Formation of Arc in Medium
Before discussing the vacuum arc, let us recall the concept of arc formation in a medium like air, gas, or any other similar medium.
Arc during closing contacts
Let us switch on a circuit breaker online. Closing of this circuit breaker connects a load to a source. During closing, the moving contact starts traveling toward the fixed contact. Ultimately, it bridges the load and source. During the travel of the moving contact, the gap between the source-side contact and the load-side contact decreases. As a result, the voltage gradient in the contact gap increases gradually. The voltage gradient means voltage per unit distance. Obviously, it is nothing but the electric field strength.
After a certain movement, the tip of the moving contact comes very close to the fixed contact. As a result, the electric field strength in the contact gap becomes very strong. Consequently, the medium between the contacts becomes ionized. That means the medium breaks down. As a result, a channel of ionized particles of the medium forms between the contacts. This channel provides a conductive path for the current to continue through the physically separated contacts. This channel of ionized particles carries the current and glows. We refer to this glowing channel as plasma. Obviously, this is the pre-closing arc.
Arc during opening contacts
Now, let us switch off a circuit breaker to withdraw a live load from the source. Here also, the same phenomenon of arcing occurs. During the opening of the contacts, the distance between the fixed contact and the moving contact surface gradually increases from zero. As a result, an arc forms as soon as the contacts separate. Because the voltage gradient is maximum when the contacts are just detouched, the voltage gradient gradually decreases with increasing distance between the contacts.
After a certain distance, the voltage gradient (electric field strength) between the contacts becomes very weak. So, it cannot sustain the arc. Finally, the arc becomes extinguished. This is the simplest theory of how an arc forms. Also, we have understood how an arc becomes in a medium.
Vacuum Arc
Now we shall discuss how an arc forms in a vacuum. A vacuum means there is no medium. One may think that there should be no arc forms in a vacuum. But that is not actually the case. In a vacuum, arcs also form, but in a totally different way. We specially refer to these arcs as a vacuum arc. Let us now discuss, one by one, the arc formation phenomenon during the closing and opening of contacts in a vacuum interrupter.
Vacuum Arc during Closing Contacts
First, we consider the case of closing contacts. When we switch on a vacuum circuit breaker, it connects a load to a source. During this switching operation, the moving contact travels toward the fixed contact.
Voltage Gradient Increase During Contact Approach
As the gap between the contacts decreases, the voltage gradient in the gap increases. At a certain distance between the moving contact surface and the fixed contact surface, the voltage gradient or electric field strength becomes very high. Under this condition, electrons begin to emit from the cathode surface. Here, we refer to the cathode as the contact that possesses negative potential at that instant.
Cathode Spot Formation and Electron Emission
We call the spot from where the electrons emit the cathode spot. These emitted electrons, due to the high voltage stress, strike the opposite contact surface (anode surface). The high-speed collisions on the anode surface generate enormous heat energy.
Metal Vapor Generation and Ionization
The high temperature at the collision spots vaporizes the metal at those locations. Subsequently, the metal vapor becomes ionized due to the presence of the high electric field. This ionized metal vapor creates a conductive path between the contacts. Therefore, the current continues to flow through this plasma channel.
Vacuum Arc Formation
The arc is actually this glowing channel of plasma. More specifically, we can say this is a vacuum arc. This arc continues until the contacts touch each other. Finally, the arc is quenched when the moving contact surface completely touches the fixed contact surface.
Role of Field Emission in Arc Initiation
In conclusion, we can say that field emission alone initiates a vacuum arc during contact closing. This differs from the opening operation, where both field emission and thermionic emission contribute to arc formation.
Vacuum Arc during Opening Contacts
Now, we come to the opening event of the vacuum interrupter. When we switch off a circuit breaker online, it disconnects a load from the source. During this opening operation, the moving contact travels back. However, the detachment of the contact surfaces is not instantaneous.
Contact Separation Process
Rather, during separation, the contact surface first decreases to the narrowest area, and then the contacts finally become separated. At the instant of final separation, the entire current tries to pass through that narrowest path. As a result, that narrow spot produces a high temperature.
Metal Vapor Formation During Separation
This temperature is so high that it vaporizes the metal at that spot. Consequently, it creates a metal vapor cloud in the surrounding area. Additionally, due to the very high voltage gradient, this metal vapor becomes ionized. As a result, it creates a plasma to continue the current.
Thermionic and Field Emission in Vacuum Arc
In more detail, we can say that electrons emit from the hot spots due to two mechanisms. First, high temperature causes thermionic emission. Second, the high voltage gradient causes field emission. These electrons ionize the metal vapor.
Therefore, we can say that during contact opening in a vacuum interrupter, both thermionic emission and field emission cause the arc. In contrast, during contact closing, field emission alone initiates the arcing.
Unique Arc Quenching Mechanism in Vacuum
The arc quenching during contact opening is unique in vacuum. Actually, at the first current zero, the metal vapor cools down. At the same time, the contacts have separated enough. Therefore, the voltage gradient between the contacts becomes weaker.
Hence, the metal vapor condenses on the contact surfaces and the contact shield. For the next half-cycle of current, no vaporized metal remains to continue the current. At that instant, the contact gap is sufficient, so thermionic emission cannot occur further. Hence, no arc forms again. Therefore, the arc is finally extinguished, and the current is ultimately interrupted.
Restriking Phenomenon and Dielectric Recovery
However, due to the restriking phenomenon, the arc can be reignited for the next one or two cycles. If the dielectric strength does not recover quickly enough, a restrike may occur. Therefore, arc extinction is successful only if the dielectric recovery is faster than the transient recovery voltage (TRV).