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Investigations of the electromagnetic interference indicate that disconnecting switching of the off-loaded busbars is one of the most important sources of electromagnetic interference in the secondary circuits of a power station. Disconnector's contacts in air-insulated substations (AIS) move slowly causing numerous strikes and restrikes between contacts. Every strike causes high-frequency currents (from a few hundred kHz to a few MHz) tending to equalize potentials at the contacts. These travelling current and voltage waves are the most important sources of electromagnetic coupling to the secondary circuits of instrument transformers, on whose ends high-speed and low-power electronic devices are connected. During disconnector switching malfunctioning of auxiliary circuits can occurred. In this paper results of measurements of overvoltages in the secondary circuits in the AIS Grabovica (Hydro Power Plant - HPP Grabovica) and AIS Kakanj are presented. In order to reduce overvoltages in secondary circuits, measures for mitigation of electromagnetic interference are presented.