In electrical engineering (automatic switches, knife switches), an electric arc is born when a loaded circuit is turned off.
Leaf Electrical Safety is expertise in arc flash study, and we know all about risk and hazard. But also we do know how to protect your team as an arc flash study provides an understanding of the hazard.
Important parameters of the electric arc:
- An arc discharge can develop only at high currents (for metal, this current is 0.5 amperes).
- The temperature in the arc shaft is significant and is about 6-18 thousand kelvins (often 6-10 thousand kelvins).
- The voltage drop at the cathode is insignificant and equals 10-20 volts.
The arc discharge is conditionally divided into three zones:
- Near-cathode
- Arc trunk (main part)
- Near-anode
Processes accompanying the birth of an electric arc
At the initial stage of separation of the main contacts, the arc is initiated during the following processes:
- Thermionic emission, or the discharge of unfavorable electrons from a heated contact surface. When the contacts are broken in the area of the last contact area, a zone with molten copper with the corresponding temperature is formed. The so-called cathode spot, a source of free electrons, causes copper to evaporate at the negative electrode. This process is influenced by: temperature and metal of contact surfaces; it is sufficient to create an electric arc but not sufficient to maintain its combustion.
- Field ionization (detachment of electrons from the cathode under the influence of a significant electric field). The air space between the contacts can be regarded as a kind of capacitor, the capacity of which is unlimited at the first instance and then decreases depending on the growing gap between the movable and fixed contact. The described capacitor is gradually recharged, and the voltage in it is comparable to the voltage of the main circuit. When the electric field intensity reaches certain levels, the surface of an unheated cathode experiences conditions that allow electrons to leave.
