Abstract: The invention relates to a current transformer (1) for measurement and protection purposes in high- or medium-voltage power supply networks which include busbars guided in support insulators, which current transformer is characterized in that a busbar (3) is surrounded, in the range of the head (21) of a support insulator (2), in a spaced-apart relation by a non-closed ring (4) of magnetically conductive material which is arranged within the material of the support insulator (2); that the magnetic field sensor (5) is arranged, also within the material of the support insulator (2), in the gap between the open ends of the ring (4); and that the connecting leads (6) of the magnetic field sensor (5) are guided through the body (22) of the support insulator (2) as far as the base (23) thereof and are capable of being tapped there.