Electrical isolation is the most important means for ensuring the safety of working with electrical installations and their uninterrupted functioning. The action of alternating current, not exceeding 0.5 mA, is practically imperceptible by the human body. According to current regulations, an alternating current with a frequency of fifty hertz, flowing through the human body, should not exceed 03 mA. Therefore, the systematic measurement of insulation resistance is a necessary measure to ensure electrical safety when working with high voltage equipment.
The most fundamental characteristic of the insulating coating in this aspect is the indicator of its electrical resistance. There are several different types of insulation used in high voltage electrical installations. The working coating is designed to ensure the normal functioning of the unit. It is selected taking into account the technical features of eachelectrical installations. Therefore, regular measurement of this type of insulation resistance is necessary to control the integrity of the coating, which ensures the normal operation of the equipment.
Additional insulation, which performs a purely protective function, is intended to eliminate even the slightest possibility of electric shock in the event of any damage to the main coating. Measuring this type of insulation resistance is key to safe electrical installations.
In addition, some types of electrical equipment use double insulation, which is a combination of the above two types of coating. A feature of this combination is that the parts of the unit accessible to touch will not acquire dangerous voltage even if one of the insulating layers (working or additional) is damaged. Measuring the insulation resistance of this type is reduced to monitoring the overall integrity of the shell and each of its components separately.
The need for regular control and measuring measures is due to factors that worsen the condition of the insulating coating and contribute to its degradation. Such detrimental factors include, first of all, electrical and mechanical damage; the influence of aggressive chemical compounds, which is not uncommon in industrial production; thermal damage.
The device for measuring insulation resistance is called a megaohmmeter. This device is intended not only to measure the resistance of an insulating material, but also to check its electrical strength (in other words, a test for the absence of electrical breakdowns). Before testing, electrical equipment must be de-energized. The very method of measuring the insulation resistance of electrical installations consists in the formation in the bowels of the device by means of a special built-in electromechanical DC generator of a certain control voltage, which is applied to the object under test. In this case, the measuring voltage must exceed that of the electrical network.
