Introduction
Arcs are central to the science of current interruption. When contacts in a circuit breaker separate, the gap that forms is bridged by an arc; a plasma channel that carries current until it is extinguished. The way this arc behaves depends heavily on the environment in which it forms. Two important categories are high pressure arcs and vacuum arcs. Each has unique characteristics that affect how circuit breakers are designed and applied.
High Pressure Arcs
High pressure arcs occur at atmospheric pressure or higher. These arcs are typically seen in gases such as air or in insulating mediums like SF₆. A high pressure arc appears as a bright, narrow column of plasma with a very hot core. Temperatures can easily reach around 6,000 K under natural cooling and may exceed 20,000 K under forced cooling.
The high pressure arc is strongly influenced by its environment. Gas pressure, type of gas, and flow conditions determine how heat is removed from the plasma and how the arc length develops. Because the arc has a relatively narrow diameter, current density is high, and the arc voltage can be significant. An important feature of high pressure arcs is that they can be shaped and controlled by external forces such as magnetic fields or high-pressure gas flows. This makes them suitable for use in designs like air-blast or SF₆ circuit breakers, where arc movement and cooling are used to extinguish the discharge quickly.
Another point of interest is the material of the electrodes. At the cathode, the current density is very high and material losses can occur due to vaporization or melting. Metals with low boiling points, like copper, tend to suffer more erosion than refractory materials such as tungsten. This electrode behavior is an important factor in determining breaker life and maintenance needs.
Vacuum Arcs
Vacuum arcs form in environments where the pressure is extremely low, typically less than 10⁻⁵ torr. Unlike high pressure arcs, the plasma of a vacuum arc comes almost entirely from the electrode material itself rather than from the surrounding gas. Because of this, the positive column of a vacuum arc is made up of metal vapor rather than ionized air or gas.
Vacuum arcs have a much lower arc voltage, typically around 40 volts, which is considerably less than high pressure arcs. The behavior of a vacuum arc is unique because it can exist in two distinct modes: diffuse and constricted. In the diffuse mode, many small cathode spots form, each carrying a small portion of the current, giving the appearance of several arcs burning in parallel. This mode is stable and favorable for current interruption, especially when the current is below about 15 kA.
As current increases, the arc can transition into the constricted mode, where one or a few concentrated spots carry most of the current. In this mode, electrode erosion is more severe, and the arc becomes harder to control. However, as the alternating current approaches its natural zero crossing, the arc can revert back to the diffuse mode, increasing the interrupter’s ability to extinguish the arc. This unique “mode-switching” behavior is one of the key reasons why vacuum circuit breakers are so effective for medium voltage applications.
Key Differences in Behavior
The most significant difference between high pressure and vacuum arcs is the source of the plasma. In high pressure arcs, the plasma is primarily from the surrounding gas, while in vacuum arcs it is almost entirely from electrode material. This changes not only the arc voltage but also how the arc responds to current, pressure, and external influences.
High pressure arcs can reach extremely high temperatures and are easier to manipulate using magnetic fields or gas flow, but they often demand careful management of electrode erosion and gas by-products. Vacuum arcs, on the other hand, operate at lower voltages, produce little to no external gas decomposition, and recover dielectric strength very quickly after current interruption. However, their performance is limited by the risk of constricted arcs at very high current levels.
Key Takeaways
Both high pressure arcs and vacuum arcs play essential roles in the design of circuit breakers. High pressure arcs are common in air and SF₆ breakers, where forced cooling and gas management are critical to performance. Vacuum arcs dominate in medium voltage applications because of their quick dielectric recovery and relatively low arc voltage. Understanding the behaviors of these arcs allows engineers to select the right type of breaker for the right application, balancing performance, durability, and environmental considerations.
Images: By Atlant, CC BY 2.5, https://commons.wikimedia.org/w/index.php?curid=294010
