The issue of determining temporary overvoltages on metal oxide surge arresters, which are installed in the block transformer field, and which occur when the generator is suddenly unloaded, is addressed. Configurations are treated: generator - block transformer - cable line, which are regularly encountered in hydroelectric power plants with engine room and block transformer located in a cavern and switchgear away in an open area. The theory of the accompanying transient state is presented and the original expression for the calculation of the amplitudes of the expected temporary overvoltages is developed. Calculation results obtained using the expression in question are regularly on the safe side. The presented calculation procedure is applied to the example of block 2 of the Zakucac HPP, as it is planned to be realized as part of the initiated process of renovation of the respective power plant.