The problem of electromagnetic compatibility (EMC) of electronic equipment has arisen together with the advent of this kind of equipment, as it receives and transmits electromagnetic radiation. The problems resulted from both internal interferences of assemblies and external emission of various origins. For decades, the problems of EMC have been the prerogative of specialists in electronics, radio engineering, and communications. Suddenly, over the last 10–15 years, this problem has become critical for the power industry. Of course, rather high electromagnetic fields have always existed in electric power facilities. However, electromechanical devices which have been applied for decades in automation, control, and relay protection were not addressed to electromagnetic fields too much as there were no significant EMC problems encountered. The last two decades has showed a sharp change-over from electromechanical to digital protective relays (DPRs) and automation. Moreover, the change-over included both the construction of new substations and power plants and the replacement of old electromechanical protective relays (EMPRs) at the old substations, built in those days when nobody assumed using microprocessor technologies, with the up-to-date DPRs. The latter have proved to be very sensitive to electromagnetic interference coming “out of thin air,” penetrating through operating power circuits, voltage circuits, and current transformers. Some malfunctions of DPR were caused by mobile phones 1 and similar types of equipment. There have been other cases, such as malfunctions of DPRs at operating capacities of the Mosenergo, Ochakovskaya, and Zubovskaya substations (all in Russia). The operating algorithm of protection was affected by lightning, excavation work nearby, electric welding, and other types of interference. The Lipetsk substation start-up was postponed for 6 months due to faults of DPRs while they spent nearly $1.5 million for the DPRs. As a result, the substation was commissioned using a set of conventional defenses. 2 In practice, the shorting on the 110 kV side can cause protection failures on the 330 kV side, and 244interference during switching of the same voltage rating penetrated inputs (through the auxiliary circuits) of the relay protection apparatus operating under the other voltage rating. 3