What Is Air Circuit Breaker And Types Of ACB
What is a Air Circuit Breaker?
A air circuit breaker is a device, which can
- Make or break a circuit manually or by remote control under normal conditions.
- Break a circuit automatically under fault conditions (like over current, Short circuit, etc).
- Make a circuit manually or by remote control under the fault conditions.
A circuit breaker is used for switching mechanism and protection of the system. Other associated devises and components are also used for this purpose associated with circuit breakers like fuses, relays, switches etc. Circuit breakers are widely used in industries as well as power system for controlling and protection of different parts of the circuit like switch gears, Transformers, Motors, Generators/Alternator etc., which leads the system stable and reliable.
Construction of Air-Circuit Breaker
The following fig shows the main and external parts of an ACB. (ABB EMax Low Voltage, Current Limiting Air Circuit Breaker and Selective (Non-Current Limiting) Air Circuit Breaker).
- OFF button (O)
- ON button (I)
- Main contact position indicator
- Energy storage mechanism status indicator
- Reset Button
- LED Indicators
- “Connection”, “Test” and “isolated” position stopper (the three-position latching/locking mechanism)
- User-supplied padlock
- Connection “,” Test “and” separation “of the position indication
- Connection (CE) Separation, (CD) Test (CT) Position indication contacts
- Rated Name Plate
- Digital Displays
- Mechanical energy storage handle
- Shake (IN/OUT)
- Rocker repository
- Fault trip reset button
The following figure shows the Internal Construction of Air Circuit Breaker
- 1. Sheet Steel Supporting Structure
- 2. Current Transformer for Protection Trip Unit
- 3. Pole Group insulating box
- 4. Horizontal rare terminals
- 5a. Plates for fixed main contacts
- 5b. Plates for fixed arcing Contacts
- 6a. Plates for Main moving contacts
- 6b. Plates for Moving Arcing contacts
- 7. Arcing Chamber
- 8. Terminal box for fixed version – Sliding Contacts for withdrawable version
- 9. Protection Trip Unit
- 10. Circuit breaker Closing and Opening Control
- 11. Closing Springs
Principle of Operation of Air Circuit Breaker
ACB has two sets of contacts main and auxiliary contacts. Each set of contact consists of a fixed contact and a moving contact. The main contact normally carries most of the load current. All the contacts are made of cadmium-silver alloy which has very good resistance to damage by arcing.
When the ACB is closed, the powerful spring is energized and the ACB is than latched shut against spring pressure. The auxiliary contact makes first & breaks last i.e. when ACB is closed, the auxiliary contact closes first then the main contact follows.
When the ACB is open, the main contact open firsts and then auxiliary contact opens. Thus the auxiliary contacts are subjected to arcing during the opening of ACB and can easily be replaced.
The main contact closing pressure is kept high so that the temperature rise in the contacts while carrying current remains within limit. Closing coil operating on D.C voltage from a rectifier is provided to close the circuit breaker by operating a push button.
How Arc Quenching is Achieved?
1.Using arcing contacts made of resistance alloy and silver tips for the main contacts. Arcing contacts close earlier and opens later than the main contacts.
2.When opening contacts have long travelled at high speed to stretch the resultant arc, which is transferred to the arcing contact.
3.Cooling and splitting of the arc is done by arc chutes which draw the arc through splitters by magnetic action and quickly cool and split the arc until it snaps. The circuit breaker opens when the arc is quenched.
Types of Air Circuit Breaker
There are four types of ACBs used in the control and protection to maintain and stable operation of switch gears and indoor medium voltage.
- Plain Break Type Air Break Circuit Breaker or Cross-Blast Air Circuit Breaker
- Magnetic Blowout Type Air Break Circuit Breaker
- Air Chute Air Break Circuit Breaker
- Air Blast Circuit Breaker
Plain air circuit breaker or Cross-Blast Air Circuit Breaker:
The circuit breaker is fitted with a chamber surrounding the contact. The chamber is known as “arc chute”. The arc is made to drive in it. The arc chute will help in achieving cooling. Arc chute is made from some refractory material. The inner walls of arc chute are shaped in such a way that arc is not only forced into close proximity, but will drive into the serpentine channel projected on arc chute wall.
The arc chute is divided into a number of small compartments by using metallic separation plates. Metallic separation plates are arc splitters and each of small compartments behave as a mini arc chute. Initial arc will split into a series of arcs this will make all arc voltages higher than system voltage. They are preferable choice in low voltage application.
Air Chute Air Break Circuit Breaker
In air chute air circuit breaker, there are two types of contacts namely “main contact” and “auxiliary or arcing contacts”. The main contacts are made of copper and the silver plates having low resistance and conducts the current in closed position. Auxiliary or arcing contacts are made of copper alloy as they are heat resistance and used to prevent from damaging the main contacts due to arcing and can be easily replaced when needed in case of wear and tear. During the circuit breaker operation, the arcing or auxiliary contacts are closed before and open after the main contacts of the circuit breaker.
Magnetic Blowout Type Air Break Circuit Breaker
Magnetic blowout air circuit breakers provide magnetic control over the arc moment to make arc extinction within the devices. The arc extinction is controlled using magnetic field provided by the current in blowout coils connected in series with the circuit being interrupted. These coils are known as “blow out the coil”. Magnetic field does not control and extinguish the arc made in the breaker, but it moves the arc into chutes where the arc is lengthened, cooled and extinguished accordingly. These kind of circuit breakers are used up to 11kV.
Air Blast Circuit Breaker:
This type of circuit breaker is used for system voltage of 245 KV, 420 KV and even more.Air blast circuit breaker has further divided into three categories:
- Axial blast breaker
- Axial blast with sliding moving contact.
Axial Blast Breaker
The moving contact is in contact. There is a nozzle orifice in fixed contact at normal closed condition of breaker. When a fault occur high pressure is introduced into the chamber. High-pressure air will flow through nozzle orifice voltage is sufficient to sustain.
Axial blast with sliding moving contact
The moving contact is fitted over a piston supported by a spring. The blast transfers arc to arcing electrode.