Dieletric Breakdown

Any material subjected to a sufficiently strong electric field can be made to conduct electricity. When this happens, dielectric breakdown takes place and the material becomes a conductor. Electrons are torn from their atoms and crash into other atoms, liberating their electrons. There is an avalanche of charge, which often starts very suddenly.

Because of this, capacitors, which use dielectrics, come with maximum voltage ratings. When a capacitor is subjected to an excessive voltage, an arc may form through a layer of dielectric, burning it or creating a short circuit. The capacitor may be rendered useless.

The maximum electric field that a material can withstand without breakdown is called it's dielectric strength. This strength is affected by temperature, impurities and other factors difficult to control, so dielectric strengths are only approximate. Some are given below.

Material

Dieletric Strength (V/m)

Polycarbonate

Polyester

Polypropylene

Polystyrene

Pyrex Glass

For example a 1mm film of polyester can withstand a voltage of about before breakdown occurs.