What Is an Electric Arc?

An arc is a gas discharge phenomenon. An instantaneous spark caused by an electric current passing through some insulating medium (such as air). An arc is a self-sustaining gas that conducts electricity (electrical conduction in an ionized gas), and most of its carriers are electrons generated by an electron emission ^[1] . The metal surface of the contact is caused by an electron emission (thermionic emission, field emission, or photoelectric emission), and the gas atoms or molecules in the gap will generate electrons and ions due to ionization (collision, photoionization, and thermal ionization). In addition, bombardment of the emission surface by electrons or ions can cause secondary electron emission. When the ion concentration in the gap is sufficiently large, the gap is electrically broken down and an arc occurs ^[1] .

The basic function of a switching appliance is to be able to open and close the circuit in the required short time, that is, it functions as a so-called switch. Mechanical switch equipment uses contacts to interrupt the circuit current. As long as the voltage exceeds 12-20V and the interrupted current exceeds 0.25-1A, a generally cylindrical gas with extremely high temperature, strong light and electrical conductivity can be generated in the contact gap (also called arc gap). Is the arc ^[2]

An arc is a gas discharge phenomenon and a plasma. Plasma is the fourth state of matter juxtaposed with solid, liquid, and gas. With 50000K as the boundary, the plasma can be divided into two categories, high temperature plasma and low temperature plasma, and the arc belongs to the latter. In fact, about 90% of the material in the universe is plasma.

Plasma is the fourth state of matter ^[4]

The arc is usually divided into three areas: cathode area, arc column area, and anode area ^[5]

1. Arcing occurs when the circuit is open ^[6]

<1> Press electricity

When using a switching appliance to cut off the current, if the circuit voltage is not less than 10-20 volts and the current is not less than 80-100mA, an arc will occur between the contacts of the 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 ^[7]

Strong conductivity, energy concentration, high temperature, high brightness, light weight, variability, etc. ^[5]

The arc is a high temperature and highly conductive free gas, which not only has a great damaging effect on the contacts, but also prolongs the time to open the circuit. ^[6]

The arc has a great impact on the safe operation of the power supply and distribution system. The structural design of the switchgear should ensure that the arc can be quickly extinguished during its operation ^[6] .

The AC arc is temporarily extinguished twice in each cycle.

The vacuum interrupter can quickly recover the gap insulation ability and the ability to withstand the transient recovery voltage of the system, and finally extinguish the arc.