What are Circuit Breakers?
- To Disjoint the system if we need maintenance or Extension
- To Disjoint the system from faulty part to healthy part.
The Maintenance work is also done by a simple switch, but if the fault current is high, we need to protect the system automatically. The simple switch does not protect from the arc so that's why we need a circuit breaker to protect the system from faults. A basic circuit breaker consists of fixed and moveable contacts connected to the quick-acting mechanism. if a circuit breaker consists of one set of fixed and moveable contact is called a single-pole circuit breaker.
if a circuit breaker consists of three sets of fixed and moveable contact and each set is connected with the single-phase supply is called a Three pole circuit breaker. Each circuit breaker consists of Arc Extinguishing Chamber for an Individual pole. we design contacts that are not welded to each other during the Arc process. There are Many Types of Circuit breakers that are available in the market in which Air circuit breakers, Oil Circuit breakers, Gas Circuit breakers etc. Below you see the "On position" and "off positions" of the contacts
A normally open(NO)state defines when its connections are open and their resistance is infinite in case ideal and current are not flown in the contacts.
.png "NO state")
Working Principle of circuit breaker
When current-carrying contacts are separate from each other the arc is produced in between them. To minimize the arc effect we used an insulating Medium for example Dielectric Oil, Sulpher Hexa Flouride(SF6) Gas and vacuum. In normal conditions, the Circuit breaker contacts are closed until the system becomes faulty. when a fault occurs in a system the Relay generates a Signal to the trip coil of the circuit breaker and energizes the trip coil. after that, we separate the contacts with the help of any mechanism. The different parts of the Circuits breaker are shown below
Parts of the circuit breaker
- Relay
- A trip coil of CB
- Trip Circuit
- Battery
- Relay contacts
- Potential Transformer(CT)
- Current Transformer(PT)
- Auxiliary Switches
- Protected Lines
Arc Initiation Process
When Circuit Breaker contact is separate the Arc is produced across the connections. due to the separation of contacts, the I²R losses occurred in the form of heat, resulting in a temperature increase. Due to an increase in temperature, the thermal emission effect starts in the system. this emission affects the medium insulation strength.
Arc Extinction Method
There are two methods of Extinction of the arc that is described below
- current Zero Method
- High Resistance Method
Current Zero Method
The other name of the Current zero method is the Low resistance method. this method is used in high-power ac circuits. we know that Every half cycle of ac supply the current is zero because of alternating supply. when the current is zero at this particular time the arc is also zero. we used this principle to avoid the arc, we used insulating material i.e. oil or gas in the contacts which result in the medium being deionised.
High Resistance method
The high resistance method is an Arc Extinction Method in which we increase the Arc Resistance until The arc current is minimised. When we increase arc resistance the flow of current is stopped in the electrode or ionized gasses. the major disadvantage of this system is Energy loss in the form of heat. the heat is in direct relation to the I2R.this method is limited in the Low ac circuit capacity and dc system. There are different methods To increase the Arc Resistance that are described below
- Lengthening The Arc
- Cooling the cross-section Area
- Splitting the Arc Arc
Lengthening The Arc
We know that according to the law of resistance, the resistance is directly proportional to the lenght. if we increase the distance between the contacts the length is increased. we also increase the length using arc runners or arc splitters. To increase the length the resistance is increased and the current is minimized. the construction of arc runners is horn shapes that are contacted to arching contacts. the tip of the arc runner is a V shape.
Cooling The Arc
To minimize the arc we use the cooling method in the circuit breaker to deionize the medium.
due to cooling the arc resistance is increased and the arc current is decreased.
Reducing the X-section Area
If we decrease the arc cross-sectional area the arc resistance increases and the arc current is
decreases.
Splitting the Arc
Splitting the arc is another Method of reducing the arc currents. in this method, we split the arc into small arc sections so every small length provides a cooling effect and a long length minimises the arc effect. the other name is arc splitter. we set these plates as verticle side.
Magnitude of Arc
An electrical discharge consists of electrical charge carriers. the more charge carrier the less resistance of the arc. the charge carrier depends on the following things
Magnitude Depending Factors
- The potential difference between the arc points
- the dielectric strength of the insulating material
- The physical properties of the insulating medium
- The current flow due to the discharge
- The temperature of the Arc
- The Quantity of insulating medium between the arc
- The Diameter of the Arc
- The Length of the Arc
There are different Temperature zone in the Arc. The first temperature zone is the Core temperature the value is between 6000 degrees to 25000 degrees. the Second zone is the ionized gasses zone which is a value between 2000 degrees to 6000 degrees. the other zone is the Hot gasses zone the temperature is 2000 degrees.
The Relationship between the Steady State arc current and Voltage is
Varc = A + Bd + (C+Dd)/Iarc
A, B,C, D is a constant value
d= Length of arc
Varc = Voltage drop in the arc
Iarc = Arc Current
Earc= Varc * Iarc*t
Where
E arc I the useable energy in the arc
t = time in ms
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