Here is a list of air-break circuit breakers: 1. Plain Break Type Air-Break Circuit Breaker 2. Magnetic Blow-Out Type Air-Break Circuit Breaker 3. Arc-Chute Air-Break Circuit Breaker.
1. Plain Break Type Air-Break Circuit Breaker:
Plain break type air-break circuit breaker is the simplest one in which contacts is made in the shape of two horns. The air initially strikes across the shortest distance between the horns and is then driven steadily upwards by the convection currents caused by heating of air during arcing and the interaction of magnetic and electric fields.
The arc extends from one tip to the other when the horns are fully separated resulting in lengthening and cooling of arc. The relative slowness of the process and the possibility of arc spreading to adjacent metal work limit the application to about 500 V and to low power circuits.
2. Magnetic Blow-Out Type Air-Break Circuit Breaker:
In a number of air circuit breakers employed in circuits up to 11 kV the arc extinction is accomplished by means of magnetic field provided by current in blow-out coils connected in series with the circuit being interrupted. Such coils are called blow-out coils, because they assist in the arc being magnetically blown out. The magnetic field itself does not extinguish the arc. It simply moves the arc into arc chutes where the arc is lengthened, cooled and extinguished. The arc shields prevent arc spreading to adjacent metal work.
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It is important to connect the coils at correct polarity so that the arc is directed upwards.
As the breaking action becomes more effective with large currents, this principle has resulted in increasing the rupturing capacities of such breakers to higher values.
Arc chute is an efficient device for arc extinction in air and performs the following three interrelated functions:
i. It confines the arc within a restricted space.
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ii. It provides magnetic control over the arc movement so as to make arc extinction within the device.
iii. It provides for the rapid cooling of arc gases to ensure arc extinction by de-ionization.
3. Arc-Chute Air-Break Circuit Breaker:
The normal arrangement of an arc-chute air-break circuit breaker employed for low and medium voltage circuits is illustrated in Fig. 10.13. There are two sets of contacts called the main contacts and arcing or auxiliary contacts. Main contacts are usually of copper and conduct the current in closed position of the breaker. They have low contact resistance and are silver plated. The arcing contacts are hard, heat resistant and usually of copper alloy.
Arcing contacts are used to relieve the main contacts from damage due to arcing. The arcing contacts are easily renewable when required. The auxiliary and arcing contacts close before and open after the main contacts during operation. Here the blow-outs consist of steel inserts in the arcing chutes. These are so arranged that the magnetic field induced in them by the current in the arc moves it upwards still faster.
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The steel plates divide the arc into a number of short arcs in series. The distribution of voltage along the arc length is not linear but is accompanied by a rather large anode and cathode drops. In case the total sum of anode and cathode drops of all short arcs in series exceeds the system voltage, conditions for quick extinction of the arc are automatically established.
When the arc comes into contact with the relatively cool surfaces of the steel plates, it gets rapidly and effectively cooled. The movement of the arc may be natural or aided by a magnetic blow-out (being employed for heavy duties up to 500 MVA at 16 kV). Thus the arc is extinguished by lengthening and increasing the voltage gradient i.e., power loss of the arc.
In this breaker relatively high arc resistance is obtained near current zeros. This effect plays an important role in having high rupturing capacity by modifying the circuit power factor near current zero such that voltage available to restrike the arc is considerably less than the peak value.
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A drawback of arc chute principle is the inefficiency at low currents where the electromagnetic fields are weak. The chute itself is not necessarily less efficient in its lengthening and de-ionizing action than at high currents, but the arc movement into the chute tends to become slower and high speed interruption is not necessarily obtained.
With the increase in operating voltage and rupturing capacity, such a breaker becomes bulky, the arc chutes become more complex and the initial cost increases. Further the time of operation is not small enough to make such a breaker suitable for modern power system.
Action:
On occurrence of fault, the main contacts dislodge first and the current is shifted to the arcing contacts. Now the arcing contacts dislodge and the arc is drawn between them. This arc is forced upwards by the electromagnetic forces and thermal action. The arc ends travel along the arc runner (arcing horns). The arc moves upwards and is split by the arc splitter plates. The arc is extinguished by lengthening, cooling, splitting etc.
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Applications:
Air-break circuit breakers are generally suitable for the control of power station auxiliaries and industrial plants. They do not require any associated equipment such as compressors, etc. As they have no oil, they are recommended where there are possibilities of fire or explosion hazards.
Air-break principle of lengthening of arc, arc runners, and magnetic blow-up is employed for dc circuit breakers up to 15 kV.