Various packings made of metallic grains have been electrically characterized. Electrical breakdown is observed in I–V curves. A well defined critical point separates the insulating and the conducting regime of the packing. The breakdown is reversible by submitting the packing to a small tap. This unusual property leads to hysteretic loops in I–V diagrams. The study of the behavior of I–V curves allows us to describe the system as a network of weak lossy contacts. The key parameter is the inner electrical field controlling the diode-like or diffusion-like state of the packing.