How to test the insulation resistance of electrical equipment correctly

Insulation resistance is an important technical index of electrical equipment, cables and transmission lines, and it is an important prerequisite to ensure their normal operation. In order to avoid leakage or short circuit accidents caused by heat, moisture, mechanical damage, pollution and aging of insulating materials, it is necessary to measure the insulation resistance of electrical equipment and power lines in time and regularly to guess whether their insulation performance meets the use requirements and nip in the bud. Insulation resistance meter (referred to as megger) is the most commonly used instrument for testing insulation resistance in electrical installation and maintenance. In this paper, megohmmeter is used as an insulation resistance testing instrument to talk about how to correctly test the insulation resistance of electrical equipment.

I. Selection and inspection of megohmmeter

1. Selection of megohmmeter

The selection of megohmmeter is mainly based on the working voltage of the measured object, the size of insulation resistance under normal conditions, the accuracy requirements of test results, the structural characteristics of the measured object and the specific conditions of the measurement site. The key technical parameters are the voltage specification, measuring range and accuracy of the megger.

(1) voltage specification selection. The selection principle of voltage specification is that the rated voltage (i.e. voltage specification) of megohmmeter should be suitable for the working voltage of the tested object. Usually, the voltage specification of megohmmeter is required to be properly higher than the working voltage of the tested object, so that the insulation defect of the tested object can be found at the highest possible voltage without damaging the insulation of the tested object. For the tested electrical appliances with working voltage below 50V, 100 ~ 250V megohmmeter should be used routinely; The working voltage is 50~ 100V, and a 250V megohmmeter should be used; When the working voltage is 100~380V, 500V megohmmeter should be selected; The megohmmeter with working voltage higher than 500V should be 1000~2500V, and the megohmmeter with working voltage not higher than 380V should be1000 V; In the power system, insulators, cables, porcelain bushings, disconnectors and other high-voltage and ultra-high-voltage equipment should use 2500~5000V megohmmeters. For electrical equipment and devices with special procedures for insulation testing, the special procedures shall prevail.

(2) Selection of measuring range. The selection principle of measuring range is that the measuring range of megohmmeter should be suitable for the insulation resistance of the measured object. Because the measuring range of various types of megohmmeters is very different, some are less than1mΩ, and some are as high as10000 mΩ, and the insulation resistance of the measured object is often distributed in the narrow range from zero megohmmeter to tens of thousands of megohmmeters, so it is necessary to choose the measuring range of megohmmeters reasonably. In addition, the scale of megohmmeter is nonlinear, so the measured value cannot fall into the dense area of scale value which is difficult to distinguish when reading. Usually, to test the insulation resistance of low-voltage electrical equipment and lines, a megohmmeter with a range of 0 ~ 200 mω or 0 ~ 500 mω should be selected; To test the insulation resistance of high-voltage electrical equipment, lines and cables, 0 ~ 2000MΩ or 0 ~ 2500MΩ megohmmeters should be selected; Used for testing insulators, UHV equipment, cables, porcelain bushings, disconnectors, etc. 1000 mΩ or 5000 mω megohmmeter should be selected for power system.

When choosing the measuring range of megohmmeter, we should also pay attention to the lower limit value on the scale, because some megohmmeters do not start from 0mΩ, such as1mΩ, 5mΩ, etc. These megohmmeters with higher lower limit values are obviously not suitable for testing electrical appliances with lower insulation resistance values, such as various low-voltage electrical equipment and power lines, so as to avoid their real insulation resistance values being covered by the initial values of megohmmeters.

(3) Selection of accuracy. The megohmmeters produced in China have accuracy grades such as 1.0, 15, 25, 2.0, 5.0, 10.0 and 20.0. In electrical installation and maintenance, 1.0 and 1.5 megohmmeters are the most commonly used, and the commonly used models are ZC 1, ZC1,ZC 13 and ZC/kloc.

(4) Other selection factors. For workplaces with strong electromagnetic interference, pointer megohmmeter should be selected; For electrical equipment, power cables, large motors, long-distance transmission lines and other objects with large capacitance, especially when measuring absorption ratio and polarization index, megohmmeters with enough short-circuit current should be selected, such as KD2676, KD2677, DMG267 1 and other models.

References:

Shaanxi schneider baoguang electric power technology co., ltd