Electrical breakdown

Under sufficient voltage, electrical breakdown can occur within solids, liquids, or gases (and theoretically even in a vacuum).

In a gas, the electric field accelerates the small number of free electrons naturally present (due to processes like photoionization and radioactive decay) to a high enough speed that when they collide with gas molecules they knock additional electrons out of them, called ionization, which go on to ionize more molecules creating more free electrons and ions in a chain reaction called a Townsend discharge.

If the voltage is low enough, breakdown may remain limited to this small region; this is called partial discharge.

So the breakdown region rapidly (within nanoseconds) spreads in the direction of the voltage gradient (electric field) from one end of the insulator to the other, until a continuous conductive path is created through the material between the two contacts applying the voltage difference, allowing a current to flow between them, starting an electric arc.

In power circuits, the sudden drop in resistance causes a high current to flow through the material, beginning an electric arc, and if safety devices do not interrupt the current quickly the sudden extreme Joule heating may cause the insulating material or other parts of the circuit to melt or vaporize explosively, damaging the equipment and creating a fire hazard.

However, external protective devices in the circuit such as circuit breakers and current limiting can prevent the high current; and the breakdown process itself is not necessarily destructive and may be reversible, as for example in a gas discharge lamp tube.

Electrical breakdown is often associated with the failure of solid or liquid insulating materials used inside high voltage transformers or capacitors in the electricity distribution grid, usually resulting in a short circuit or a blown fuse.

Dielectric breakdown is also important in the design of integrated circuits and other solid state electronic devices.

The dielectric strength of capacitors limits how much energy can be stored and the safe working voltage for the device.

The partial discharge is a local ionization and heating of the area, degrading the insulators and metals nearest to the defect.

Ultimately the partial discharge chars through a channel of carbonized material that conducts current across the gap.

Possible mechanisms for breakdown in liquids include bubbles, small impurities, and electrical super-heating.

The process of breakdown in liquids is complicated by hydrodynamic effects, since additional pressure is exerted on the fluid by the non-linear electrical field strength in the gap between the electrodes.

In liquefied gases used as coolants for superconductivity – such as Helium at 4.2 K or Nitrogen at 77 K – bubbles can induce breakdown.

In oil-cooled and oil-insulated transformers the field strength for breakdown is about 20 kV/mm (as compared to 3 kV/mm for dry air).

Regions of intense voltage gradients can cause nearby gas to partially ionize and begin conducting.

Lightning is an example of an immense spark that can be many miles long and thunder produced by it can be heard from a very large distance.

The free ions in and around the arc recombine to create new chemical compounds, such as ozone, carbon monoxide, and nitrous oxide.

Partial breakdown of the air occurs as a corona discharge on high voltage conductors at points with the highest electrical stress.

High-voltage apparatus is designed with rounded curves and grading rings to avoid concentrated fields that precipitate breakdown.

Corona can also occur naturally as "St. Elmo's Fire" at high points such as church spires, treetops, or ship masts during thunderstorms.

The main advantage of ozone is that any residual overdose decomposes to gaseous oxygen well before the water reaches the consumer.

Although corona discharge is usually undesirable, until recently it was essential in the operation of photocopiers (xerography) and laser printers.

Many modern copiers and laser printers now charge the photoconductor drum with an electrically conductive roller, reducing undesirable indoor ozone pollution.

An example is the corona treatment of plastic materials which allows paint or ink to adhere properly.

Commercial spark gaps use this property to abruptly switch high voltages in pulsed power systems, to provide surge protection for telecommunication and electrical power systems, and ignite fuel via spark plugs in internal combustion engines.