What is a Vacuum Circuit Breaker : Working & Its Applications

The Vacuum interrupter technology was first introduced in the year of 1960. But still, it is a developing technology. As time goes on, the size of the vacuum interrupter has reduced from its early 1960’s size due to different technical developments in this field of engineering. A circuit breaker is a device that, interrupts an electric circuit to prevent unwarranted current, caused by a short circuit, typically resulting from an overload. Its basic functionality is to interrupt current flow after a fault is detected.  This article discusses an overview of the vacuum circuit breaker and its working. To know more about Circuit breakers read this article Types of Circuit Breakers and Its Importance.


What is a Vacuum Circuit Breaker?

A vacuum circuit breaker is a kind of circuit breaker where the arc quenching takes place in a vacuum medium. The operation of switching on and closing of current carrying contacts and interrelated arc interruption takes place in a vacuum chamber in the breaker which is called a vacuum interrupter.

Vacuum Circuit Breaker
Vacuum Circuit Breaker

A vacuum that is used as the arc quenching medium in a circuit breaker is known as a vacuum circuit breaker because vacuum gives high insulating strength due to superior arc quenching properties. This is suitable for most standard voltage applications because, for higher voltage, vacuum technology was developed however not commercially feasible.


The operation of current-carrying contacts & related arc interruption take place within a vacuum chamber of the breaker, which is known as a vacuum interrupter. This interrupter includes a steel arc chamber within the center of symmetrically placed ceramic insulators. The maintenance of vacuum pressure within a vacuum interrupter can be done at 10– 6 bar. The vacuum circuit breaker performance mainly depends on the material used for current-carrying contacts like Cu/Cr.

Working Principle

The vacuum circuit breaker working principle is, once the circuit breaker contacts are opened within the vacuum, then an arc can be generated among the contacts through the metal vapors ionization in the contacts. But, the arc can be quenched easily as the electrons, ions & metallic vapors are generated throughout the arc quickly condense over the outsides of the CB contacts, so the dielectric strength can be quickly recovered.

The most important feature of a vacuum is that once the arc is generated within the vacuum, then it can be extinguished quickly because of the quick improvement rate in the dielectric strength of the vacuum.


Contact Materials

The contact material of VCBs must follow the following properties.

  • High density
  • Contact resistance must be less
  • Electrical conductivity is high to pass usual load currents without overheating.
  • Thermal conductivity is high to dissipate quickly the large heat produced throughout arcing.
  • The thermionic function must be high to allow early arc destruction.
  • The tendency must be low to weld
  • Less current chopping level
  • High arc resist capability
  • A boiling point must be high to diminish arc erosion.
  • Gas content must below to make sure longer service life
  • Low vapor pressure must be sufficient to decrease the quantity of undividable metal vapor within the chamber.

Construction of Vacuum Circuit Breaker

The vacuum circuit breaker comprises a steel arc chamber in the center-symmetrically arranged ceramic insulators. The pressure inside the vacuum interrupter is maintained below 10^-4 torr.

The material used for current-carrying contacts plays an important role in the performance of the vacuum circuit breaker. Alloys like, Copper-bismuth or copper-chrome is the ideal material to make VCB contacts.

Vacuum Circuit Breaker Construction
Vacuum Circuit Breaker Construction

From the figure shown above, the Vacuum circuit breaker consists of fixed contact, a moving contact, and a vacuum interrupter. The moving contact is connected to the control mechanism by stainless steel bellow. The arc shields are supported o the insulating housing such that they cover these shields and is prevented from condensing on the insulating enclosure. The possibility of a leak is eliminated due to the permanent sealing of the vacuum chamber for that a glass vessel or ceramic vessel is used as the outer insulating body.

Working of  Vacuum Circuit Breaker

The sectional view of a vacuum circuit breaker is shown in the figure below when the contacts are separated due to some abnormal conditions, an arc is struck between the contacts, the arc is produced due to ionization of metal ions and depends very much on the material of contacts.

The arc interruption in vacuum interrupters is different from other types of circuit breakers. The separation of contacts causes the release of vapor which is filled in the contact space. It consists of positive ions liberated from the contact material. The vapor density depends on the current in the arc. When the current decreases, the rate of vapor release decreases, and after current zero, the medium regains its dielectric strength if the vapor density is reduced.

When the current to be interrupted is very small in a vacuum, the arc has several parallel paths. The total current is divided into many parallel arcs that repel each other and spread over the contact surface. This is called a diffused arc which can be interrupted easily.

At high values of current, the arc gets concentrated in a small region. It causes rapid vaporization of the contact surface. The interruption of the arc is possible if the arc remains in a diffused state. If it is quickly removed from the contact surface, the arc will be re-strike.

Arc extinction in vacuum breakers is greatly influenced by the material and shape of the contacts and the technique of considering metal vapor. The path of the arc is kept moving so that temperature at any one point will not be high.

After the final arc interruption, there is a rapidly building up of dielectric strength which is peculiar of the vacuum breaker. They are suitable for capacitor switching as it will give a re-strick-free performance. The small current is interrupted before natural current zero, which may cause chopping whose level depends on the material of contact.

Current Chopping

The current chopping in the vacuum circuit breaker mainly occurs within oil circuit breakers as well as in air due to the instability of the arc column. In vacuum circuit breakers, current chopping mainly depends on the pressure of vapor as well as the properties of electron emission in the contact material. So, the level of chopping also influenced by the thermal conductivity, when the thermal conductivity is less, then the chopping level will below.

It is possible to decrease the present level at which chopping occurs by selecting a contact material to provide ample metal vapor to let the current approach to an extremely low value, however, this is not often done because it affects the dielectric power badly.

Properties of Vacuum Circuit Breakers

The insulating medium of the vacuum circuit breaker is high for arc extinction as compared with other types of circuit breakers. The pressure in the vacuum interrupter is around 10-4 torrent which includes very few molecules within the interrupter. This circuit breaker has mostly two extraordinary properties like the following.

As compared with other insulating media employed in the circuit breakers, this circuit breaker is a superior dielectric medium. It is superior as compared with other media apart from SF6 & air because these are used at high pressure.

Once an arc is opened separately by moving the contacts within a vacuum, then a break will occur at the main current zero. By the interruption of this arc, their dielectric strength will increase up to a thousand times as compared with other kinds of breakers.

These properties will make the circuit breakers more proficient, less weight as well as less cost. The life span of these circuit breakers is high as compared with other circuit breakers & they don’t need any maintenance.

vacuum circuit breaker parts are vacuum interrupter, terminals, flexible connections, support insulators, operating rod, tie bar, common operating shift, operating corn, locking cam, making spring, breaking spring, loading spring, and main link.

There are different types of vacuum circuit breakers are available based on the manufacturers which are discussed below.

Mitsubishi Vacuum Circuit Breaker

These circuit breakers are manufactured by Mitsubishi Electric. They provide high safety, reliability & protection of the environment. Mitsubishi VCBs have the following features.

  • Products lineup range is wide
  • No requirement for the six particular dangerous materials.
  • The material name is illustrated over the main plastic parts
  • The structure is foldable to mount the frame
  • Easy maintenance

Siemens Vacuum Circuit Breaker

The Siemens vacuum circuit breakers are SION 3AE5 which are used in all typical switching applications like in industrial networks & medium-voltage power distribution that ranges from short-circuit currents & switching load to busbar sections or connecting network. Their solid structure including the least depth & width dimensions will help to decrease the necessity of different panels.

So, these circuit breakers are obtainable through an optional grounding switch for plug-in versions & fixed mounting. The main features of this circuit breaker include the following.

  • Very simple to install within air-insulated medium voltage switchgear
  • Reliability is high
  • Design is Compact
  • Remote switching through the remote control unit
  • Planning costs are low
  • Service life is long
  • Maintenance is easy

Vacuum Circuit Breaker Testing

Generally, circuit breaker testing is mainly used to test both the separate switching mechanisms’ performance as well as the overall tripping system’s timing. Once vacuum interrupters are designed otherwise utilized infield, then there are mainly three kinds of tests are used to authenticate their function like contact resistance, high potential withstand & the leak-rate test.

Difference between Vacuum Contactor Unit and Vacuum Circuit Breaker

A vacuum circuit breaker trips through an error like an earth fault, short circuit, over/under voltage. A contactor is usually performed in series through a fuse that provides to avoid fault current. The main difference between the vacuum contactor unit and the vacuum circuit breaker are listed below based on different characteristics.

Vacuum Circuit Breaker Vacuum Contactor Unit
Switching capacity is, it switches currents from low-values to

complete system short-circuit current

Switch currents from very low values to

Disrupting capacity of vacuum contactor without fuses. Fuses work for higher currents as compared with the interrupting ability of the vacuum contactor only, up to the

disrupting capability of the fuse

Endurance is high for mechanicalEndurance is extremely high for mechanical like 1,000,000 processes for up to 630A
Endurance is high for electrical is high like a vacuum that ranges from10k – 50k actions at rated continuous current. For vacuum, it is 30 to 100 operations at full short-circuit rating.Extremely high switching continuous current ranges from 450,000 to 1,000,000 actions for up to 630 A. Switching short-circuit current, endurance data not established in short-circuit

current break that needs substitution of fuses

These are not applicable for extremely high endurance applications.These are used for the operations of extremely frequent switching
It is operated electricallyIt operates electrical only
It is mechanically latched because CB remains closed on system voltage loss.Usually, the vacuum contactor unlocks once

system voltage is lost; vacuum contactor will lock once system voltage comes back

 

It uses protective relaysIt uses protective relays for overload protection & fuses for the protection of short-circuit
Short circuit let through energy is lowShort circuit let through energy is low
Remote operation is suitableRemote operation is suitable
Control power is used for the operation of CB, protective relays,  & space heatersControl power is used for the operation of contactor, protective relays & space heaters
It uses a larger areaIt uses less area
Its cost is highIts cost is moderate
Its maintenance is mediumIts maintenance is low.

Advantages of VCB

Vacuum offers the utmost insulating strength. So it has extremely superior arc quenching properties than any other medium.

  • The vacuum circuit breaker has a long life.
  • Unlike Oil Circuit Breaker (OCB) or air blast Circuit Breaker (ABCB), the explosion of VCB is avoided. This enhances the safety of the operating personnel.
  • No fire hazard
  • The vacuum CB is fast in operation so ideal for fault clearing. VCB is suitable for repeated operation.
  • Vacuum circuit breakers are almost maintenance-free.
  • No exhaust of gas to the atmosphere and Noiseless operation.

Disadvantages of VCB

  • The main disadvantage of VCB is that it is uneconomical at voltages exceeding 38 kV.
  • The cost of the breaker becomes excessive at higher voltages. This is due to the fact that at high voltages (above 38 kV) more than two numbers of the circuit breaker are required to be connected in series.
  • Moreover, VCBs production is uneconomical if produced in small quantities.

Applications of Vacuum Circuit Breaker

The vacuum circuit breaker is today recognized as the most reliable current interruption technology for medium voltage switchgear. It requires minimum maintenance compared to other circuit breaker technologies.

The technology is mainly suitable for mainly medium voltage applications. For higher voltage vacuum technology has been developed, but it is not commercially feasible. Vacuum circuit breakers are used in metal-clad Switchgear and also in porcelain housed circuit breakers.

Thus, this is all about Vacuum Circuit Breaker (VCB) working and applications. We hope that you have got a better understanding of this concept. Furthermore, any doubts regarding this concept or to implement any electrical and electronics projects ideas, please give your feedback by commenting in the comment section below. Here is a question for you, What is the working principle of VCB?

One Comment

  1. Zaw Min lwin says:

    Many many thanks toyour knowledge sharing

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