Introduction
In modern medium-voltage power systems, vacuum circuit breakers play a dominant role in safe and efficient current interruption. Engineers prefer these breakers for their robustness, low maintenance requirements, and high dielectric recovery strength. A vacuum circuit breaker, also known as VCB, uses the unique properties of a vacuum to quench electrical arcs. Unlike SF6 or oil-based circuit breakers, VCBs are clean and compact.
This article mainly focuses on the working principle of vacuum circuit breakers from a technical standpoint.
Fundamentals of Arc Interruption
Before analyzing the operation of a vacuum circuit breaker, it is essential to understand how an arc forms in the vacuum within the interrupter chamber. The interrupter chamber, also known as a vacuum interrupter (VI), is a ceramic-made enclosure inside which power contacts make and break. When contacts within the VI separate under load, the conductive path is temporarily maintained through an electrical arc. This arc must be extinguished quickly to isolate the fault and prevent equipment damage or safety hazards.
In most circuit breakers, arc quenching is achieved by controlling the medium in which the arc exists. For example, air, oil, and SF6 gas circuit breakers rely on gas flow, insulation, or chemical recombination to suppress the arc. In contrast, a VCB employs a high vacuum—typically between 10-6 and 10-7 torr—to extinguish the arc.
Vacuum Interruption Mechanism
A vacuum interrupter (VI) consists of one fixed and one movable power contact. These contacts are usually made from a copper-chromium (CuCr) alloy, selected for its superior arc resistance, low welding tendency, and minimal contact erosion.
Contact Separation and Arc Initiation
When a fault condition is detected, the trip command actuates the operating mechanism, causing the moving contact to separate from the fixed contact. The formation of the arc in vacuum during contact separation is an interesting topic. According to general physics, when two surfaces detach from each other, they cannot separate instantly. During separation, the contact area gradually decreases, eventually reducing to a single point before the contacts are fully disconnected.
The fault current attempts to continue flowing through that point at the last instant of contact separation. This causes a large current to flow through a very narrow path at that instant. This phenomenon generates a huge amount of heat energy instantly. Moreover, the surrounding vacuum means a no-pressure zone. As a result, the metal of the contact surfaces instantly gets vaporized. The electrostatic stress in the contact gap instantly ionizes the metal vapour emitted from the contacts, creating a short-duration arc. This is referred to as a vacuum arc.
This is how a vacuum, without having a medium for ionization, provides a conductive plasma needed to sustain the arc for at least a small moment. This arc is typically a metal vapor arc, and its characteristics depend heavily on the electrode material, current level, and contact geometry.
As the current passes through its natural zero-crossing point (in AC systems), the density of the plasma diminishes rapidly. The metal vapour condenses onto the surrounding surfaces of the interrupter, causing the arc to extinguish almost instantaneously. The dielectric strength of the vacuum recovers well before the next voltage peak. Ideally, during the next voltage half-cycle, the contacts have already been separated; hence the phenomenon of contact detaching and vapor formation will not repeat. So ideally, the current gets finally interrupted just after the first zero crossing.
But practically, due to restriking, the arc may be reignited for one or two additional cycles. However, re-ignition of the arc is virtually impossible during the interruption of normal current. Now, it is clear that the rate of dielectric recovery is very high in a vacuum circuit breaker. Generally, it is on the order of 10⁸ to 10⁹ V/m·s. This is the primary advantage of the vacuum medium.
Operating Mechanisms of VCB
The movement of the moving contact in the vacuum interrupter is handled by an electromechanical mechanism. A spring-charged, motor-operated, and solenoid-actuated mechanism is commonly used for this purpose. To ensure such fast recovery of dielectric strength, the opening speed of the moving contact must be sufficiently high. Therefore, the mechanisms must ensure high-speed contact separation (typically 1–2 m/s).
The fast recovery makes VCBs particularly effective in interrupting short-circuit currents, where voltage transients can be severe. In addition to sufficient speed, the mechanism must be able to prevent bouncing and welding.
Current Chopping
One limitation of vacuum interrupters is current chopping. Current chopping means the interruption of current before its natural zero crossing. This is especially problematic in inductive circuits. Due to high inductance, a sudden current interruption can cause high overvoltages. These overvoltages can break the reignited arc. This is why vacuum circuit breakers are not recommended for highly inductive circuits.
Restriction for High Voltage Applications
In vacuum, arc extinction relies on quick dielectric recovery. At higher voltages (>66 kV), the gap between contacts needs to be larger to withstand the voltage. A larger gap requires a longer traveling time of the moving contact. The prolonged travel reduces the effectiveness of vacuum in quickly recovering dielectric strength. This is why VCBs cannot be used for higher voltage systems (>66 kV).
Modern VCBs are rated to interrupt fault currents up to 63 kA and operate at voltages up to 36 kV. Their making and breaking capacities, mechanical endurance (often >10,000 operations), and thermal stability make them suitable for a wide range of industrial, commercial, and utility applications.