The invention relates to a circuit breaker, comprising a fuse unit (6) with a fusible element (7) made from a conducting material (8), such as Ag, Cu or Al, provided with a series of regularly spaced doping sites (9). The conducting material bears a layer of a first compound (10) in direct contact on the above sites, said compound containing a doping material such as In or Ge and forming mixed crystals, which contain the conducting material (8) and the doping material in a fixed stoichiometric ratio such as Ag2In, for example, and is separated from the first by a stable phase boundary. The doping sites (9) weaken the fusible conductor by lowering the melting point to beneath 250 DEG C, such that an arc is quickly formed on a short circuit occurring, even though the electrical resistance per unit length thereof is possibly only a few percent greater than in the other region. The fusible element (7) has a continuous coat of an inflammable material (12), with an ignition temperature preferably lower than the melting point of the first compound (10).