Discharge or breakdown occurs when the stress due to electric force exceeds the cohesion force in the material. Normally the threshold electric field is of the order of 107 V/m between a dielectric element and a metallic element (see [38]). In practice failures occur on microscopic scale surfaces like edges, spikes where electric fields are the strongest but difficult to measure or to estimate. Therefore, empirical criteria based on more macroscopic potential measurements are generally used.
Because the microscopic fields depend on the structure of the material, the macroscopic potential at which discharge is observed can depend on sample geometry, type of discharge, environment, material properties and surface state. Inverted voltage gradient discharges have been observed to start when potential differences of over 100 volts are applied.
For a dielectric adjacent to a more positive metal (normal potential gradient), the critical voltage is around 1kV. Electric fields of 106 to 107V/m normally lead to ESD but the discharge does not propagate along the surface (i.e. lead to flash-over) without a substantial transverse electric field and so surface potentials of greater than 1kV are an additional characteristics for these discharges. Discharges initiated by the dense plasma environment in low Earth orbit can occur with potential differences above around 100V (see [7]).