The current transformer includes a magnetic circuit made of magnetic material that is intended to be placed around a primary conductor, and a secondary winding that is wound onto a portion of the magnetic circuit in order to deliver a secondary current to processing circuits. In this current transformer the magnetic circuit includes at least one device for varying the magnetization of a portion of the magnetic circuit according to the temperature in order to limit or to decrease the magnetic flux in the magnetic circuit when the temperature of the magnetic circuit increases. The protection device and the electrical circuit breaker include such a transformer.