Breakdown depends on many factors, especially thermal ones, and is a function of the time of application of the p.d.
A dielectric material must possess:
(a) a high insulation resistivity to avoid leakage conduction, which dissipates the capacitor energy in heat;
(b) a permittivity suitable for the purpose - high for capacitors and low for insulation
(c) a high electric strength to withstand large voltage gradients, so that only thin material is required. It is rarely possible to secure optimum properties in one and the same material.
A practical dielectric will break down (i.e. fail to insulate) when the voltage gradient exceeds the value that the material can withstand. The breakdown mechanism is complex.
With gaseous dielectrics (e.g. air and hydrogen), ions are always present, on account of light, heat, sparking, etc. These are set in motion, making additional ionisation, which may be cumulative, causing glow discharge, sparking or arcing unless the field strength is below a critical value.
Field strength of the order of 3MV/m is a limiting value for gases at normal temperature and pressure. The dielectric strength increases with the gas pressure.
The polarisation in gases is small, on account of the comparatively large distances between molecules. Consequently, the relative permittivity is not very different from unity.
When very pure, liquids may behave like gases. Usually, however, impurities are present. A small proportion of the molecules forms positive or negative ions, and foreign particles in suspension (fibres, dust, water, droplets) are prone to align themselves into semiconducting filaments: heating produces vapour, and gaseous breakdown may be initiated. Water, because of its exceptionally high permittivity, is especially deleterious in liquids such as oil.
Solid dielectrics are rarely homogeneous, and are often hygroscopic. Local space charges may appear, producing absorption effects; filament conducting paths may be present; and local heating (with consequent deterioration) may occur.