Electrical breakdown

thumbnail|right|180px|Electrical breakdown in an [[electric discharge showing the ribbon-like plasma filaments from a Tesla coil.]]

In electronics, electrical breakdown or dielectric breakdown is a process that occurs when an electrically insulating material (a dielectric), subjected to a high enough voltage, suddenly becomes a conductor and current flows through it. All insulating materials undergo breakdown when the electric field caused by an applied voltage exceeds the material's dielectric strength. The voltage at which a given insulating object becomes conductive is called its breakdown voltage and, in addition to its dielectric strength, depends on its size and shape, and the location on the object at which the voltage is applied. Under sufficient voltage, electrical breakdown can occur within solids, liquids, or gases (and theoretically even in a vacuum). However, the specific breakdown mechanisms are different for each kind of dielectric medium.

Electrical breakdown may be a momentary event (as in an electrostatic discharge), or may lead to a continuous electric arc if protective devices fail to interrupt the current in a power circuit. Electrical breakdown can cause catastrophic failure of electrical equipment and fire hazards.

Explanation

Electric current is a flow of electrically charged particles in a material caused by an electric field, usually created by a voltage across the material. The mobile charged particles which make up an electric current are called charge carriers. In different substances different particles serve as charge carriers: in metals and some other solids some of the outer electrons of each atom (conduction electrons) are able to move about in the material; in electrolytes and plasma it is ions, electrically charged atoms or molecules, and electrons that are charge carriers. A material that has a high concentration of charge carriers available for conduction, such as a metal, will conduct a large current with a given electric field, and thus has a low electrical resistivity; this is called an electrical conductor. A material that has few charge carriers, such as glass or ceramic, will conduct very little current with a given electric field and has a high resistivity; this is called an electrical insulator or dielectric. All matter is composed of charged particles, but the common property of insulators is that the negative charges, the orbital electrons, are tightly bound to the positive charges, the atomic nuclei, and cannot easily be freed to become mobile.

However, when a large enough electric field is applied to any insulating substance, at a certain field strength the number of charge carriers in the material suddenly increases by many orders of magnitude, so its resistance drops and it becomes a conductor.

Mechanisms

Breakdown mechanisms differ in solids, liquids, and gases. Breakdown is influenced by electrode material, sharp curvature of conductor material (resulting in locally intensified electric fields), the size of the gap between the electrodes, and the density of the material in the gap.

Solids

In solid materials (such as in power cables) a long-time partial discharge caused by a defect such as a crack or bubble in the material typically precedes breakdown. 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.

Liquids

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). Despite the purified oils used, small particle contaminants are blamed.

Gases

Electrical breakdown occurs within a gas when the dielectric strength of the gas is exceeded. Regions of intense voltage gradients can cause nearby gas to partially ionize and begin conducting. This is done deliberately in low pressure discharges such as in fluorescent lights. The voltage that leads to electrical breakdown of a gas is approximated by Paschen's law.

Partial discharge in air causes the "fresh air" smell of ozone during thunderstorms or around high-voltage equipment. Although air is normally an excellent insulator, when stressed by a sufficiently high voltage (an electric field of about 3 MV/m or 3 kV/mm), air can begin to break down, becoming partially conductive. Across relatively small gaps, breakdown voltage in air is a function of gap length times pressure. If the voltage is sufficiently high, complete electrical breakdown of the air will culminate in an electrical spark or an electric arc that bridges the entire gap.

The color of the spark depends upon the gases that make up the gaseous media. While the small sparks generated by static electricity may barely be audible, larger sparks are often accompanied by a loud snap or bang. 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.

Persistent arcs

If a fuse or circuit breaker fails to interrupt the current through a spark in a power circuit, current may continue, forming a very hot electric arc (about 30 000 degrees C). The color of an arc depends primarily upon the conducting gasses, some of which may have been solids before being vaporized and mixed into the hot plasma in the arc. The free ions in and around the arc recombine to create new chemical compounds, such as ozone, carbon monoxide, and nitrous oxide. Ozone is most easily noticed due to its distinct odour.

Although sparks and arcs are usually undesirable, they can be useful in applications such as spark plugs for gasoline engines, electrical welding of metals, or for metal melting in an electric arc furnace. Prior to gas discharge the gas glows with distinct colors that depend on the energy levels of the atoms. Not all mechanisms are fully understood.

thumb|Voltage-current relation before breakdown

The vacuum itself is expected to undergo electrical breakdown at or near the Schwinger limit.

Voltage-current relation

Before gas breakdown, there is a non-linear relation between voltage and current as shown in the figure. In region 1, there are free ions that can be accelerated by the field and induce a current. These will be saturated after a certain voltage and give a constant current, region 2. Region 3 and 4 are caused by ion avalanche as explained by the Townsend discharge mechanism.

Friedrich Paschen established the relation between the breakdown condition to breakdown voltage. He derived a formula that defines the breakdown voltage (<math>V_\text{b}</math>) for uniform field gaps as a function of gap length (<math>d</math>) and gap pressure (<math>p</math>).

: <math>V_\text{b} = {Bpd \over \ln\left({Apd \over \ln\left(1 + {1 \over \gamma}\right)}\right)}</math>

Paschen also derived a relation between the minimum value of pressure gap for which breakdown occurs with a minimum voltage.

Corona discharges are also used to modify the surface properties of many polymers. An example is the corona treatment of plastic materials which allows paint or ink to adhere properly.

Disruptive devices

thumb|right| Dielectric breakdown within a solid insulator can permanently change its appearance and properties. As shown in this [[Lichtenberg figure]]

A disruptive device is designed to electrically overstress a dielectric beyond its dielectric strength so as to intentionally cause electrical breakdown of the device. The disruption causes a sudden transition of a portion of the dielectric, from an insulating state to a highly conductive state. This transition is characterized by the formation of an electric spark or plasma channel, possibly followed by an electric arc through part of the dielectric material.

If the dielectric happens to be a solid, permanent physical and chemical changes along the path of the discharge will significantly reduce the material's dielectric strength, and the device can only be used one time. However, if the dielectric material is a liquid or gas, the dielectric can fully recover its insulating properties once current through the plasma channel has been externally interrupted.

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. Spark-gap transmitters were used in early radio telegraph systems.