Cut-off chamber of a lightning protector, characterized in that on the one hand, it comprises a wall made of a porous ceramic substance, ensuring the discharge and the cooling of the gases at the periphery of the chamber, at the time of the ignition of an arc between two elctrodes combined in that chamber and that on the other hand, a means for reinforcing the dielectric strength of the wall is arranged perpendicular to the zones subjected to the highest potential gradients.