Air - Blast circuit breakers employ high pressure Air – Blast as an arc quenching medium. The contacts are opened in a high flow of air blast established by the opening blast valve.

1. Air – Blast Circuit Breakers
Air - Blast circuit breakers employ high pressureAir – Blast as an arc
quenching medium. Thecontactsareopened in a high flow of air blast
established by theopening blast valve. The air blast cools thearc and
sweeps awaythe arcing productstothe atmosphere. It rapidlyincreases
the dielectric strength ofthe medium betweencontactsand then
prevents from re-establishing thearc. Consequently, the arc is
extinguished and theflow of current is also interrupted.
Large Hydro Power Plant
Advantages of Air – Blast Circuit Breakers
The air – blast circuitbreaker’sadvantagesaregivenbelow:
(i) Therisk of fire is eliminated.
(ii) The growth of dielectric strength isso rapid that finalcontact needed a
very small gap for arc extinction. It reducesthe size of the device.
(iii) The arcing timeisvery small due to the quickbuild-up of dielectric
strength betweenboth contacts. That’swhy, thearc energyis only a
fractionof that inoil circuit breakers, thusresulting inless burning of
contacts.
(iv) Cause of lesser arc energy, the air-blast circuit breakersareverysuitable
for conditionswhere frequent operationsarerequired.
(v) The energy supplied for the arc extinctionisgained from high-pressure
air and is independent of the current interruption.
(vi) Short and consistent arc duration.
(vii) The burning of contactsisless due to less arc energy (since the arc
durationisshort and consistent).
(viii) Less maintenanceisrequired.

2. (ix) High-speed operation.
(x) The facilityof high-speed reclosure.
Disadvantages of Air – Blast Circuit
Breaker
The disadvantagesof air – blast circuit breaker aregivenbelow :
(i) The air has manypropertiesone is the relatively inferior arc
extinguishingproperty.
(ii) The air-blast circuit breakersarevery sensitive circuitbreakersthan
others to the variationsintherateof rise of restriking voltage.
(iii) Maintenanceisrequired for the circuit breaker for compressor plant
which supplies theair-blast.
(iv) The air leakageat the pipeline fittingscreateproblems.
(v) Current chopping.
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Applications of Air – Circuit Breaker
The air blast circuit breakersareusing wideapplicationsinhigh voltage
installations. Themajorityofthese circuit breakersfor voltagesup to 110
kV are of this type.
Types of Air – Blast Circuit Breakers
The classificationofair – blast circuit breakersare depending upon the
directionof air-blast inrelationtothe arc,

3. (i) Axial – blast type
In which the arc – blast is directed along the arc path.
(ii) Cross – blast type
In thisair – blast is directed at right anglesto the arc path.
(iii) Radial – blast type
In thistype of circuitbreaker air –blast is directed radially.
(i) Axial – Blast Air Circuit Breaker
The figureshows the essential componentsof a typical Axial-Blast Air
Circuit Breaker. Thefixed and moving contactsofthe circuit breaker
are held in the closed positionby spring pressureunder normal
conditions. Theair reservoir of the circuit breaker isconnected tothe
arcing chamberthrough anair valve. This valve remainsclosed during
normalconditionsbut when the fault occurson the system it opens
automaticallyby the tripping.

4. When a fault occurs, the tripping impulse causes opening of the air valve
which connects the air reservoir to the arcing chamber. The high-pressure
air entering the arcing chamber which pushes away the moving contact
against the spring pressure. The moving contacts are separated and an arc
is struck between them. At the same time, the high-pressure air blast flows
along the arc and takes away the ionized gases along with it. Consequently,
the arc is extinguished and interrupted the flow of the current.
It shouldbe noted that in these types of circuit breakers, the contact
separation required for interruption is generally small almost (1-75 cm or
so). These small gaps may constitute inadequate clearance for the normal
service voltage. Cause of that, an isolating switch is incorporated as a part
of such a circuit breaker. After the fault interruption, this switch opens
immediately to provide the necessary clearance for insulation.
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(ii) Cross – Blast Air Circuit Breaker

5. In the cross – blast air circuit breaker, an air-blast is directed at right
angles (90o) to the arc. The cross-blast lengthens and forces the arc into a
suitable escarpment for arc extinction. The below Figure shows the
essential parts of a typical cross-blast air circuit breaker. When the moving
contacts are withdrawn, an arc is struck between both fixed and moving
contacts. The high contact pressure cross-blast forces the arc into a chute
consisting of arc splitters and baffles. The splitters help to increase the
length of the arc and baffles provide improved cooling. The result is that the
arc is extinguished and the current flow is interrupted. Because the air blast
pressure is similar for all currents, the inefficiency at low currents is
eliminated. The final gap for interruption is great enough to give normal
insulation clearance therefore, a series isolating switch is not necessary.
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