Abstract: Transformer, especially a measurement transformer, for detecting faults on electrical cables, for example.Transformer, comprising an elastically deformable split laminated cylindrical toroidal magnetic circuit (3), a screw-type assembly member (5), a coil (4) constituting its secondary and a casing (6) comprising a rigid casing part (19, 20) having side by side a housing (23) for the coil (4) and a housing (24) for the assembly member (5), the coil and the assembly member being fitted to a first end (8) of the toroidal magnetic circuit (3), and a deformable casing part (21) of annular ring segment shape one end of which is joined to the rigid casing part. The assembly member (15) includes an abutment (16) for defining the position of the first end (8) of the toroidal magnetic circuit and the deformable casing part has a length such that its second end cannot be locked to the rigid casing part unless the second end (9) of the toroidal magnetic circuit is fully engaged in the assembly member (5).