Several forms of writing the Biot-Savart-Laplace law and formulas obtained using modern methods for calculating magnetic fields are given. It is mentioned that there are inaccuracies in the interpretation of the Biot-Savart-Laplace law. It is shown that in the electric power industry a simple formula based on the use of the Biot-Savart-Laplace law and the experimentally obtained correction coefficients can be used to calculate the magnetic fields produced by alternating currents in the buses and in power lines. The nomogram and the table for their determination are presented. Streszczenie. Niekiedy można spotkać niedokładności w interpretacji prawa Biot-Savart-Laplace. Stosowane są poprawki do obliczania pola magnetycznego wytwarzanego przez prad przemienny. Zaprezentowano program ułatwiający określenie tych poprawek. O prawie Biota-Savarta_Laplace w zastosowaniu do obliczania wysokonapieciowych instalacji pradu przemiennego

This publication is a continuation of many years of research on the development of the basis for building a unique resource-saving relay protection, which does not use current transformers to obtain information about the primary current. The authors propose to consider the possibility of building relay protection devices with the use of reed switches as sensitive elements. However, an important issue remains the study of reed switches parameters with the passage of time and the number of actuations and returns.

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A study of the functioning of reed switches under the influence of a magnetic field created by a current in a conductor in a transient mode with the presence of an aperiodic component has been carried out. A well-known method for determining current using reed switches was implemented. At the same time, it was determined that the originally formulated method did not give the required result within the limits of errors. This is most likely due to the peculiarities of the mechanism of movement of the reed switch contacts. Alternatively, the measurements were taken to take the return currents instead of the pick-up currents and the time between the return times. They are more stable. Simulation is performed, experimental determination of the value of surge current by measuring time is carried out. The main element of the created installation was the power transformer coil with low active and high inductive resistance. As part of the study, the reed switches were placed in a magnetic field with an aperiodic component, as in the transient mode. This study will show the applicability of reed switches for the construction of relay protection devices that will not need current transformers to obtain information about the primary current in the conductor. In the course of the research, it was found that the error in determining the magnitude of current was no more than 10 %. Using microprocessors, it is possible to build relay protection devices with a speed of up to 20 ms. This result makes it possible to build new devices. Since in the well-known developments, it was only said about determining the magnitude of current in a steady state. When building relay protection devices on reed switches, without using current transformers, it will be possible to build backup protections that duplicate not only the devices themselves, but also the primary measuring transformers with other sensitive elements. This will improve the reliability of the power supply.