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The switching operations of circuit breakers and disconnectors in substations can generate very fast transients, including transient earth voltage rise and intense electromagnetic radiation, causing severe disturbance of on-site electronic devices in substations and seriously affecting the development of smart power equipment. The power supply of an on-site electronic device provides a path along which the disturbance couples with the device. In this article, we investigated the coupling mechanism and measures to suppress this disturbance. We found that, for most on-site electronic devices, their power supply includes a power line that connects the exterior and interior of the shielding box, greatly reducing the shielding effectiveness and inducing disturbance from the outside to the inside. This article proposes a shielding–crossing isolation transformer as the power supply of which the primary winding is outside and the secondary winding is inside of the shielding box, without a power line across the shielding box. This arrangement proved to deliver the high performance in terms of eliminating disturbance from the power supply. This article provides an understanding of the mechanism according to which the electromagnetic disturbance couples through the power supply. A method to determine the electromagnetic compatibility of on-site electronic devices in a substation is also proposed. [ABSTRACT FROM AUTHOR]