protective materials

All electrical wiring, and this applies not only to wires, but also to parts of electric motors, must be distinguished by high elasticity and strength throughout the entire period of operation. Insulators with these optimum properties are the following materials:

1. Rubber. It is made from both artificial and natural materials. For example, butadiene and butyl rubbers. The advantage of its use lies in its simple manufacture, flexibility and ease of application to the conductor. The disadvantage is aging and rapid wear. Exposure to temperature causes the substance to dry out and disintegrate.
2. Plastic. This type of insulation is made of cross-linked polyethylene (EPS). According to its characteristics, it is best suited for use in conjunction with high-voltage cables. The advantages of this insulator are resistance to alkaline and acidic environments, high humidity, strength. Specially used additives make the insulator resistant to cracking and increase the melting point. Distinguish plastic by density and degree of elasticity.
3. Polyvinyl chloride (PVC). It has high thermal stability and the ability not to change its dielectric parameters at high voltages. But at the same time, this material is destroyed by exposure to ultraviolet radiation, therefore it is used only indoors. The manufacturing process is the cheapest compared to other types of insulators.
4. Paper. Although it is itself a natural insulator, it is rarely used due to its low breakdown voltage. Most often, it is additionally impregnated with special varnishes that increase its insulating properties, hygroscopicity and reduce vapor permeability. In the manufacture of fibers used cellulose, cotton, cane or asbestos.
5. Fluoroplastic. It is considered the most reliable material, but differs in the complexity of its application to the conductor. It is difficult to damage it mechanically, and it is completely inert to chemical attack. Withstands temperatures from -90 to 250 degrees.

Dielectric characteristics

During the operation of electrical appliances, insulation is influenced by various factors. These include electrical stress, mechanical and thermal stress. DC voltage denotes a value acting for a long time not exceeding 15% for networks up to 220 kV, 10% for 330 kV, and 5% for networks up to 500 kV and more. In addition, there is internal overvoltage and atmospheric overvoltage. The first appears in emergency situations or switching processes, characterized by a short exposure time (up to 10 seconds) and a large amplitude.

The second one occurs during lightning strikes and lasts for hundredths of a second, but has an amplitude of the order of a million volts.

Insulation is distinguished by its appearance into internal and external ones. The first is characterized by the ability to heal itself. That is, it completely restores its properties after an electrical breakdown. The external protection is directly exposed to the influence of air, but at the same time additionally uses its dielectric properties.

The main characteristics of insulation required to assess its effectiveness include:

1. Resistance. This is the most important parameter that is measured when checking it. It is he who determines the safety of the operation of electrical installations and lines. Its measurement is carried out at a constant current of a certain value.
2. The dielectric constant. In conductors, the presence of capacitance is extremely undesirable, and the insulation should be as low as possible. It is characterized by the degree of polarization, that is, the influence of polarized particles on the resulting intensity.
3. Dielectric loss angle. Determines the power loss. It is calculated by the formula: Pa = U²* 2 * π * f * C * tan φ, where: tan φ - depends on the applied potential difference. Measuring this parameter and comparing it with the previous value makes it possible to conclude about the degree and rate of aging of the insulator.
4. Electrical strength. It is characterized by the value of the breakdown voltage, that is, the value at which the breakdown occurs.

In addition to electrical parameters, insulators also have physicochemical characteristics: viscosity, heat resistance class, softening and dropping points. And also resistance to frost, ozone and nitrogen.

Normative documents

The safety provided by the insulation must guarantee the dielectric properties. These requirements are provided for by various standards and GOST. Failure to comply with them leads to possible damage and risk. The requirements for the technical characteristics of the insulator are quite stringent, they are as follows:

• ensuring the reliability of work in the event of various kinds of overvoltage;
• creation of conditions for safe human work;
• limiting exposure to radio frequency interference;
• avoidance of power losses.

The main normative documents governing the requirements and terms of verification are the current editions:

These documents provide comprehensive information on the frequency of measurements and the acceptability of their results. So, in accordance with PTEEP, namely paragraph 2.12.17, the insulation condition should be checked at least once every three years. At the same time, the conditions under which measurements are carried out annually have been determined. For example, in rooms with high humidity, or where there are chemical liquids.

The timing of measurements can additionally be established by intersectoral labor protection rules, fire safety, orders of departmental ministries. But at the same time, they should not contradict regulatory documents: GOST, PUE, PTEEP.

All inspection results must be documented and approved by the appropriate technical services. When checking electrical wiring, in addition to resistance measurements, strength tests can be assigned. In addition, hardness and flammability are often investigated.

Testing methodology

According to the PUE, only certified specialists with calibrated devices in the GOST standard should carry out an insulation test for electrical resistance.

The analysis of the data obtained is allowed for persons of electrical personnel dealing with insulation issues with a special education.

Megohmmeters designed for various DC voltages are used as a measuring device: 100 V, 500 V, 1 kV, 2.5 kV. In devices of the old model, a mechanical generator is used, the work of which is based on the principle dynamos, while modern testers use electronic converters and autonomous power supplies.

The very same measurement of the insulation resistance of electrical wiring is as follows:

• The connecting line and the measuring device are visually inspected.
• The measured line is disconnected from the power source, all electrical installations, instruments and other circuits.
• Possible residual charge is removed from the line for a few minutes by means of grounding.
• The megohmmeter sets the measurement area according to the expected value.
• Check in progress. To do this, first the meter probes are closed, and the test button is pressed, and then they open and the operation is repeated. In the first case, the device should show zero, and in the second, infinity.
• Tests are performed on each phase of the wire relative to others temporarily earthed, or between phase and earth.
• The readings are recorded only after a certain time (about 1 minute), when all transients are over and the measuring needle takes a stable position.
• If the measurement limit was chosen incorrectly, the charge is removed from the wires again, and the measurements are repeated.
• After the end of the test, the result is entered into a table indicating the test method.

As soon as the testing is completed, the residual charge is removed from the tested line or equipment by temporary grounding. The person performing this operation must be on an insulated base and wear dielectric gloves. Measurements are carried out at a temperature of 25 ± 10 ° C and an air humidity of about 80%, unless other requirements are stipulated by the technical conditions.

Nuances of tests

The purpose of the measurements is to establish the possibility of breaking through the insulation by high voltage, but without the risk of damage to it at the time of testing. During testing, according to GOST 12.3.019.80, the safety of work must be ensured. Insulation diagnostics with voltage over 1 kV is performed by two persons with a tolerance group of at least class 4. Before starting work, it is necessary to make sure that there is no contact of people with the measured line, while touching the tester with live parts is also strictly prohibited.

Each cable has its own insulation resistance standard. According to PTEEP p. 6.2 and PUE clause 1.8.37, for power cables designed over 1 kV, the resistance must be at least 10 MΩ, below 1 kV - 0.5 MΩ. Thus, insulation measurement is a very important and complex process, which takes into account the requirements of various regulatory documents. In this case, all the results must be correctly documented, and the test itself must be performed by certified specialists.