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BASIC KNOWLEDGE - CIRCUIT BREAKER What is a Circuit Breaker? Definition, types, and more

From Venus Kohli Reading Time: 15 min

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Circuit breakers are now standard household equipment that protects electrical systems from fire and overcurrent. This protective switchgear saves lives, time, and money for power system maintenance. The article explains circuit breakers in detail along with their working principle, types, uses, applications, replacement, and tests.

This image shows a Voltage switchboard with circuit breakers.
This image shows a Voltage switchboard with circuit breakers.
(Source: romaset -

1. What is a Circuit Breaker?

A Circuit Breaker was first mentioned by Thomas Edison in his 1879 patent application. However, a group of Swiss companies Brown, Boveri & Cie. patented the world’s first miniature circuit breaker (MCB) in 1924.

Circuit Breaker Definition

A Circuit Breaker is a type of mechanical switchgear that automatically disconnects the circuit to protect the elements of a power system from the damaging effects of overcurrent. In simple words, a circuit breaker disconnects a circuit under faulty conditions by interrupting the overcurrent flow and reconnects the circuit again when conditions become normal.

2. What does a Circuit Breaker do?

A circuit breaker “makes” and “breaks” a circuit automatically under faulty conditions such as a short circuit or overload. When the current value becomes large enough to cross the predetermined value specified by the manufacturer, the current is termed overcurrent. The circuit breaker automatically senses the overcurrent and disconnects the circuit to break the fault current flow and prevent damage such as fire.

Apart from opening the circuit, a circuit breaker should be able to close the circuit during the faulty condition. It carries the overcurrent for a time period of T until the fault is recovered in the power system. Circuit breakers isolate the faulty components of the power system from further damage and resume operation post-repair of the faulty components. However, a circuit breaker must not damage itself within the limit of its power rating.

Three duties of a Circuit Breaker

  • The circuit breaker should be capable of opening the faulty circuit and breaking the faulty current flow.
  • The circuit breaker must remain closed during the faulty condition.
  • Circuit breakers must carry fault current for a time period until the fault is cleared.

3. How does a Circuit Breaker work?

What is the Arc effect in a circuit breaker?

Arc effect is observed when heavy current flows through the non-conductive path developed between two separating metal contacts inside a circuit breaker. The electric arc is produced when an ionized gas with molecules that have lost one or more electrons generates plasma-the fourth state of matter. The plasma in the electric arc results in visible light (just like a spark) that provides a path for heavy current flow.

Image one. This Image shows an effect similar to the arc effect.
Image one. This Image shows an effect similar to the arc effect.
(Source: BORIS -

When these metal contacts begin to separate, the contact area and pressure decrease. This in turn increases resistance by a small value of about 1 ohm. The large value of fault current (in the order of mega ampere) produces a heavy potential difference of the same order. The electronics dislodge from the cathode surface towards the anode. Due to a decrease in pressure and an increase in heat, the gas in the circuit breaker ionizes.

The ionized gas contains positive ions and free electrons. Due to their small size, free electrons move at a higher velocity toward the anode compared to positive ions. The free electrons moving towards the anode collide with molecules that knock off more electrons and increase the heat. The kinetic energy and velocity of the electrons increase, leading to more collisions. This effect is known as field emission because electrons are emitted due to the electric field. The current due to positive ions moving toward the cathode is very less compared to the current produced by the flow of electrons. The current path between two separating metal contacts consisting of plasma is termed an electric arc. In short, the electric arc provides a path for the heavy current to flow without any interruption.

The second reason for electric arc generation is thermionic emission. The vibration of molecules may absorb the heat energy and knock off electrons from the atom. The decrease in contact area due to the separation of metal contacts increases the current density, leading to increased temperature and vibration of molecules. The increased temperature boosts the ionization of the gas inside the circuit breaker.

Methods of Arc Extinction in a Circuit Breaker

Arc voltage is in phase with arc current. As time increases, the arc current starts dissipating as heat. The arc current is inversely proportional to the arc resistance that increases with the distance between the metal contacts.

As per Ohm’s law and formula of resistance, the arc resistance depends upon

RArc = p LArc / AArc
RArc = Resistance of the arc
LArc = Length of the arc
AArc = Area of the cross-section of the arc

The electric arc can be extinguished by reducing the diameter and increasing the length of the arc. Another way of arc quenching is by decreasing the potential difference between the contacts.

Reducing the degree of Ionization

The conductance of the arc depends directly upon the number of ionized electrons/cm3. By removing the charged particles, an electric arc can be extinguished. The temperature may increase high enough to damage the electrical equipment. An electric arc can be quenched by cooling the internal structure of the circuit breaker which results in the recombination of the particles. The second way to quench the arc is to introduce a gas that may absorb negatively charged particles (electrons).

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High resistance method

In this method, the arc resistance is increased by increasing the length of the arc. The method is used in low-voltage AC circuit breakers and low-medium voltage DC circuit breakers because it is physically difficult. Usually, in cases of high voltages problems arise due to the need for large separation between the contacts. Reducing the diameter of the arc will increase the arc resistance. Other methods to increase the arc resistance are cooling and splitting the arc.

Low resistance or current zero method

In this method, the strength of the dielectric medium in between the metal contacts is built up rapidly compared to the potential difference between them. The arc fails to restrike and gets suppressed. It can be achieved by recombining the ionized particles or introducing another gas to generate new particles inside the circuit breaker. However, this method is only used for high-voltage AC circuit breakers and high-voltage DC circuit breakers with artificial current zeros. The distance between the contacts can be as low as 20 mm.

Circuit Breaker explained in detail

A circuit breaker has two types of contacts- a fixed and moving contact. These contacts are termed electrodes as one of them is a cathode and the other is an anode. The fixed contact stays in the same position while the moving contact moves away or towards the fixed contact depending upon the operating conditions. Both coils are connected to each other via a spring mechanism.

Image two. Internal structure of a circuit breaker.
Image two. Internal structure of a circuit breaker.
(Source: Venus Kohli)

The current transformer is connected to the transmission line for stepping down the high value of current into a low value for measurement and operation purposes. The primary coil of the current transformer is connected to the transmission line and the secondary coil is connected to the circuit breaker’s auxiliary relay.

Image three. Closed circuit breaker under normal operating conditions.
Image three. Closed circuit breaker under normal operating conditions.
(Source: Venus Kohli)

As soon as the faulty enters the transmission line, the secondary coil of the current transformer energizes. The relay of the circuit breaker gets energized after sensing the faulty current and sends the respective signals. The relay coil generates an MMF (magnetomotive force) that closes the trip coil circuit. The trip coil of the moving contact gets energized by generating an MMF. It pulls the moving contact away from the fixed contact and opens the circuit breaker. This movement opens the circuit to break the overcurrent flow.

Image four. Open circuit breaker under faulty conditions.
Image four. Open circuit breaker under faulty conditions.
(Source: Venus Kohli)

When the voltages are high, a strong electric arc is produced between the contacts. A strong mechanism is needed to quench the electric arc because simply opening the circuit does not guarantee its extinguishing.

What happens after the electric arc extinguishes?

After the electric arc extinguishes, the circuit breaker must perform reclosure for normal operation. Usually, two or more circuit breakers are connected in series. Whenever overcurrent passes in the system, one of the circuit breakers should be opened for repair and another one should be closed to allow faulty current flow. The circuit breaker re-closes after the damage gets fixed. However, a large value of current produces heat that may melt the contacts of the circuit breaker- a condition that must be avoided.

Circuit Breaker ratings

  • Rated Voltage: Rated Voltage is the highest phase-to-phase voltage that the circuit breaker is capable of withstanding without damage. It is always more than the nominal voltage and is expressed in KV rms.
  • Rated Frequency: Maximum frequency for which the circuit breaker operates without failure.
  • Operating Sequence: The operating sequence of a circuit breaker is “CO”- closing followed by a rapid opening. Another sequence can be CO-t1-C— closing followed by a rapid opening that sustains for time t1. After time t1, the circuit closes, and so on.
  • Standard Duty Cycle: The standard duty cycle of a circuit breaker is a sequence of two operations in a time period between two operations. For eg: O-t1-CO-t2-CO.
  • Breaking capacity or rated short-circuit breaking current: The breaking capacity of a circuit breaker is defined as the current breaking capability of a circuit breaker at a certain recovery voltage under specific conditions. In simple words, breaking capacity is the rms value of the fault current at the instant of contact separation initiation.
  • Making Capacity or Rated Short-Circuit Making Current: The making capacity of a circuit breaker is defined as the maximum capability of a circuit breaker to withstand faulty current and close against the forces. In simple words, making capacity is the value of peak current during the first half cycle of current after its closure of the contacts. It includes the DC component of the peak current as well.
  • Short-Time Rating: The short-time rating is defined as the time period for which the circuit breaker allows faulty current flow while being safely closed. The circuit breaker should neither be tripped nor damaged during the short-term rating.
  • Short-Time Current Rating: The short-time current rating is defined as the rms value of the current for which the circuit breaker is closed for the time period without any damage.
  • Nominal Current Rating: The nominal current rating is the rms value of the continuous current of the circuit breaker without temperature rise exceeding specified conditions.

4. Circuit Breaker types

Circuit breakers are classified into multiple types depending upon the mediums used for arc extinction and voltage-withstanding capacity.

Image five. Circuit breaker types based on medium and voltage handling capacity.
Image five. Circuit breaker types based on medium and voltage handling capacity.
(Source: Venus Kohli)

However, there are four main mediums for circuit breakers:

  • Oil
  • Air
  • Vacuum
  • Sulphur Hexafluoride

Higher dielectric strength of the insulating medium enables arc quenching mechanism with efficiency.

Image six. Dielectric strength of different circuit breaker media.
Image six. Dielectric strength of different circuit breaker media.
(Source: Venus Kohli)

Category 1: Low Voltage Circuit Breakers (V < 1000 Volts)

  • Miniature Circuit Breaker (MCB): MCB (Miniature Circuit Breaker) is one of the most popular household circuit breakers that operate for low voltages. It can be turned on and off manually and works through magnetism. The bimetallic strip in the MCB senses the overcurrent and releases a mechanical latch. MCB comes in various types such as A, B, C, D, G, H, and K.
  • Air Break Circuit Breaker: Air Break circuit breakers are used in low voltage applications with a less contact life of about 6 short circuits. This circuit breaker is of two types- plain air break and magnetic blow-out air break circuit breakers. The air break circuit breaker employs the method of high résistance interruption for arc quenching. The resistance of the arc is increased by reducing the area of the cross-section and increasing the length of the arc. Arc can also be quenched by cooling and spitting it inside the circuit breaker.

Category 2: Medium Voltage Circuit Breakers (1 kV - 33 kV)

  • Minimum Oil Circuit Breaker (MoCB): Oil circuit breakers generally use transformer oil as an insulating medium to extinguish the arc. It is because oil is a good dielectric medium having a high dielectric strength of 110 kV/cm. The ions meant for producing the arc react with oil to release gasses like hydrogen (70-80%), methane, ethylene, and acetylene. Since hydrogen is a good conductor of heat, the hydrogen bubble near the contact cools down the system to promote de-ionization for the production of the electric arc. The turbulence of oil in the arc path is another factor that forbids its production. The contact life of such circuit breakers is around 6 short circuits and is more likely to be replaced frequently.
  • Vacuum Circuit Breaker: A vacuum circuit breaker uses a medium with pressure less than that of the atmospheric pressure i.e. pressure less than 760 mm of Mercury. Unit Torr (1 mm of Hg) is used to measure such low pressures. In vacuum circuit breakers, a vacuum arc quenching medium of 10-5 to 10-7 Torr is used. The dielectric and insulating strength of the medium is the highest among other circuit breaker mediums. The presence of the vacuum allows micro projections to produce metal ions and form an electric arc. The vacuum breaker is fast enough to interrupt the faulty current within the very first cycle. The explosion possibility of the vacuum circuit breaker is NIL and it offers a contact life of about 100 short circuits.

Category 3: High Voltage Circuit Breakers (33 kV - 220 kV) and Extremely High Voltage Circuit Breakers (V > 400 kV)

  • Air Blast Circuit Breaker: Air blast circuit breakers are used as a replacement for oil for medium, high, and extremely high voltages. However, air blast circuits are frequently used for high voltages of more than 110 kV. High-pressure air or compressed air is used as a method for quick arc quenching instead of other gasses like nitrogen, carbon dioxide, and hydrogen. The choice of air instead of other gasses reduces the cost and size of the circuit breaker. Due to the presence of air instead of oil, there is no risk of fire. The contact life of air blast circuit breakers is around 25 short circuits and they offer re-closures (reuses).
  • SF6 Circuit Breaker: Sulphur Hexafluoride Circuit Breaker uses SF6 gas for extinguishing the arc in high and extremely high voltage applications. The sulfur hexafluoride gas has high dielectric strength and electronegative properties of absorbing the free electrons. The gas produces negative ions which are relatively slower than free electrons to enable ionization for arc generation. The contact life of SF6 circuit breakers is around 25 short circuits and the same gas can be used after operation. Other features include a low risk of fire, and non-explosive, noble, and non-poisonous properties.

5. Circuit Breaker advantages

  • Circuit breakers operate in groups and protect multiple electrical equipment and switches in a power system.
  • Circuit breakers are fast in operation and ensure continuity in the power supply.
  • The circuit breaker extinguishes the electric ark and prevents re-striking.
  • Circuit breaker protects the electrical equipment it is supplying power to from overcurrent.
  • Circuit breaker operates within no-load, full-load, and faulty conditions.
  • The circuit breaker performs switching functions manually. It opens and closes a circuit upon manual input at the time of repair or replacement of electrical equipment.
  • Circuit breakers can be reliably used for high-voltage applications.
  • Circuit breakers are better than mechanical fuses because they offer multiple reuses.
  • Circuit breakers require auxiliaries like relays and motors for full operation.
  • Circuit breakers save replacement time and reduce maintenance costs.

6. Circuit Breaker uses

What is a Circuit Breaker used for?

Replacement of Fuse

Compared to another switchgear-like fuse, a circuit breaker is smaller in size and can be automatically reset for repeated operation. Fuses are made up of metal and are placed in the circuit for protection. Whenever heavy current flows through the circuit, the fuse melts or blows up. A fuse can be used only one time and cannot be reset post-repair of faults in the circuit. Every time a new fuse must be added to the circuit for a one-time operation. Moreover, fuses have slow capabilities to determine the time to start operation and are used for low-voltage applications. On the other hand, circuit breakers are fast enough to operate in faulty conditions. A circuit breaker can be automatically reset post-repair of the circuit and ensures continuous supply. Using a circuit breaker instead of a fuse eliminates the cost of replacement, saves time, and enables high-voltage operation.

Circuit Breaker as a Switch

The circuit breaker can also act as a switch that manually turns on and turns off the power supply to electrical equipment in the power system. It can be used as a switch to manually turn on and turn off the supply for repair or replacement purposes. When power system components like transformers, coils, and other equipment or circuit breakers need to be repaired or replaced, a circuit breaker isolates those components. A circuit breaker breaks the current flowing to the device and disconnects the device for further procedures. Post repair or replacement, circuit breakers can be switched on for regular operation. A normal switch cannot be used in faulty conditions because it needs a manual turn-off. One cannot judge that the overcurrent flow has passed design limitations. Circuit breaker automatically “senses” overcurrent and breaks the circuit immediately.

Circuit Breaker applications

  • Circuit breakers are used in switching different kinds of loads in households, commercial, buildings, and industrial places.
  • Circuit breakers are used as switchgear to protect electrical equipment in household and commercial areas from faulty current, overcurrent, short-circuit, and fire.
  • Circuit breakers are used in feeder circuits and motors.

7. How to test a Circuit Breaker?

There are multiple ways to test a circuit breaker using an analyzer, ohm-meter, multimeter, and many more devices. Each type of circuit breaker operates differently and requires different tests. Several tests like the mechanical test, thermal test, dielectric test, short-circuit test, voltage test, and resistance test. The correct method must be chosen by a certified electrician or an engineer for safety purposes. However, one can test circuit breakers for the potential difference with a digital multimeter.

The general test for a circuit breaker

Step 1: Dry all the areas near the electric panel. Clean the wet floor as well and wait for a few minutes.

Step 2: Open the electric panel and choose the circuit breaker for the test.

Step 3: Uncover the circuit breaker using a screwdriver.

Step 4: Turn off the power supply for all the devices and appliances in association with the circuit breaker.

Step 5: Put the multimeter settings to “AC voltage”.

Step 6: Touch the red lead of the multimeter to the circuit breaker terminal screw (hot wire) and the other black lead to the ground screw.

The multimeter displays the voltage across the breaker screw.

Step 7: If the potential difference is zero, the circuit breaker is faulty and needs a replacement.

Step 8: Change the multimeter settings to ohm (resistance).

Step 9: Touch one lead of the multimeter to the supply and another to the screw.

Step 10: The resistance reading should be zero at the time of turn-off. A value of resistance indicates a faulty circuit breaker that needs a replacement.

8. How to replace a Circuit Breaker?

Replacing a circuit breaker is a professional task that must be handled by a licensed electrician. However, a beginner can safely replace a circuit breaker by following a few simple steps.

Step 1: Turn off the entire power of the house for safety purposes. Beginners should be extra careful. However, professional electricians perform this task without turning the power off.

Step 2: Carry a flashlight.

Step 3: Locate the circuit breaker panel and turn it off.

Step 4: Remove the breaker panel plate covering the circuit breaker with a screwdriver.

Step 5: Locate the faulty circuit breaker and turn it off as well.

Step 6: Start removing all the wires and remove the circuit breaker.

Step 7: Disconnect all the wires from the circuit breaker.

Step 8: Connect all the wires to the new circuit breaker.

Step 9: Insert the new circuit breaker in the same place.

Step 10: Turn on the panel and power supply.


Circuit breaker test references:


Circuit breaker replacement reference:


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