Electrical breakdown
In electronics, electrical breakdown or dielectric breakdown is a process that occurs when an electrically insulating material, 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. However, the specific breakdown mechanisms are different for each kind of dielectric medium.
Electrical breakdown may be a momentary event, 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
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 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. This is called electrical breakdown. The physical mechanism causing breakdown differs in different substances. In a solid, it usually occurs when the electric field becomes strong enough to pull outer valence electrons away from their atoms, so they become mobile, and the heat created by their collisions with other atoms releases additional electrons. In a gas, the electric field accelerates the small number of free electrons naturally present 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. As these examples indicate, in most materials breakdown occurs by a rapid chain reaction in which mobile charged particles release additional charged particles.
Dielectric strength and breakdown voltage
The electric field strength at which breakdown occurs is an intrinsic property of the insulating material called its dielectric strength. The electric field is usually caused by a voltage applied across the material. The applied voltage required to cause breakdown in a given insulating object is called the object's breakdown voltage. The electric field created in a given insulating object by an applied voltage varies depending on the size and shape of the object and the location on the object of the electrical contacts where the voltage is applied, so in addition to the material's dielectric strength, the breakdown voltage depends on these factors.In a flat sheet of insulator between two flat metal electrodes, the electric field is proportional to the voltage divided by the thickness of the insulator, so in general the breakdown voltage is proportional to the dielectric strength and the length of insulation between two conductors
However the shape of the conductors can influence the breakdown voltage.
Breakdown process
Breakdown is a local process, and in an insulating medium subjected to a high voltage difference begins at whatever point in the insulator the electric field first exceeds the local dielectric strength of the material. Since the electric field at the surface of a conductor is highest at protruding parts, sharp points and edges, for a conductor immersed in a homogeneous insulator like air or oil, breakdown usually starts at these points. In a solid insulator, breakdown often starts at a local defect, such as a crack or bubble in a ceramic insulator. If the voltage is low enough, breakdown may remain limited to this small region; this is called partial discharge. In a gas adjacent to a sharp pointed conductor, local breakdown processes, corona discharge or brush discharge, can allow current to leak off the conductor into the gas as ions. However, usually in a homogeneous solid insulator after one region has broken down and become conductive there is no voltage drop across it, and the full voltage difference is applied to the remaining length of the insulator. Since the voltage drop is now across a shorter length, this creates a higher electric field in the remaining material, which causes more material to break down. So the breakdown region rapidly spreads in the direction of the voltage gradient 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.Electrical breakdown can also occur without an applied voltage, due to an electromagnetic wave. When a sufficiently intense electromagnetic wave passes through a material medium, the electric field of the wave can be strong enough to cause temporary electrical breakdown. For example a laser beam focused to a small spot in air can cause electrical breakdown and ionization of the air at the focal point.
Consequences
In practical electric circuits electrical breakdown is usually an unwanted occurrence, a failure of insulating material causing a short circuit, possibly resulting in a catastrophic failure of the equipment. 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. If the current supplied by the external circuit is removed sufficiently quickly, no damage is done to the material, and reducing the applied voltage causes a transition back to the material's insulating state.Lightning and sparks due to static electricity are natural examples of the electrical breakdown of air. Electrical breakdown is part of the normal operating mode of a number of electrical components, such as gas discharge lamps like fluorescent lights, and neon lights, zener diodes, avalanche diodes, IMPATT diodes, mercury-vapor rectifiers, thyratron, ignitron, and krytron tubes, and spark plugs.
Failure of electrical insulation
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. Electrical breakdown can also occur across the insulators that suspend overhead power lines, within underground power cables, or lines arcing to nearby branches of trees.Dielectric breakdown is also important in the design of integrated circuits and other solid state electronic devices. Insulating layers in such devices are designed to withstand normal operating voltages, but higher voltage such as from static electricity may destroy these layers, rendering a device useless. The dielectric strength of capacitors limits how much energy can be stored and the safe working voltage for the device.
Mechanisms
Breakdown mechanisms differ in solids, liquids, and gases. Breakdown is influenced by electrode material, sharp curvature of conductor material, the size of the gap between the electrodes, and the density of the material in the gap.Solids
In solid materials 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. Despite the purified oils used, small particle contaminants are blamed.