A numerical model employing FDTD based particle-in-cell code (PIC) is used to study the discharge mechanism across a gas/solid interface. The simulation is carried out with and without an insulator in the anode cathode gap at a pressure of 1atmosphere and a ramp voltage with a rise time of 5ns and pulse duration of 50ns is used as the input pulse. The profiles of electron densities, avalanche growth, phase profile of electrons, and the gap voltage are calculated with and without an insulator to study the mechanism of discharge in presence of an insulator. Spacer efficiency of 81%is obtained which closely agrees with the existing experimental data.