Circuit Breakers - Part 1

Classification of Circuit Breakers

Circuit breakers are used in a wide range of applications. They are used in many environments and can handle currents and voltages of different ranges.

Circuit breakers are classified on the basis of different criteria. Some of the classifications are below

Based on the interrupting medium

Air circuit breaker
Oil circuit breaker
SF6 circuit breaker

Based on type of action

Automatic circuit breakers
Non-automatic circuit breakers

Based on the method of control

Locally controlled circuit breakers
Remotely controlled circuit breakers (remote control can be mechanical, pneumatic or electrical)

Based on the type of mounting

Panel mounted circuit breakers
Remote from panel mounted circuit breakers
Rear of panel mounted circuit breakers

Based on location

Outdoor circuit breakers
Indooor circuit breakers

Based on voltage

Low voltage circuit breakers
Medium voltage circuit breakers
High voltage circuit breakers

Air Circuit Breakers

Air Circuit Breakers are Circuit breakers where air is used as the medium of extinguishing the arc.  The air is usually compressed and kept in a cylinder.  When the breaker operates and the contact separation occurs, the arc is driven into special arc chutes by means of compressed air which is blown through specially designed nozzles. 

Air Circuit Breakers are mostly used in the LV range.  They can interrupt currents of several thousand amperes. 

Air Circuit breakers are provided with an inherent current sensing mechanism like the thermo-magnetic release.

Arc Chute used in Air Circuit Breakers

The Arc chute is a component which is used to weaken and quench the arc.  The Arc Chute contains a number of splitters which split the arc into a number of sections.  This increases the length of the arc and quenches it.  The arc which is formed during the separation of the fixed and the moving contact is driven into the chute by means of the pressurized air.  The arc chute is usually made of composite refractory materials. 

Air Circuit Breakers are available in both Three pole and Four pole versions.

Air Circuit breakers are used widely in the industry for the protection of facilities and transmission lines.  They are also used for protection of electric equipment such as transformers, motors, etc.  Air circuit breakers are also used in mines and on board ships. 

Vacuum Circuit Breakers

Vacuum Circuit breakers are a very popular type of circuit breaker used in the industry. Vacuum Circuit Breakers or VCBs as they are popularly known use vacuum as the quenching medium.

Vacuum Circuit breakers work by opening the contacts in an evacuated chamber. The vacuum in the chamber ensures that there are very few ionisable molecules which can sustain the arc. The arc is, thus, unable to sustain itself and is extinguished.

Vacuum circuit breakers have been used up to 36 kV and can interrupt up to 4000A. When the arc is initiated during contact separation, metallic vapour is produced from the contacts. If the contacts are of soft metal, very little vapour is produced. While this ensure quick arc extinction. The current waveform can get chopped and lead to high voltage transients. If too soft metals are used in the contacts, the arc would cause erosion of the material. Hence, the contacts are made of materials which are neither too soft or hard. Common materials used are alloys Copper-bismuth or copper-chrome.

Live Tank circuit breakers and Dead tank circuit breakers

Live Tank circuit breakers are circuit breakers in which the interrupting chamber is at the line potential. The interrupting chamber should therefore be provided with insulated supports. The centre of gravity of these circuit breakers is higher, hence live tank circuit breakers need extra support for seismic capability (ability to withstand earthquakes)

In dead tank circuit breakers, the interrupting chamber is at ground potential. The conductors enter the interrupting chamber through insulated bushings. Maintenance activities are easier to conduct as the interrupting chamber is at ground level. Seismic capability is higher as the interrupting chambers are at ground level. 

Generator Neutral Breaker

The Generator Neutral Breaker is used in systems, which are grounded through low resistances or solidly grounded (without a resistance). In such systems, the fault current in the line due to an earth fault will be high.

The current flowing through the equipment due to an earth fault can be limited if a breaker is connected in series with the neutral. This breaker is opened simultaneously with the armature and the field breaker. This will bring the fault current to zero quickly.

Pre Insertion Resistors in Circuit Breakers

Circuit breakers used in switching of long transmission lines have a resistors which is pre-inserted between the contacts before the contacts are closed. This resistor is called the Pre-insertion resistor. The function of this resistor is to limit the initial charging current of the line. The resistance of the line is around 500 ohms.

Once the closing command is given to the breaker, the resistor is first connected across the contacts. This resistance in series limits the line current. A few milliseconds later, the contacts are closed. 

While opening the breaker, the pre-insertion resistor is first disconnected before the contacts are opened by the circuit breaker. Pre-insertion resistors are also used in lines which have transformers to limit the high inrush current.