Circuit breaker

Circuit breaker (CB):

CB is nothing but a special kind of switch, which can be operated manually and automatically. In power system network, the load is increasing day by day. All power system equipment was design for a specific range of current. If system current (current following through the equipment) is increased beyond this range, equipment may damage and cost of this equipment is high. So, it important to protect the equipment and CB plays important role in protection engineering.

When a fault occurs in the system, an excessive amount of current will flow through the system. This current will thousand times higher than the rated current, that will depend on the type of fault. In the overload condition, current will increase, but in this case, current will be a few times higher than the rated current. The equipment or the network of the power system which we want to protect that will be connected in series with CB and relay. CB cannot sense the fault, the relay is used to sense the fault and if the current is increased from pre-specified value, it will give a signal to CB and CB will operate. This is how fault protection scheme works in the power system.

Working of CB:

CB has two arms (contacts); fixed arm and moving arm. If these contacts are in contact (ON) that means the circuit is close and if these contacts aren’t in contact (OFF) that means the circuit is open. Contacts will be ON or OFF, that depends on the operating condition of the system. CB is ON in normal condition and it will manually or automatically OFF in abnormal conditions. A handle connects with the moving arm and this handle is used for the manual operation of CB.

Working of CB looks easy, but it is very difficult in case of higher voltage (in kV). When CB is turned on or off, at that time spark occurs between the contacts. The reason behind the spark is a very high potential difference between the arms. One arm has zero voltage and another arm has rated voltage (kV) at a time of switching. This potential difference is sufficient to ionize the air between the arms and create a spark between the two contacts. Very high amount of current will flow through the contacts and this current is sufficient to damage the contacts. Arc quenching medium is used the contacts to avoid this condition. When CB is turn ON or OFF, arc quenching medium comes to the contacts and quench the arc. SF6, transformer oil, air etc are used as a quenching medium. CB can classify according to arc quenching medium.

Classification of CB:

 According to arc quenching medium, classification of CB is as below;

              1)      SF6 CB: A CB which uses SF6 (sulphur hexafluoride) as an arc quenching medium, that is known as an SF6 circuit breaker. SF6 CB is advanced and most used CB compare to another CB. SF6 has very good insulating property and very high electronegativity.  When an arc occurs, free electrons are produced because of ionization process. These free electrons are colliding with SF6 gas molecules. SF6 molecules have a tendency to absorb the free electrons. SF6 has very good heat transfer capability. The high temperature is produced due to arc and SF6 has the ability to reduce the temperature. SF6 is 100 times more efficient arc quenching medium compared to air. This CB is used for the voltage range of 33 kV to 800 kV. The disadvantage of this CB is that SF6 has a global warming potential that is 23,900 times worse than CO2.

             2)      Oil CB: Oil is used as a dielectric medium in this type of CB. This oil is known as transformer oil or mineral oil. This type of CB is a very oldest type of CB and rarely used in present power system network. The oil has good dielectric property and acts as the best liquid insulator. The fixed contact and moving contact are immersed in the insulating medium of mineral oil. When circuit breaker turns ON and OFF, the arc will initialize between the contacts. The oil is vaporized and decompose in hydrogen gas. This gas is produced in form of bubbles and these bubbles are produced around the arc and prevent restricting of the arc after current reaches zero crossing of the cycle. Two types of oil circuit breakers are available; bulk oil circuit breaker and minimum oil circuit breaker. Oil CB is used for the voltage range of 1 kV to 300 kV.

            3)      Air CB: This type of CB uses atmosphere air as an arcing medium so this CB is known as air circuit breaker or air blast circuit breaker. This CB is not used for very high voltage. It used for low or medium voltage level. This CB is preferable compared to oil CB because this CB is safer. There are chances for oil blasting which is very dangerous. In developed countries, air circuit breaker is used instead of oil CB. Working principle of air CB is different compared to another CB. It creates an arc voltage for interruption of the arc. Arc voltage is a minimum voltage required to maintain the arc.

·         Arc voltage can be increased by cooling of arc plasma.

·         Arc is a divide into a number of the series path.

·         By increasing the length of the arc.

Two types of air blast CB are available; plain air CB and air blast CB.

               4)      Vacuum CB: Vacuum is used as a dielectric medium, so this type of CB is known as vacuum CB. This CB technology is not fully developed. The performance of CB is depending on the material used for the contacts. Generally, copper/chrome is used for good performance. The vacuum has very good dielectric properties. The dielectric strength of vacuum is eight times higher than air and four times higher than SF6 gas. When breaker operates, the production of an arc is due to the ionization of metal ions. The metallic vapor produces electron and ions during the arc. So, the arc will be extinguished quickly. Vacuum CB is used in the distribution network and voltage range is 22 kV to 66 kV. Generally, this CB is used in a rural area.  

Classification according to voltage level,

             1)      Low voltage CB: This type of CB is used in the domestic, commercial and industrial application. The voltage level around 1 kV is considered as a low voltage CB. It includes MCB, MCCB, and RCCB.

MCB stands for the miniature circuit breaker. MCB is an electromechanical device. MCB provides protection against overcurrent and overload. When a fault occurs in the system, it will produce a high amount of current. This condition is known as overcurrent. If the load is increased, system current will increase. 

This condition is known as overload. MCB can operate manually and it will operate automatically in case of overload and overcurrent. In normal condition, a force produced by a current caring coil is less than the force produced by the spring. In overcurrent condition, the force of current caring coil is greater than the force of spring and it will responsible for the trip. This is called as magnetic tripping. 

In case of overload, bimetallic strip is used. The displacement occurs in bimetallic strip due to overload. This tripping is called thermal tripping.

MCCB stands for Moulded Case Circuit Breaker. MCCB is used for 250 A to 800 A current. 

         2)      High voltage CB: High voltage CB is not used in the industrial, residential and commercial application. This type of CB is used in the range of kV. Generally, high voltage CB is used in power system transmission and distribution. In power system network, CB is used to isolate faulty part from rest of the healthy system. 

    For example, a 220-kV transmission line is connected to power system network and fault occur in this line for any reason. So, it is necessary to isolate this line from the rest of the network. Two CB are connected at both ends of the transmission line and in this condition both CB open and isolate the faulty line.