Purpose. Analyze the existing problems in the relay-current protection system of electrical installations of 0.4 kV auxiliary substations of nuclear power plants, which do not allow the implementing the "long-range backup" mode, as well as to increase the sensitivity of relay protection devices to remote short-circuit currents by using additional criteria for identifying emergency modes in order to ensure selectivity and and protection against remote redundancy failures. Methodology. Method of system analysis and synthesis, as well as the theory of electromagnetic transient processes in electric power systems for diagnostics of emergency modes of operation of distribution electrical circuits. Findings. The article shows the need and provides scientific and technical justification for proposals to modernize relay-current protection systems for 0.4 kV electrical installations using digital technologies to implement the requirements of "long-range backup". A scientifically sound technical solution is provided for upgrading circuit breakers using microprocessor protection devices, the output circuits of which affect independent electromagnetic tripping devices of these circuit breakers. This solution allows for an in-depth analysis of processes in electrical circuits and the implementation of "long-range backup" by building high-speed selective protection and increasing the sensitivity of the protection to short-circuit currents. As a result of modernization of electrical installations of 0.4 kV NPP auxiliary substations due to implementation of new types of relay-current protection, the following is possible: significant reduction of protection response time at all stages between the source and receiver of electric power, both in the normal mode and in the "long-range backup" mode, and, accordingly, significant reduction of thermal effects on elements of electrical installations both from the flowing short-circuit current and from the effect of an electric arc; significant increase in sensitivity of protection to remote short-circuit currents, which will eliminate both cases of possible protection failure and its false operation. After modernization of the entire protection system due to the use of microprocessor protection devices, the existing structure of the protection system will be completely preserved without replacing switches of all stages, which will allow significant savings in time and financial costs compared to other modernization options. Originality The article presents a technical solution for upgrading circuit breakers with microprocessor protection devices, in which the output circuits act on independent electromagnetic releasing mechanisms of these devices. Practical value. The development allows increasing the reliability of emergency protection automation, as well as fire safety of auxiliary substations of nuclear power plants with a voltage of 0.4 kV.