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
When a circuit breaker is switched on to connect a load with a source, the moving contact starts traveling toward the fixed contact to bridge the load and source. As the gap between the source-side contact and the load-side contact decreases, the voltage gradient increases gradually. The voltage gradient means voltage per unit distance, which is nothing but the electric field strength. After a certain position of the moving contact tip in front of the fixed contact, the electric field strength becomes so strong that the medium between the contacts gets ionized. That means the medium breaks down. As a result, a channel of ionized particles of the medium is formed between the contacts. This channel provides a conductive path for the current to continue even though the contacts are still physically separated. This channel of ionized particles, called plasma, carries the current and glows. This is known as the pre-closing arc.
Arc during opening contacts
During the switching off of a live load from the source by a circuit breaker, the same phenomenon occurs. During the opening of the contacts, the distance between the fixed contact surface and the moving contact surface gradually increases from zero. As a result, an arc forms as soon as the contacts separate, and the voltage gradient gradually decreases with the increasing distance between the contacts. After a certain distance, the voltage gradient—that is, the electric field strength—between the contacts becomes so weak that it cannot sustain the arc, and finally, the arc is extinguished. This is the simplest theory of how an arc is formed and how it gets quenched in a medium.
Vacuum Arc
Now we shall discuss how an arc forms in a vacuum. As vacuum means no medium, one may think that there will be no arc formation in vacuum. But that is not actually the case. In vacuum also, arc forms, but in a totally different way, and this arc is referred to 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 to connect a load with a source, during switching on, the moving contact travels toward the fixed contact. At a certain distance of the moving contact surface from the fixed contact surface, the voltage gradient or electric field strength becomes so high that some electrons start emitting from the cathode surface. Here, we refer to the cathode as the contact which possesses negative potential at that instant. The spots from where these electron emissions take place are called cathode spots. These emitted electrons, due to the high voltage stress, strike the opposite contact surface, i.e., the anode surface. Due to the high-speed collisions on the opposite contact surface, a huge amount of temperature is generated. This high temperature causes the metal at those spots to be vaporized. The vaporized metal or metal vapor, due to the presence of a high electric field, gets ionized. This ionized metal vapor creates a conductive path between the contacts. Therefore, the current continues to flow through this plasma channel. This glowing channel of plasma is known as an arc—more specifically, a vacuum arc.
That arc continues until the contacts touch each other. When the moving contact surface completely touches the fixed contact surface, the arc is finally quenched. In conclusion, we can say that the vacuum arc is initiated only due to field emission.
Vacuum Arc during Opening Contacts
Now, we come to the opening event of the vacuum interrupter. When we switch off a circuit breaker to disconnect a load from the source, the moving contact starts traveling back. The detachment of the contact surfaces is not instantaneous; rather, during separation, the contact surface first decreases to the narrowest area and then finally separates. At that instant—i.e., the instant of final separation—the entire current tries to pass through that narrowest path. As a result, a high temperature is generated at that narrow spot. The temperature is so high that the metal at that spot gets vaporized and creates a metal vapor cloud. Due to the very high voltage gradient, this metal vapor gets ionized and creates a plasma to continue the current.
In more detail, we can say that, first, due to high temperature (thermionic emission), as well as due to the high voltage gradient (field emission), electrons start emitting, and these electrons ionize the metal vapor. So, we can say that during contact opening in a vacuum interrupter, the arc is initiated due to both thermionic emission and field emission, unlike during contact closing, where field emission alone initiates the arcing.
Again, during the opening of the contacts, the arc quenching is also unique. Actually, at the first current zero, the metal vapor cools down, and at that time the contacts have separated enough, so the voltage gradient between the contacts becomes weaker. Hence, the metal vapor condenses on the contact surfaces and contact shield. For the next half-cycle of current, no vaporized metal remains to continue the current. As the contacts are already separated enough, there is no further thermionic emission and no further field emission. Hence, no arc forms again. Therefore, the arc is finally extinguished, and the current is ultimately interrupted.
Although, 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. So arc extinction is successful only if the dielectric recovery is faster than the transient recovery voltage (TRV).