According to one embodiment, a voltage capacitive transformer (58) is provided with two capacitors (C1, C2) connected in series between a point of a high voltage line (62) and a reference point such as the earth (148) and two impedences (C3, C4) connected in series with the two capacitors. A medium voltage electromagnetic detector (95) is connected between the common point (94) of the two capacitors and the common point (145) of the two impedences. A voltage electronic detector (160) is connected to the terminals of the series circuit formed by the two impedences, the values of which are selected so that the instantaneous voltage across the terminals of the electronic detector is always a constant fraction of the voltage measured between the line and the reference point. Application to the control of fast static protections from a voltage capacitive transformer with medium voltage electromagnetic circuit.