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Automatic Switch•The simplest Circuit Interruption device is knife switch•By closing the spring against the action of a spring we have an automatic device in which the energy for opening the contacts is stored by the closing operation.•Here only a small force and a short time is needed to open the switch by means of a simple latch, which when it is tripped, releases the stored energy of the spring and opens the contacts.•The switch now become a circuit breaker which can interrupt the current under predetermined condition•This is used in very low voltage circuit.

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Air Break Circuit Breaker

• The arc interruption process of air-break Circuit Breakers is based on the natural deionization of gases by a cooling action.

• The arc is resilient and can be stretched, and has resistance which can be increased by length

• Hence the increase of arc resistance is so high that the short-circuit current drops and the current and voltage are drop in to phase.

• Reducing the phase difference between the system voltage and the short-circuit current assures that when the arc current is interrupted at its zero value, the recovery voltage has very low value at this instant.

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• So that the restriking voltage is not allowed to reach two-times the value of the system peak voltage, which occurs in most of the cases, when current is interrupted

• The energy dissipated in the arc is however high and this limits the application of high –resistance interruption to low and medium power a.c circuit breake, it can also used in low and medium powe d.c circuit breaker.

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• Methods of increasing arc resistance1) Arc lengthening: Resistance is approximately

proportional to the length of the arc.2) Arc Cooling: The voltage required to maintain

ionization increases with a decreases of temperature so that cooling effectively increases the resistance.

3) Arc splitting: An appreciable voltage is absorbed at the two contact surfaces so that if the arc can be split into a number of small arcs in series, the voltage available for actual arc is reduced

4) Arc Constraining: if the arc can be constrained into a very narrow channel the voltage necessary to maintain it is increased.

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Plain break air Circuit Breaker• Simplest type of CB• Contacts are made in shape of two Horn• The arc initially strikes across the shortest distance

between the horns, but it is then driven steadily upwards by the convection current set up due to 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 arc lengthening and cooling. The relative slowness of the process and the possibility of the arc spreading to adjacent metal work limits the application to about 500V and to lower power circuits.

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Oil Circuit Breaker• Oil CBs are extensively employed in our power systems.• Here arc energy is used to crack the oil molecules,

hydrogen gas may be generated which can be used to sweep, cool and compress the arc plasma and so deionize it in a self-extinguishing process.

• Earlier it was thought that the oil acted as an insulator when the breaker was closed and then it flowed in to the arc gap when the contacts separated and smothered the arc.

• But actually it was quite impossible in the very short time of ½ to ¼ cycle available for oil to flow and smoother the arc.

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Arc rupture under oil• By immersing the interrupting contacts in oil, the

production of an arc during contact separation cannot be prevented.

• However the heat of arc immediately evaporates the surrounding oil and dissociates it into carbon and a substantial volume of gaseous hydrogen at high pressure.

• Hydrogen gas has high heat conductivity and this results in cooling the arc column and the contacts, which in turn increases the ignition voltage and thus the arc extinguishes.

• Hydrogen by its high heat conductivity cools the arc so fast that the voltage required for reigniting is 5-10 times as high that required for air and so it is best suited for interruption.

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• Advantages of oil • During arcing, the oil acts as a producer of hydrogen gas

which helps in extinguish the arc.• It provides the insulation for the live exposed contacts

from the earthed portions of the container.• It provides the insulation between the contacts after the

arc has been finally extinguished and there has been time for the oil to flow in to the gap between contacts.

• Disadvantages of oil• It is inflammable and may cause fire hazards, if a

defective oil circuit breaker should fail under pressure and cause an explosion.

• Possibility of its forming an explosive mixture with air.• Because of the decomposition of oil in the arc, the oil

becomes polluted by carbon particles, which reduce its dielectric strength

Hence it requires periodical maintenance &replacement.

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Evolution of Oil Circuit Breaker• Plain Break Oil Circuit Breakers.• Arc control Circuit Breakers• Minimum Oil Circuit Breakers

• Plain Break Oil Circuit Breaker• In early days Plain Break Circuit Breaker were used,

which do not have a special arc extinction system.• In this type the arc is confined only within the oil tank.• Deionization of arc is due entirely to turbulence and

increase of pressure.

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• For successful interruption, a comparatively long arc-length is essential so that the turbulence in the oil caused by the pressure generated by arc, may assist in quenching it.

• Because of the absence of effective control over the arc, the arcing times and the amount of energy releases before interruption often vary over a wide range, thus making a large factor of safety necessary, in the design of the tank.

• The tank must be weather tight to keep moisture out.• The head of oil above the contacts must be large enough

to prevent the escape of gas in the form of a column to the surface.

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• If the contacts are separated at high speed, the arc length can be greatly increase, so interruption can easily occur.

• If the speed of contact travel is slow the welding of contacts may result on short-circuit interruption.

• Performance of plain break oil CB depends on the following main feature]

• Length of break• Speed of contact movement• Head of oil above contacts• Clearance to earthed metal adjacent to contacts

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• By increasing the size of tank, head of oil, length of break and insulation, it is possible to obtain breakers for higher voltages and larger interrupting capacities

• This type of CB are preferred up to 11 KV and 250MVA

• Plain break CB with two interruptions in series is as shown in fig

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Arc Control CB• The bulk oil CBs generally employed in our power

systems are of this type.• Here the gases produced during arcing are confined to

small volumes by the use of an insulating, rigid arc chamber surrounding the contacts.

• Thus higher pressures can be developed to force the oil and gas through or around the arc to extinguish it.

• These small, high pressure resistant chambers are known as arc control pots or sometimes as explosion pots.

• Apart from their efficiency of arc interruption these explosion pots have substantially brought down the risks of fire hazard.

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Types of arc control pots

• There has been a continuous development in the types of arc control pots.

• The two basic forms of which are shown in fig.

• As the arc itself develops the pressure necessary to produce the blast for arc extinction it is to be expected that the pressure will increases as the arc current increases.

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• This increased pressure gives a more powerful blast and a greater electrical strength per unit length of arc path, with the result that the restriking voltage transient can be withstood by shorter gaps between moving and stationary contacts.

• Fig shows typical characteristics of such a pot.

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• For a given pot, the maximum arc duration attained at X is mainly function of the arc magnitude and rate of rise of restriking transient voltage

• The maximum pressure attained at Y is mainly a function of the magnitude of the current and is independent of the restriking transient voltage.

• Thus the limiting voltage rating is reached when the pot fails to clear a short-circuit at region X, whilst the limiting breaking-current rating is reached when the pot bursts at Y.

• The design of high-voltage pots such that the there is sufficient pressure at X to ensure that the arc is positively broken and yet that the pressure at Y is not high enough to burst the pot.

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• Modified arrangements of pot are shown in fig• In fig a arc takes place in two stages: arc first appears in

the upper chamber and builds up high pressure: when the spring of the intermediate contact unit is fully extended the lower contacts opens in oil already at high pressure and so arc is removed

• The same principal is use in cross blast CB as in fig b

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• In fig c the vents are placed immediately adjacent to the stationary contacts, with this arrangement high current can be extinguished with minimum travel after separation, it also improves the low –current operation.

• In technical word this arrangement increases the voltage rating at X without decreasing current rating at Y

• In fig d the arc in the upper chamber forces the piston A against the oil in the lower chamber and also forces some oil down the moving contact; as soon as the contact has passed through the piston the pressure in the upper chamber forces the piston down the expels oil form the lower into the upper chamber through the orifice in the piston thereby causing considerable turbulence and rapid deionization.

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• In fig e Oil is delivered by pressure due to the initial break to several points along the arc path and flow axially with the arc before being discharged.

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• Still in other forms of explosion pots, to avoid explosion at very high currents, pressure limiting valves are used to protect the so called elastic arc chambers; at very low currents, a mechanically driven piston guarantees sufficient flow, (in fig )

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• Alternative arrangement is the deion grid as shown in fig.

• The grid surrounds the arcing contacts and consists of a series of insulating plates

• They are made of asbestos having interspersed plates of magnetic material.

• Plates are so disposed and vented that the arc is moved laterally into oil pockets by the electromagnetic force due to the flux in the magnetic grids, where it vaporizes the oil.

• The resulting gases can only escape by sweeping across the arc path.

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Minimum Oil CB• For higher voltages and higher breaking capacities large

amounts of oil are required and the size of the bulk oil CB becomes inordinately large.

• The MOCB uses solid materials for insulating purpose and uses just enough oil for arc quenching.

• The interruption device is enclosed in a tank of insulating material, the whole of which is at line voltage in normal operation.

• These are also known as live tank breaker.• The MOCB may be self-blast type, external type or a

combination of the two.• In the self blast type CB the force with which the arc is

quenched adapt itself to the current to be broken.

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• Larger the short-circuit current more is the oil

decomposed .So the arc quenches effectively as current increases in self blast type MOCB.

• However gas pressure in the chamber is also increase with increase in the short-circuit current, which impose a mechanical stress limitation on the CB.

• The limit of breaking capacity is therefore not set by the principal of arc quenching but by mechanical strength of the arcing chamber.

• But with modern insulating materials such as glass fiber reinforced, synthetic resins, used for arcing chamber the MOCBs are able to meet easily the increased fault level of the systems

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• Two type of venting are used in the arcing chamber, axial venting and radial venting.

• In axial venting the gases sweep the arc in longitudinal direction while in radial venting the arc is blown in transverse direction.

• Axial venting has the advantage that it generated high pressure and also has high dielectric strength.

• Axial venting is used where low current is to be interrupted at high voltage.

• Radial venting generates low pressure and has low dielectric strength, it is mainly suitable for heavy currents at low voltages.

• Some times a combination of both is used so that arcing chamber is equally efficient at low as well as at high currents.

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• These arcing chamber has also disadvantage that at very low currents, called critical current, they have a long arcing period, this critical current lie between 10 to 100A

• To eliminate critical currents, in addition to self blast, separate oil injection devices are also used, which help in making arcing time virtually independent of the value of interrupted current.

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• Various arrangements of arcing chambers used in MOCB are as shown in fig

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Single and multibreak construction

• Circuit breaker may comprise a single pair of contacts or a number of pairs in series operating simultaneously.

• Single-break is the simplest arrangement, a single-break is used about 66KV and 1000 MVA.

• Multibreak construction has the following advantages.• 1) the transient restriking voltage is divided over a no of

breaks so that only a fraction arrears across each.• 2) each break can be separately tested at a voltage

lower than that of the whole breaker as a whole ( unit testing)

• 3) the effective speed of arc lengthening is increased in proportion to the no of breaks.

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• The main difficulty with multibreak CB is to ensure that the restriking voltage distributes itself evenly across the breaks.

• Multibreak type of CBs have been developed of installations where rupturing capacities are high and very high speed operations are possible.

• The construction of multibreak type CB is based on the unitized design principle in which one unit consists of a vertical support insulator surrounded by a single or a pair of interrupting units in Y formation.

• By connecting the units in series, requirements for the network for different voltages and rupturing capacities can be met.

• This type has 2 to 3 cycles as total break time.• The arcing time is 10 to 15ms.