Arc is a gas discharge phenomenon, an instantaneous spark produced by the passage of electric current through some insulating medium (such as air).
When a switching appliance is used to disconnect the current, if the circuit voltage is not less than 10-20 volts and the current is not less than 80-100 mA, an arc will occur between the contacts of the electrical appliance.
Therefore, before understanding the structure and working conditions of switching appliances, let's first take a look at how they are generated and extinguished.
The formation of arc is a process in which neutral protons (molecules and atoms) are dissociated between contacts. When the switch contacts are separated, the distance between the contacts is very small and the electric field intensity E is very high (E = U/d). When the electric field intensity exceeds 3×10^6V/m, the electrons on the cathode surface will be pulled out by the electric field force to form free electrons in the contact space. This dissociation method is called: strong electric field emission.
The free electrons emitted from the cathode surface and the few original electrons between the contacts accelerate toward the anode under the action of the electric field force, constantly colliding with neutral points on the way. As long as the velocity v of the electron is high enough and the kinetic energy A=1/2mv^2 of the electron is large enough, it is possible to eject electrons from neutral protons to form free electrons and positive ions. This phenomenon is called collisional dissociation. The newly formed free electrons also accelerate towards the anode and will also collide with the neutral point and become dissociated. The result of the continuous collision dissociation is that the contacts are filled with electrons and positive ions, which have a large electrical conductivity; under the applied voltage, the medium is broken down to generate an arc, and the circuit is turned on again.
The gap between the contacts where the arc burns is called the arc gap. After the arc is formed, the high temperature between the arc gaps causes the electrons on the cathode surface to obtain enough energy and emit outwards, forming a thermal electric field emission. At the same time, under the influence of high temperature (the temperature maintained in the center part of the arc can reach more than 10,000°C), the irregular thermal movement speed of the neutral points in the gas increases. When neutral particles with sufficient kinetic energy collide with each other, they will be dissociated to form electrons and positive ions. This phenomenon is called thermal dissociation.
As the distance between the contacts increases, the electric field intensity E between the contacts gradually decreases. At this time, the arc combustion is mainly maintained by thermal dissociation.
Between the contacts of the switching appliance, while the dissociation process occurs, a de-dissociation process that reduces the charged particles also occurs.