Disruptive discharge

Electrical breakdown or dielectric breakdown is when current flows through an electrical insulator when the voltage applied across it exceeds the breakdown voltage. This results in the insulator becoming electrically conductive. Electrical breakdown may be a momentary event (as in an electrostatic discharge), or may lead to a continuous arc if protective devices fail to interrupt the current in a power circuit.

Under sufficient electrical stress, electrical breakdown can occur within solids, liquids, gases or vacuum. However, the specific breakdown mechanisms are different for each kind of dielectric medium.

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.

Breakdown mechanisms differ in solids, liquids, and gasses. 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.

In solid materials (such as in power cables) a long-time partial discharge typically precedes breakdown, degrading the insulators and metals nearest the voltage gap. Ultimately the partial discharge chars through a channel of carbonized material that conducts current across the gap.

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