Commonly used test formulas for electricians

Calculation is an important part of the daily installation and maintenance practice of electricians. In view of the characteristics of the electrical industry, that is, some calculations are not required to have high accuracy, but are often required to be completed quickly on site. For example, to determine the cross-section of the low-voltage power grid power supply conductor, the cross-section of the power supply conductor should be selected according to the heating conditions (long-term allowable load current) and checked according to the voltage allowable deviation. Now most design institutes use computers to complete such calculations, but the effect is not ideal. If it is at the construction site, it is necessary to check a large number of reference books and spend a lot of time on manual calculations. For this reason, the cumbersome data in the calculation process force electricians and electrical technicians to strive to seek simplified formulas and empirical formulas for electrical calculations. In order to achieve the purpose of electricians: being able to perform quick calculations, oral calculations and mental calculations on site, greatly reducing the calculation workload, such as the "insulated conductor. Estimation method for the allowable current carrying capacity of power cables for long-term continuous operation", "Estimation of the current carrying capacity of overhead conductors with different wire diameters" and "Empirical formula for selecting the cross-section of low-voltage overhead line conductors" introduced in this chapter; 3-1 Quick calculation method
insulation resistance of oil-immersed power transformer windings Measuring the insulation resistance of power transformer windings is to check the insulation status of oil-immersed transformers. It is a simple and universal method, which can effectively detect moisture, whether the lead wire is grounded, etc. The measured insulation resistance value can be compared with the results of each measurement to determine whether it meets the requirements; or it can be compared and judged according to the allowable value (standard value) specified in the "Electrical Equipment Preventive Test Regulations" issued by the former Ministry of Water Resources and Electric Power (such as at the time of handover or after overhaul). However, the regulations stipulate that the allowable value of the insulation resistance of power transformer windings (MΩ) is too much data, which is not easy to remember. Here is a quick calculation method for on-site measurement of transformer insulation resistance-the insulation resistance judgment formula when converted to 20°C. The regulations stipulate that the allowable value of the insulation resistance of the transformer winding ( Standard value, MΩ) is shown in Table 3-1. As we all know, when comparing the values ​​of insulation resistance, it should be converted to the same temperature. It can be seen from Table 3-1 that for every 10°C drop in temperature, the insulation resistance value increases by about 1.5 times. According to this principle, various insulation resistance values ​​above 20°C can be converted to the insulation resistance values ​​at room temperature, and it can be immediately determined whether they meet the allowable insulation resistance value at 20°C (the relationship between insulation resistance and temperature is very close. When the temperature rises, the insulation resistance value decreases, and vice versa, the insulation resistance value increases. Therefore, when using insulation resistance measurement to measure the insulation condition of the transformer, it is necessary to convert it to a certain specified temperature for comparison. The judgment formula is: Where Rt20 - insulation resistance converted to 20°C, MΩ; Rt1 - insulation resistance measured at temperature t1, MΩ; t1 - temperature when measuring insulation resistance, ℃; t20 - normal temperature 20°C;