Study of Circuit breaker

1. Study of Circuit breaker By:- Ankit bansal UE104006 EEE

2. Introduction • Inspired by the works of American scientist Joseph Henry and English scientistMichaelFaraday, the circuit breaker was invented in 1836 by an American, Charles Grafton. • The American National Standards Institute (ANSI) defines a circuit breaker as: “A • mechanical switching device, capable of making, carrying and breaking currents • under normal circuit conditions. Also capable of making and carrying for a • specified time and breaking currents under specified abnormal circuit conditions, • such as those of a short circuit.” • A circuit breaker is an equipment which is designed to protect an electric circuits from damage caused by short circuit or overload. • The basic circuit breaker working principle is that it is an automatically operated electric switch.Itdetects the fault condition and interrupt the continuity to stop the flow of electricity immediately. • It is not like a fuse which operates once and need to be replaced ,a circuit breaker can be reset (either manually or automatically) to resume it’s normal operation.

3. Why do we need Circuit Breakers? • In every country the there will be a standard value for the voltage distributed by the power plants. In INDIA it is 240 Volts. Though this is a constant value, the value of the resistance and current will always vary according to our connections. • When we buy electrical components like bulbs and tubes, their company’s provide a standard resistance value for their effective working. This value, often referred to as the load, is the real reason for the appliances to work. • The components are designed in such a manner that the total voltage, current and the resistance across it are always constant. • If the current through a component increases, the heat may increase causing the melting down of the wires inside. This causes a contact between the neutral and phase wires. • The fire thus produced may even cause the whole building to collapse. To avoid this, circuit breakers are used.

4. Construction of Circuit Breaker

5. Frame:-The frame has a molded case exterior. Its primary function is to provide a rigid,mechanically strong, insulated housing in which the other components are mounted. • Operating Mechanism:- The operating mechanism provides the means of opening and closing the circuit. It has a three-position switch (“on,” “off,” and “tripped”). It can only be in the “tripped” position as a result of an overcurrent. When power is removed manually, it can only be switched to the “off” position. This makes it is easy for a maintenance person to determine why power has been cut. • Trip Unit:- The trip unit is the brain of the miniature circuit breaker. It activates the operating mechanism in the event of a prolonged overload or short circuit. This type of circuit breaker uses a thermal magnetic mechanism. This is the predominant trip unit technology used in the domestic market. A bimetal and an electromagnet work together to provide overload and short-circuit protection. • Contacts:-When an overload or short circuit situation occurs, the contacts open to break the current flow. When this happens, an electrical arc is formed. The arc continues until the first possible zero point in the AC cycle. The zero point is the weakest point in the AC cycle and will not support the continuance of an arc. By breaking the arc, flow is stopped. This is called zero point construction.

6. Working principle of circuit breakers • Miniature Circuit Breakers are based on thermal magnetic technology. The protection is Provided by combining a temperature sensitive device (bimetal) and a current sensitive electromagnetic device. Both components trigger the mechanism mechanically. • Tripping Mechanisms • A tripping mechanism is an assembly within the circuit breaker molded case that causes the circuit breaker to open automatically under sustained overload or short circuit conditions. • The tripping mechanisms in two- and three-pole circuit breakers operate such that an overcurrent on any pole of the circuit breaker will cause all poles of the circuit breaker to open simultaneously. • Thermal Trip:- • The thermal trip element of a circuit breaker is a root-mean-squared (rms) current sensing device. • The thermal element, or bimetal, is constructed from metals with dissimilar rates of expansion bonded together.

7. The thermal element responds to overloads by reacting to the heat generated both by the current flowing through the circuit breaker and by the heat contribution from ambient conditions. The bending force of the bimetal causes the circuit breaker to trip. • The deflection of the bimetal is predictable as a function of current and time. This is the inverse time tripping characteristic of the thermal element, i.e., the tripping time decreases as the magnitude of the current increases.

8. Magnetic Trip:- • The magnetic (instantaneous) trip element uses an electromagnetic assembly to trip the circuit breaker instantaneously (with no intentional delay) at or above a predetermined current value. • During a short circuit of sufficient magnitude, the high-level current passing through the conductor rapidly increases the magnetic field of the electromagnet that attracts the armature. As the armature is drawn toward the electromagnet, it initiates an unlatching action and opens the circuit breaker contact.

9. Arc phenomenon • when a short circuit occurs, the contacts of circuit breaker begin to separate. • The potential difference between the contacts is sufficient to ionize the air or oil or gas. • This ionized air acts as conductor and ARC is struck between them even though contacts of breaker are opened.

10. Types of circuit breaker • based of insulating medium employed in the circuit breakers , CB’s are of 4 types:- • Oil Circuit Breakers • Air Blast Circuit Breakers • Vacuum Circuit Breakers • SulphurHexafluoride (SF6) Circuit Breakers

11. SULPHUR HEXAFLUORIDE (SF6) CB • A circuit breaker in which the current carrying contacts operate in Sulphur Hexafluoride or SF6 gas is known as an SF6 Circuit Breaker. • SF6 has excellent insulating property. SF6 has high electro-negativity. That means it has high affinity of absorbing free electron. • Whenever a free electron collides with the SF6 gas molecule, it is absorbed by that gas molecule and forms a negative ion. • The attachment of electron with SF6 gas molecules may occur in tow different ways, • 1) SF6 + e = SF6 - 2) SF6 + e = SF5 - + F

12. These negative ions obviously much heavier than a free electron and therefore over all mobility of the charged particle in the SF6 gas is much less as compared other common gases. We know that mobility of charged particle is majorly responsible for conducting current through a gas. • SF6 gas is approximately 100 times more effective arc quenching media than air. Due to these unique properties of this gas SF6 Circuit Breaker is used in complete range of medium voltage and high voltage electrical power system. • Working of SF6 Circuit Breaker • the arc energy is utilized to develop pressure in the arcing chamber for arc quenching. Here the breaker is filled with SF6 gas at rated pressure. • There are two fixed contact fitted with a specific contact gap. A sliding cylinder bridges these to fixed contacts

13. The cylinder can axially slide upward and downward along the contacts. There is one stationary piston inside the cylinder which is fixed with other stationary parts of the breaker, in such a way that it can not change its position during the movement of the cylinder.  • As the piston is fixed and cylinder is movable or sliding, the internal volume of the cylinder changes when the cylinder slides.

14. During opening of the breaker the cylinder moves downwards against position of the fixed piston hence the volume inside the cylinder is reduced which produces compressed SF6 gas inside the cylinder. The cylinder has numbers of side vents which were blocked by upper fixed contact body during closed position. • As the cylinder move further downwards, these vent openings cross the upper fixed contact, and become unblocked and then compressed SF6 gas inside the cylinder will come out through this vents in high speed towards the arc and passes through the axial hole of the both fixed contacts. The arc is quenched during this flow of SF6 gas.

15. Advantages: Disadvantages: • SF6 breakers are costly due to high cost of SF6 . • Sealing problems arise , Imperfect joints give rise to leakage of gases. • After every operation SF6 gas has to be reconditioned, so additional equipment is required. • Due to superior arc quenching property, SF6 circuit breakers have very short arcing time. • Since the dielectric strength of SF6 is 2 to 3 times than that of air , these breakers can interrupt larger currents • The SF6 circuit breaker gives noiseless operation due to its closed gas circuit . • Risk of fire is eliminated because SF6 gas is non-flammable.

16. Special applications • Ground Fault Interrupter Circuit Breakers:- • This breaker has a solid state trip unit. It detects ground currents (which are small short circuits from one phase to ground), and trips to protect both people and equipment. • Ground fault interrupter (GFI) circuit breakers go one step further to detect problems due to electricity being "grounded" by something that's not part of the electrical system. They're used in places that are exposed to water; for instance, they're what prevent you from getting electrocuted even if you get a hairdryer wet.

17. By:- Ankitbansal UE104006 EEE