The Vacuum Circuit Breaker – Its Inception, How It’s Built, and How It Works

Electrical components like circuit breakers are brilliant feats of engineering, but they can be difficult to understand. Luckily for you, we’ve done our homework on vacuum circuit breakers, and we’re here to tell you all about them.

A circuit breaker is a piece of equipment that switches off an electric circuit to stop an overload of unnecessary current (also known as a power surge) from happening after a fault is identified, preventing possible appliance damage or a fire. A vacuum circuit breaker is where the arc quenching (i.e. heat treating) occurs within a vacuum medium. The process of turning on and off current-carrying contacts and corresponding arc interruption occurs in a vacuum chamber within the breaker, which is known as a vacuum interrupter.

The Vacuum interrupter technology was first revealed in 1960. Despite having been around for 60 years, it is still an evolving technology. The vacuum interrupter’s size has shrunk gradually over the years thanks to various technical advances in this engineering field.

How Vacuum Circuit Breakers (VCBs) are Built

The vacuum circuit breaker consists of a steel arc chamber in the centered and symmetrically positioned ceramic insulators. The pressure within the vacuum interrupter is kept lower than 10^-4 (.0001) Torr (a unit of pressure used in measuring vacuums). For a frame of reference, a perfect vacuum is 0 Torr, and outer space—which is as close to a perfect vacuum as can be achieved—has a vacuum pressure as high as 10^-6 Torr and as low as 10^-17 Torr.

The substances used for current-carrying contacts have a significant effect on the vacuum circuit breaker’s performance. Alloys such as copper-bismuth or copper-chrome are the best materials to create VCB contacts from.

The vacuum circuit breaker is comprised of a stationary contact, a moving contact, and a vacuum interrupter. The moving contact is attached to the control apparatus by a stainless steel bellow. The arc shields are reinforced to the insulated housing in a way that they cover these shields and deter condensation. The potential for a leak is removed thanks to the permanent, airtight closing of the vacuum chamber, for which a glass or ceramic container is used as the outer insulating body.

How VCBs Work

In the vacuum circuit breaker where the contacts are disconnected because of some unusual conditions, an arc is struck amid the contacts; the arc is created as a result of ionization of metal particles and is heavily reliant on the contacts’ material.

The arc interruption in vacuum interrupters is not the same as other kinds of circuit breakers. The disconnection of contacts results in the release of vapor, which fills in the contact area. It is made of positive ions released from contact material. The vapor density is determined by the current in the arc. When the current dwindles, the vapor release rate drops. After current zero, the medium recovers its dielectric strength if the vapor density has decreased.

When the current to be interrupted in a vacuum is very tiny, the arc has multiple parallel routes it can take. The total current is separated and distributed into numerous parallel arcs that repel each other and extend over the contact surface. This is known as a diffused arc, which can be effortlessly interrupted.

At high current values, the arc gets intensified in a small area. This results in quick vaporization of the contact surface. There is potential for interruption of the arc if it stays in a diffused state. If it is removed rapidly from the contact surface, the arc will be re-struck.

The contacts’ material and shape, along with the method of taking into account metal vapor, considerably affects arc extinction in vacuum breakers. The arc’s trail is kept moving so that the temperature won’t be too high at any given point in time.

After the concluding arc interruption, there is a quick buildup of dielectric strength, which is particular to the vacuum breaker. They are appropriate for capacitor switching because they will provide a re-strike free performance. The lesser current is interrupted prior to natural current zero, which might result in chopping, and whose level is contingent on the contact’s material.

The Benefits of VCBs

Vacuum circuit breakers provide the best insulating strength, so it has exceptional arc quenching properties surpassing any other medium. They have several other benefits, too:

  • VCBs have a longer lifespan compared to other circuit breakers.
  • VCBs do not need any extra filling or periodic refilling of oil or gas.
  • There is no explosion in VCBs, making them different from an Oil Circuit Breaker (OCB) or air blast Circuit Breaker (ABCB). This increases the operating personnel’s safety.
  • There is no fire hazard.
  • VCBs are swift in operation, making them great for fault clearing.
  • VCBs are acceptable for repeated use.
  • VCBs need virtually no maintenance.
  • No gas exhaust is released into the atmosphere.
  • It is silent while operating.
  • VCBs have quick recovery of high dielectric strength on current interruptions that merely a half rotation or less arcing happens after proper contact separation.
  • The breaker unit is tightly packed and self-contained and can be installed in any necessary orientation.
  • The two previous bullet points above combined with the economic benefit provided are reasons why VCB has high acceptance.

The Weaknesses of VCBs

  • The main drawback of VCBs is that they stop being economical at voltages higher than 38 kilovolts (kV).
  • Additionally, VCB production is also not economical when manufactured in small batches.
  • The VCB’s price tag becomes outrageous at voltages higher than 38 kV because more than two breakers are necessary to be connected in series.
  • VCBs require high technology for the manufacturing of vacuum interrupters.
  • VCBs require extra surge suppressors for the interruption of low magnetizing currents in a particular range.
  • Loss of vacuum as a result of transit damage or failure renders the whole interrupter useless, and it cannot be fixed on-site.

VCB Applications

These days, the vacuum circuit breaker is known as the most dependable current interruption technology for medium voltage switchgear. It needs little maintenance, unlike other circuit breaker technologies.

The technology is mostly appropriate for medium voltage applications. Vacuum technology has been created for higher-voltage applications, but this technology isn’t commercially achievable. Vacuum circuit breakers are utilized in metal-clad switchgears as well as in porcelain-contained circuit breakers.

Thanks to the short gap and outstanding recovery of vacuum circuit breakers, they are very beneficial as very high-speed-making switches in various industrial applications.

When there is high voltage and the current to be interrupted is low, vacuum circuit breakers have a definitive advantage over other types of breakers.

For lesser-fault-interrupting capacities, the price is small compared to other interrupting devices.

Due to the least requirements of maintenance, these breakers are quite suitable for systems that need voltage from 11 kV to 33 kV.

If you have questions about vacuum circuit breakers that weren’t answered here, contact Electric Control. You can call us at 1-800-521-0197, or email us at ElectricControl@CBSalesNE.com.

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