How do vacuum circuit breakers work?

Written by stuart withers Google
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How do vacuum circuit breakers work?
Circuit breakers are essential safety features of electrical circuits. (Jupiterimages/Brand X Pictures/Getty Images)

The vacuum circuit breaker is one of many circuit breaker types and serves a vital function in medium to high-voltage applications. Circuit breakers are responsible for stopping the flow of electricity in an electrical circuit. They are used as a safety device when currents in a circuit become too high, and also as a way of safely cutting off electricity currents to perform maintenance work. Without circuit breakers, there would be no way to control and stem the electrical current flowing through electrical appliances. An uncontrollable current can lead to fires, electric shocks, and even death.

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Electricity basics

Before we get into the workings of circuit breakers, we need to understand the basic principles of electricity. Electricity can be divided into voltage, current, and resistance. Voltage is a force of pressure that makes electrical charges move through a conductor (such as wire). Current is the rate at which these charges flow through the conductor and resistance explains the natural resistance of a given conductor, which is dependent on the conductive material and its size and shape. So, for example, if you increase the voltage or decrease the resistance, then more electrical charge will flow, and vice versa. These basic principles of electricity are what vacuum circuit breakers use to stop the flow of current.

Electrical circuits

Electrical circuits are designed with safety as a paramount feature. After all, high voltage electricity can be fatal, and electricity is all around us, from overhead wires to your plug socket and household appliances. A key safety feature of electrical circuits is to have a wire that connects to the ground. Electricity seeks to ground itself, allowing for grounding wires to be used to stabilise the voltage and current in a circuit. Nevertheless, from time to time, grounding the electricity can be compromised. Circumstances such as accidently cutting through a wire or an appliance overheating can stop electricity being dissipated in the ground, causing excess and dangerous amounts of voltage and current in the circuit.

Circuit breaker basics

Circuit breakers are basically designed to interrupt the flow of current. All circuit breakers contain metal contacts that are held together by a spring-loaded mechanism. When these metal contacts are touching, the circuit is complete and current can flow. However, all circuit breakers also contain both heat and magnetic sensors, which can be used to detect when too much current is flowing in a circuit. As soon as the sensors detect that something is wrong with the temperature or magnetic fields and the current is too high, they trip the spring-loaded mechanism which pulls the metal contacts apart. Once the metal contacts are separated, the circuit is broken, and no more current can flow.


Electricity will always flow down the path of least resistance, which means if there’s a good conductive material nearby, electricity will always choose this path over a more resistive, less conductive path. As the metal contacts are good conductors, arcing can occur for a split second after the contacts have been separated. Arcing is essentially electricity trying to follow the path of least resistance, bridging itself between the metal contacts before they become too far apart. Even a split second of arcing can be enough to make the air surrounding the circuit ionised, causing it to be electrically conductive. Electrically conductive air can give the electricity a chance to reform the circuit. Because of this, it’s highly important for medium to high-voltage circuits to prevent the air becoming electrically conductive by using materials that cannot be ionised.

The vacuum breaker

The vacuum breaker uses the power of a vacuum to stop ionisation of the air. A vacuum is a term that describes completely empty space, devoid of any particles of matter that can conduct any significant amounts of electricity. This makes vacuums perfect for use in circuit breakers, as there’s no way the circuit can be maintained once the metal contacts have separated. The contacts are surrounded with enough of a vacuum to dissipate any unwanted electrical arcing, making the breaker safe for use in applications where higher voltages and currents are often used.

Vacuum breaker uses

Due to their ability to break medium to high-voltage circuits, vacuum circuit breakers are commonly found in commercial and industrial settings. They can be used on power and transmission lines, and are also commonly used as a trip mechanism in large electrical cables, transformers, capacitors and motors. Modern technology is seeing the vacuum circuit breaker come in ever more compact designs. The more compact these circuit breakers get, the greater the potential they have to be used in a wider range of practical applications.

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