A vacuum circuit breaker(VCB) is a kind of breaker that utilizes a vacuum as an arc extinction medium. The fixed and moving contact is enclosed within a vacuum interrupter that is permanently sealed. When the contacts separate in a high vacuum, the arc is extinguished. The vacuum circuit breaker is primarily used for medium voltage ranging from 6.6 kV to 33 kV.
The vacuum circuit is superior to other types of circuit breakers due to its high-insulating medium for arc extinction. The pressure inside the vacuum interrupter is around 10-4 torr, which means there are very few molecules present in the interrupter. The vacuum circuit breaker has two main properties that make it an exceptional choice.
- Compared to other insulating media used in circuit breakers, vacuum is a superior dielectric medium. It is better than all other media except air and SF6, employed at high pressure.
- When the contacts of an electrical circuit are separated in a vacuum, an arc is created. This arc is interrupted at the first current zero, resulting in an increase in dielectric strength that is thousands of times greater than other types of breakers.
The aforementioned properties make the breakers more efficient, less bulky, and cheaper, with a longer service life and requiring almost no maintenance.
Construction of Vacuum Circuit Breaker
The construction of circuit breakers is divided into three parts: fixed contacts, moving contact, and an arc shield located within the arc interrupting chamber. It is simpler in construction than other circuit breaker types. The diagram of the vacuum circuit breaker is shown below.
The outer casing of a vacuum circuit breaker is typically made of glass. This is because the glass casing allows for easy inspection of the breaker from the outside after it has been operated. If the glass casing becomes milky or cloudy in appearance, it is an indication that the breaker is losing its vacuum.
The contacts of the circuit breaker, both fixed and moving, are positioned inside the arc shield. When the circuit is interrupted, the pressure inside the vacuum interrupter is maintained at around 10-6 torr. The distance that the moving contacts travel varies between 5 to 10 mm, depending on the voltage at which the circuit is operating.
The vacuum circuit breaker uses stainless steel metallic bellows to move the contacts. The design of the metallic bellows plays a crucial role in the lifespan of the vacuum circuit breaker, as it must perform repeated operations satisfactorily.
Working of Vacuum Circuit Breaker
When a fault occurs in the system, the contacts of the circuit breaker move apart, creating an arc between them. As the current-carrying contacts are pulled apart, their connecting parts experience a high temperature, which causes ionization. This ionization fills the contact space with a vapor of positive ions that are discharged from the contact material.
The density of vapor is dependent on the current flowing through the circuit. As the current wave decreases, the vapor release rate also falls. Once the current reaches zero, the medium regains its dielectric strength, provided that the vapor density around the contacts has been reduced. As a result, the arc does not restrike again because the metal vapor is quickly removed from the contact zone.
Current Chopping in Vacuum Circuit Breaker
Dependence of current chopping in vacuum circuit breaker on vapor pressure and electron emission properties of contact material. The thermal conductivity influences the current chopping level – lower thermal conductivity results in a lower chopping level.
It is possible to lower the current level at which chopping occurs by choosing a contact material that produces enough metal vapor to allow the current to drop to a very low or zero value. However, this is seldom done because it negatively impacts the dielectric strength.
Vacuum Arc recovery of Vacuum Circuit Breaker
The high vacuum has very high dielectric strength. The arc is extinguished quickly with no current and the dielectric strength is established rapidly. This increase in dielectric strength occurs because the vaporized metal that is localized between the contacts diffuses rapidly due to the absence of gas molecules. After interrupting an arc, the recovery strength is 1 kV/µs in the first few microseconds for an arc current of 100A.
Due to these unique attributes, vacuum circuit breakers can easily handle the severe recovery transients associated with short-line faults.
Components of a Vacuum Circuit Breaker (VCB)
A Vacuum Circuit Breaker (VCB) has several key components with conventional circuit breakers, but it features a vacuum interrupter, which is crucial for arc quenching. The primary components are described below:
1. Fixed Contact
- This is the stationary contact through which electricity enters the circuit.
- It remains in place whether the circuit is open or closed.
2. Movable Contact
- Positioned opposite the fixed contact, it allows current to flow when touching the fixed contact.
- During a fault or trip event, it retracts to break the circuit and stop current flow.
3. Vacuum Interrupter
- This is the core component of a VCB.
- It houses the fixed and movable contacts in a sealed vacuum environment.
- When the contacts separate, an arc is formed, which is quickly extinguished due to the vacuum’s excellent insulating properties.
4. Actuator Mechanism
- This mechanism is connected to the tripping system and the movable contact.
- When a fault is detected, it operates to pull the movable contact away from the fixed contact, thereby interrupting the circuit.
Properties of Circuit Breaker Contact Material
The contact material used in the VCB (vacuum circuit breaker) must possess the following properties.
- The material must have high electrical conductivity to carry normal load currents without overheating.
- For optimal performance, the material used for the contact should have a high density and low resistance.
- The material should have high thermal conductivity to quickly dissipate the heat generated during arcing.
- The material needs to possess a high arc withstand capacity and a low current chopping level.
Operating Mechanism of Vacuum Circuit Breaker (VCB)
The vacuum circuit breaker (VCB) operates with high efficiency due to several design advantages:
- It extinguishes the arc without the need for any external insulating medium.
- The moving contact is extremely lightweight.
- It travels only a short distance during the opening process.
- The switching speed is relatively slow, requiring minimal energy to operate.
These factors collectively reduce the energy demand of the VCB’s operating mechanism. The energy required is instantly available through the relay’s interlock signal.
VCBs achieve this motion through built-in mechanisms, such as pre-loaded operating springs. The breaker also includes essential components like latches, nozzles, contact interlocks, and safety devices. These ensure the contacts remain securely closed under normal conditions.
The VCB performs a single-break operation, meaning only one interruption point per phase. The VCB requires minimal maintenance beyond standard servicing because of simple and robust design
Vacuum Circuit Breaker (VCB) Testing
Testing a vacuum circuit breaker (VCB) helps assess both the performance of its switching mechanism and the accuracy of its tripping operation.
After installation and during field use, engineers perform three main tests to ensure the vacuum interrupter functions correctly:
- Contact Resistance Testing – Measures the resistance across closed contacts to detect wear, corrosion, or misalignment.
- High Potential Withstand Testing (HiPot) – Applies a high voltage to verify the insulation strength and arc-quenching capability of the interrupter.
- Leak-Rate Testing – Detects any loss of vacuum inside the interrupter, which could compromise its effectiveness.
These tests ensure the VCB operates reliably under normal and fault conditions.
Applicable Standards for Vacuum Circuit Breaker (VCB)
Standards play a crucial role in ensuring the safety, reliability, and performance of vacuum circuit breakers (VCBs) in electrical systems. Several international organizations and regulatory bodies have developed specific guidelines and specifications that manufacturers and users must follow. Below are some of the key standards applicable to VCBs:
- IEC 62271-100
Defines requirements for AC circuit breakers (including VCBs) with rated voltages above 1 kV up to and including 52 kV. It focuses on performance, testing, and safety criteria. - ANSI C37.04
Specifies the ratings, construction, testing, and operation of indoor and outdoor vacuum circuit breakers with rated voltages up to 38 kV. Widely used in North America. - ISO 9001
Establishes the quality management system (QMS) standards for organizations involved in the design, manufacturing, and testing of VCBs. It ensures consistent product quality and customer satisfaction. - NEMA AB 3
Provides performance ratings and construction requirements for indoor medium-voltage vacuum circuit breakers with voltages between 2.4 kV and 38 kV. - IEEE C37.013
Sets performance and testing standards for high-voltage AC (HVAC) circuit breakers, including vacuum types, rated above 1,000 V. It ensures high reliability in high-power applications.
Advantages of Vacuum Circuit Breaker
Rapid recovery of high dielectric strength occurs on current interruptions that last only half a cycle or less after proper contact separation.
- Vacuum circuit breakers do not require any additional oil or gas and do not need periodic refilling.
- Rapid recovery of high dielectric strength occurs on current interruptions that last only half a cycle or less after proper contact separation.
- The breaker unit is designed to be compact and self-contained and easily installed in any required orientation.
- Due to the aforementioned reasons and the economic advantages they offer, vacuum circuit breakers have gained widespread acceptance.
Disadvantages of Vaccum Circuit Breaker
- The production of vacuum interrupters requires high-tech requirements.
- Additional surge suppressors are required to interrupt low magnetizing currents within a specific range.
- Transit damage or failure causing loss of vacuum causes the entire interrupter useless, as it cannot be repaired on-site.
Applications of Vacuum Circuit Breaker
Vacuum circuit breakers are used in the following applications
- Due to their short response time and efficient recovery, vacuum circuit breakers are highly useful as high-speed switching devices in various industrial applications.
- These breakers demonstrate clear superiority over other breakers when the voltage is high, and the current to be interrupted is low.
- Interrupting devices with low fault capacities are less expensive than others.
- Due to low maintenance requirements, these breakers are suitable for systems requiring 11 to 33 kV.
Frequently Asked Questions (FAQs) on Vacuum Circuit Breaker
1. What is a vacuum circuit breaker used for?
A vacuum circuit breaker (VCB) is used to interrupt and isolate electrical circuits during fault conditions such as short circuits or overloads. It ensures the safe operation of high-voltage systems by breaking the circuit using a vacuum as the arc extinction medium.
2. What is a vacuum breaker?
A vacuum breaker is a type of circuit breaker that uses vacuum inside the interrupter chamber to extinguish the arc when contacts separate. It provides high dielectric strength and rapid arc quenching, making it ideal for medium-voltage applications.
3. Why is VCB used?
VCBs are used because they:
- Offer fast arc extinction
- Require low maintenance
- Are compact and reliable
- Are suitable for frequent switching operations
- Operate safely in harsh environments
4. What are vacuum circuit breaker applications?
VCBs are commonly used in:
- Power distribution systems
- Industrial plants
- Mining operations
- Railway traction systems
- Substations and switchgear up to 36 kV
- Transformer protection and motor switching
5. What is a Vacuum Circuit Breaker?
A Vacuum Circuit Breaker (VCB) is a type of circuit breaker where the interruption of current takes place in a vacuum medium. When the circuit breaker contacts separate, an arc is formed, which is quickly quenched in the vacuum due to the absence of ionizable material.
6. Where is VCB used?
VCBs are typically used in:
- Medium-voltage switchgear (up to 36 kV)
- Indoor/outdoor substations
- Industrial applications requiring frequent operations
- Power plants and commercial facilities
7. What is the principle of a vacuum breaker?
The principle of a VCB is arc extinction in a vacuum. When contacts separate, the arc is created but is quickly extinguished because:
- The vacuum has no ionizable particles
- Electrons and ions quickly recombine
- High dielectric strength recovers immediately
8. What is the operating voltage range of a vacuum circuit breaker?
VCBs are designed for medium-voltage applications, typically in the range of 3.3 kV to 36 kV. Some advanced models may be available for higher voltages in specialized applications.
9. What is the life of a VCB?
The operational life of a VCB breaker depends on usage and maintenance but typically includes:
- Mechanical life: Up to 30,000 operations
- Electrical life: Up to 10,000 operations at full load
VCBs are known for their long service life and low maintenance requirements.
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