  In an external electric field, a system of conductive grains embedded in a
dielectric matrix becomes unstable and relaxes towards a conductive state. We
describe and discuss the elementary acts of this relaxation. When the grains
packing density is large, the relaxation is controlled by narrow gaps
separating neighboring grains. The electric fields in some gaps are enhanced,
relative to the external field, by the factor R/l, R being the grain radius and
l the width of the gap. These enhanced fields trigger dielectric breakdown in
the gaps. Both breakdown events and grain motions increase the permittivity of
the composite; advancement of the breakdown process leads to global failure of
insulation. The non-linear and hysteretic charge-discharge behavior of the
system is determined by the main parameters characterizing the breakdown, the
delay time relative to electric field increase, and the lifetime of breakdown
conductivity after the supporting current has vanished.
