Purpose. To study the relation between leakage current and time, then to identify some limits of grid parameters to ensure electrical safety as well as explosion safety corresponding to all operating modes of leakage relay in Vietnam underground mines. Methodology. In underground coal mines of Vietnam, 660 V grids are characterized by ungrounded neutral point electric system. They require very serious and strict operation conditions including electrical safety and explosion safety. In daily operation, the earth leakage currents (earth fault currents) must be lower than the allowance limit. However, when utilizing many new power electronic devices such as variable speed drive (VSD), the quantity of connected equipment changes due to the change in capacitance current. After measuring the grid parameters, leakage currents are computed in terms of 3 compensating levels of earth fault relays. The resulting values of currents are used to build up limit curves. Findings. Curves of the graphs present the relation between the leakage current and the length of transmission cables associated with various connecting equipment. The curves may be used to operate the electric system safely and effectively. Originality. The proposed equations and simulation in Matlab may be applied to all underground mining grids with different input parameters. Practical value. Operating curves of the graphs are used to identify the most suitable grids parameters which could ensure electrical safety (keeping the leakage current within the allowance limits). It could help operators or technicians optimize the structure of grid and ensure its safety.

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