Considered is one of the parameters of the electromagnetic environment in the electric power substation, i.e. transient over-voltage that occur in the secondary circuits at switching manoeuvres with disconnectors. Described are transmission modes of transient over-voltage to secondary circuits and the formation of transient over-voltage. The theoretical background of passive means and the use of passive means in substation for reducing interference transmission are shown. According to the computing model the impact of ground network material and cable type of secondary circuit on transients are shown. The result of numerical modelling shows that the level of over-voltage in the cable with the shield is lower than that of the cable without shield. In a retrospective statistical analysis of the measurement results from different substations the effects of passive means are studied. As expected it stands out the lowest over-voltages are on the secondary circuit cables with shield. Impact of other passive means on over-voltages is less pronounced. The results of the analysis indicate several unknown parameters. From the accompanying data of measurement results, it was not possible to determine the unknown influencing parameters. However, it can be concluded that the additional parameters required are at least the quality of the implementation of passive means and more detailed information about instrument transformers. The criterion for evaluation of electromagnetic compatibility assurance in electric power plants is designed. The criterion is based on a statistical analysis of the measured transient over-voltages and immunity type test value of secondary equipment.