Air break circuit breaker | Types of Circuit Breakers

There are five Types of Circuit Breakers and In this article we will speak about

1. Air break circuit breaker
2. Air-Blast Circuit-Breaker
3. Bulk Oil and Minimum Oil Circuit Breaker
4. Sulphur Hexafluoride (SF6) Circuit Breaker and SF6 Insulated Metalclad switchgear
5. Vacuum circuit breaker

Air break circuit breaker 

The air at atmospheric pressure is used as an arc extinguishing medium in Air-Break Circuit-Breakers. These circuit breakers employ the high resistance interruption principle. 

• The arc is rapidly lengthened by means of the arc runners and arc chutes and the resistance of the arc is increased by cooling, lengthening and splitting the arc. 
• The arc resistance increases to such an extent that the voltage drop across the arc becomes more than the supply voltage and the arc gets extinguished. 
• Magnetic field is utilized for lengthening the arc in high voltage air break circuit-breakers.

Air-break circuit breakers are used in dc. circuit and ac. circuits up to 12KV.

The air-break circuit breakers are generally indoor type and installed on vertical panels or indoor draw-out type switchgear.

AC air circuit breaker are widely used in indoor medium voltage and low voltage switchgear. 

Typical reference values of ratings of air-break circuit breaker are :

• 460 V , 400-3500 A, 40-75 KA
• 3.3 KV , 400-3500 A, 13.1-31.5 KA
• 6.6 KV , 400-2400 A, 13.1-20 KA 

Construction of Air-break Circuit-breaker 

In the air-break circuit-breaker the contact separation and arc extinction takes place in air at atmospheric pressure. As the contacts are opened, arc is drawn between them. The arc core is a conducting 'path' of plasma. The surrounding medium contains ionized air. By cooling the arc, the diameter of arc core is reduced. The arc is extinguished by lengthening the arc, cooling the arc and splitting the arc. The arc resistance is increased to such an extent that the system voltage cannot maintain the arc and the arc get extinguished. 

This type of breaker is used for medium and low voltages.

 

the normal arrangement of an air break circuit breaker.

An air circuit breaker for low voltage (less than 1000 volts) power distribution switchgear

 

Construction of Air-break Circuit-breaker

 

The 10 ampere DIN rail-mounted thermal-magnetic miniature circuit breaker is the most common style in modern domestic consumer units and commercial electrical distribution boards throughout Europe. 

The design includes the following components:

1. Actuator lever - used to manually trip and reset the circuit breaker. Also indicates the status of the circuit breaker (On or Off/tripped). 
2. Actuator mechanism - forces the contacts together or apart. 
3.  Contacts - Allow current when touching and break the current when moved apart.
4. Terminals
5. Bimetallic strip.
6. Calibration screw - allows the manufacturer to precisely adjust the trip current of the device after assembly.
7. Solenoid
8. Arc divider/extinguisher

 There are two sets of contacts : Main contacts (1) and Arcing contacts (2). 

• Main contacts conduct the current in closed position of the breaker. They have low contact resistance and are silver plated. 
• The arcing contacts (2) are hard, heat resistance and are usually of copper alloy. 
• While opening the contacts, the main contacts dislodge first. The current is shifted to the arcing contacts. 
• The arcing contacts dislodge later and arc is drawn between them (3). 
• This arc is forced upwards by the electromagnetic forces and thermal action. The arc ends travel along the arc Runners (Arcing horns). 
• The arc moves upwards and is split by arc splitter plates (arc chutes) (5) as shown by the arrow (4). 
• The arc extinguished by lengthening, cooling, splitting etc. In some breakers the arc is drawn in the direction of the splitter by magnetic field. 

Furthermore, air-break circuit-breaker have been developed with current limiting feature, magnetic blow-up of arc, etc.

Air break ac circuit breakers are widely used for industrial switchgear, auxiliary switchgear in generation station. Air break principle employing lengthening of arc, arc runners, magnetic blow-ups are used for dc circuit breaker up to 15 KV.

 

Miniature and Molded-Case Circuit Breaker

These are used extensively in low voltage domestic, commercial and industrial applications. They replace conventional fuses and combine the features of a good HRC fuse and a good switch. 

the figure below gives the internal details of a thermal magnetic miniature circuit breaker. For normal operation, it is used as a switch. During over loads or faults, it automatically trips off. The tripping mechanism is actuated by magnetic (part 7) and thermal sensing devices (part5) provide within the MVB.  

The internal details of a 10 ampere European DIN rail mounted thermal-magnetic circuit breaker 

Figure contents :

1.  Actuator lever : used to manually trip and reset the circuit breaker. Also indicates the status of the circuit breaker (On or Off /tripped). Most breakers are designed so they can still trip even if the lever is held or locked in the On position. This is sometimes referred to as "free trip" or "position trip" operation.
2.  Actuator mechanism - forces the contacts together or apart. 
3.  contacts - Allow current to flow when touching and break the flow of current when moved apart.
4.  Terminals
5.  Bimetallic strip
6. calibration screw - Allows the manufacture to precisely adjust the trip current of the device after assembly.
7.  Solenoid
8.  Arc divider / extinguished  

Typical Rating of MCB :

Current Rating : 

• 5, 10, 15, 20, 30, 40, 50, 60 Amp. 
• also, 0.5, 0.75, 1, 2, 2.5, 3, 3.5, 6, 7.5, 8, 10, 12, 25, 35, 45, 55 Amp.

Voltage Rating : 

• 240 V/415 V AC; 50 V/11 V DC 

Rupturing Capacity : 

• AC: 3 KA at 415 V
• DC: 3 KA at 50 V ( non-inductive),
• DC:1 KA at 110 V (non-inductive).

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