  A theory of far-from-equilibrium transport in arrays of tunnel junctions is
developed. We show that at low temperatures the energy relaxation ensuring
tunneling current can become a cascade two-stage process. First, charge
carriers lose their energy to a bosonic environment via non-phonon energy
exchange. The role of such an environment can be taken by electromagnetic
fluctuations or dipole excitations (electron-hole pairs). The environment, in
its turn, relaxes the energy to the thermostat by means of phonon irradiation.
We derive the current-voltage characteristics for the arrays and demonstrate
that opening the energy gap in the spectrum of the environmental excitations
completely suppresses the tunneling current. The consequences of the cascade
relaxation in various physical systems are discussed.
