Breakdown voltage

An electric field causes a large current to flow, so metals have low resistivity, making them good conductors.

Applying a voltage causes only a very small current to flow, giving the material a very high resistivity, and these are classed as insulators.

If the voltage applied across a piece of insulator is increased, at a certain electric field strength the number of charge carriers in the material suddenly increases enormously and its resistivity drops, causing a strong current to flow through it.

Breakdown occurs when the electric field becomes strong enough to pull electrons from the molecules of the material, ionizing them.

The voltage gradient may vary at different points across the object, due to its shape or local variations in composition.

Electrical breakdown occurs when the field first exceeds the dielectric strength of the material in some region of the object.

The breakdown quickly spreads in a conductive path through the insulator until it extends from the positive to the negative contact.

[4] In standard conditions at atmospheric pressure, air serves as an excellent insulator, requiring the application of a significant voltage of 3.0 kV/mm before breaking down (e.g., lightning, or sparking across plates of a capacitor, or the electrodes of a spark plug).

A detailed derivation, and some background information, is given in the article about Paschen's law.

Many small-signal transistors need to have any breakdown currents limited to much lower values to avoid excessive heating.

Power transformers, circuit breakers, switchgear and other electrical apparatus connected to overhead transmission lines are exposed to transient lightning surge voltages induced on the power circuit.

Electrical apparatus will have a basic lightning impulse level (BIL) specified.

This is the crest value of an impulse waveform with a standardized wave shape, intended to simulate the electrical stress of a lightning surge or a surge induced by circuit switching.

For high-voltage transmission lines, the impulse level is related to the clearance to ground of energized components.