Abstract
Non-Ohmic behaviour is studied in models having about 1000 sites, for the case of R-hopping. In contrast with the percolation approach, for a strong interaction with phonons, the numerical analysis predicts a decrease in the differential conductivity with increasing field, just obove the Ohmic regime. In order to study the failure of the percolation approach, the field dependence of the percolation cluster, the current cluster and the charge distribution in the model are examined.