Abstract <p>Emergency modes of network operation that occur during short circuits and due to improper operation of relay protection and automation devices can be the cause of the occurrence and development of ferroresonance processes, including in power transformers and reactors. When a ferroresonance occurs, the vibration of the magnetic core of the transformer or reactor and the harmonic distortions of the mains voltage and current increase instantaneously compared to the duration of the heating process of electrical equipment above the permissible temperature. Harmonic distortion of voltage and current can be caused by the nonlinear nature of the consumer’s load. Therefore, a rational way to detect the ferroresonance process in power transformers and reactors is to detect it through a change in the vibration level of the magnetic core. The article discusses a device for detecting the ferroresonance process in power transformers and reactors by measuring the vibration level of a magnetic core. A design of a device for detecting a ferroresonance process at high magnetic field strengths and elevated temperatures is proposed. The adopted technical solutions make it possible to reduce noise and interference in the information channel of the device by shielding the useful signal using thin-film metallization, reduce the heating of the sensitive element of the device by Foucault currents, and reduce remagnetization by eliminating massive metal assembly units and performing an inertial mass in the form of a package of metal sheets glued together with high electrical resistivity, using a piezoelectric plate and a magnetic platform made of materials with a high Curie temperature.</p>