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Switching and ControlDevices


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Are included all types of switches, fuses,circuit breakers and a large number of otherdevices, the purposes of which are to make orbreak, or change the connections in an electric

circuit, under either normal or abnormalconditions.

Switching and ControlDevices


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SWITCHESFUSES

BREAKERS


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The most familiar form of switch is a manuallyoperated electromechanical device with oneor more sets of electrical contacts. Each set ofcontacts can be in one of two states: either

'closed' meaning the contacts are touchingand electricity can flow between them, or'open', meaning the contacts are separatedand non-conducting.

SWITCHES


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Electrical Switches.


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Knife switches

A knife switch is a type of switch used to control the

flow of electricity in a circuit. It is composed of a hinge whichallows a metal lever, or knife, to be lifted from or inserted into

a slot or jaw.

CLASSES OF SWITCHES


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Disconnecting Switches

for power circuits it is necessary to use some type ofcircuit breaker to open or close the circuit. In order to isolatethe circuit breakers, it is generally considered good practice toconnect the knife switches in series with the circuit breakers.


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Air-break Switches

An air break switch comprising one or more fixedcontacts and one or more movable contacts which aremovable between open and closed positions, and a coilwhich, preferably in conjunction with magneticallysusceptible material acting as a flux director.


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Control Switches

This class include all switches that are used to controlthe operation of other equipment.

A clothes dryer using a load control switch to reduce peak


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Auxiliary Switches

A switch actuated by the main device (such as a circuitbreaker) for signaling, interlocking, or other purposes. In thisclass are included all switches or contactors that are actuatedby some other control switch or device.


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Oil Switches

An oil switch is a high-voltage switch whose contactsare opened and closed in oil. The switch is actually immersedin an oil bath and contained in a steel tank.


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Magnetic-impulse Switches

This type of switch, the arc is extinguished by blowingit magnetically into arc chutes where it is lengthened cooled,and interrupted. The magnetic effect is produced by the circuitcurrent which is passed through suitable coils, setting up astrong magnetic field across the space between the switchcontacts as they are opened.


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In electronics and electrical engineering a fuse (from theLatin "fusus" meaning to melt) is a type of sacrificialovercurrent protection device. Its essential component is ametal wire or strip that melts when too much current flows,which interrupts the circuit in which it is connected. Short

circuit, overload or device failure is often the reason forexcessive current.

A fuse interrupts excessive current (blows) so that furtherdamage by overheating or fire is prevented. Wiringregulations often define a maximum fuse current rating forparticular circuits. Overcurrent protection devices areessential in electrical systems to limit threats to human lifeand property damage. Fuses are selected to allow passageof normal current and of excessive current only for short

periods.

FUSES


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Cartridge Fuses

Come in a cylindrical shape and have contacts points ateach end. They have a fuse link connected to the two endsand protect the circuit from over-current. It is composed ofstrong fiber casing inside of which is enclosed a fuse wiregenerally an alloy of lead.

CLASSES OF FUSES


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Transformer Fuse Block and Cutout

The proper protection of distribution circuits has for yearsbeen recognized as best fulfilled by means of fuses.Distribution transformers are as a general rule placed onpoles, towers, or in manholes and in any automatic protectionwould be too entirely complicated and expensive. Fuses offera simple and cheap method of protection, It is common

practice tom place these fuses in the secondary of suchtransformers, thereby protecting the transformers againstshort circuits or overloads. There are two types of fuses that

are generally used for these purpose.


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Expulsion Fuse

For higher voltages such as found in power circuits ormain feeders, there is often a demand for a fuse, on accountof its simplicity. The expulsion type-fuse has been developedfor such service. This device is consists of a hollow tube, madeof some heat-resisting substance such as fiber with a lining ofasbestos or some other material, through which is passed a

fuse wire. One end of the tube is closed and connected to theline; the other end is opened and allows the fuse wire toproject out and connect to the other terminal.


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A circuit breaker is a complex circuit-breaking device withthe following duties:

a. Makes or breaks both normal and abnormal currents.

b. Appropriately manages the high-energy arc associated

with current interruption. The problem has become moreacute due to interconnection of power stations resulting invery high fault levels.\

c. Effects current interruption only when it is called upon todo so by the relay circuits. In fact they are required to trip

for a minimum of the internal fault current and remaininoperative for a maximum of through fault current.

d. Rapid and successive automatic breaking and making toaid stable system operation

e. Three pole (3-pole) and single pole (1-pole) auto-reclosingarrangement

Circuit Breakers


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In addition to the making and breaking capabilities, acircuit breaker is required to do following tasks under the

following typical conditions:

a. Short-circuit interruption

b. Interruption of small inductive currents

c. Capacitor switching

d. Interruption of short-line faulte. Asynchronous switching


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A circuit breaker is a switching i.e. current interruptingor making device in switchgear. In more proper words, a

circuit breaker is defined as a piece of equipment whichcan do any one of the following tasks:

a. Makes or breaks a circuit either manually or byremote control under normal conditions

b. Breaks a circuit automatically under faultconditions

c. Makes a circuit either manually or by remotecontrol under fault conditions


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A Circuit Breaker with InternalStructure


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The primary function of the circuit breakers mechanism is toprovide the means for opening and closing the contacts.Initially, this seems to be a rather simple andstraightforward requirement. However, when one considersthe fact that most circuit breakers, once placed into service,

will remain in the closed position for long periods of time,and yet on the few occasions when they are called upon toopen or close, they must do so reliably, without any delay orsluggishness, then one realizes that the demands on the

mechanisms are not as simple as was first thought.

Operating Principle

Arc Phenomenon


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Based on Voltage

Low

Medium High/Extra High

Ultra High

Classification of CircuitBreakers

Arc Phenomenon


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Based on Location

Indoor

Outdoor

Based on External Design

Dead Tank

Live Tank

Based on Interrupting media

Air Break

Air Blast

Oil

SF6


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In such circuit breakers, some insulating oil (i.e., transformeroil) is used as an arc quenching medium. The contacts areopened under oil and an arc is struck between them. Theheat of the arc evaporates the surrounding oil anddissociates it into a substantial volume of hydrogen gas at a

high pressure. This large volume of the hydrogen gaspushes the oil away from the arc.

Oil Circuit Breakers


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Advantages:

1. Oil absorbs the arc energy to produceshydrogen gas during arcing. The hydrogen hasexcellent cooling properties and helps

extinguish the arc. (In addition to hydrogen gas,a small proportion of methane, ethylene, andacetylene are also generated in oildecomposition.)

2. The oil provides insulation for the liveexposed contacts from the earthed portions ofthe container.

3. Oil provides insulation between the contactsafter the arc has been extinguished


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Disadvantages:

1. Oil is inflammable and may cause firehazards. When a defective circuit breaker failsunder pressure, it may cause an explosion.

2. The hydrogen generated during arcing, whencombined with air, may form an explosive

mixture.

3. During arcing, oil decomposes and becomespolluted by carbon particles, which reduces itsdielectric strength. Hence, it requires periodic


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Bulk Oil Circuit Breakers

These circuit breakers use a large quantityof oil. The oil has to serve two purposes:

1. It extinguishes the arc during opening ofcontacts.

2. It insulates the current conducting parts fromone another and from the earthed tank.

Types of Oil CircuitBreakers


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Minimum Oil Circuit Breakers

These circuit breakers use a small quantityof oil. In such circuit breakers, oil is used onlyfor arc extinction; the current conducting parts

insulated by air or porcelain or organicinsulating material.

In these circuit breakers, the oilrequirement can be minimized by placing the

interrupting units, in insulating chambers at livepotential, on an insulator column.


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These circuit breakers employ a high pressure air-blast as an arc quenching medium. The contactsare opened in a flow of air-blast established by theopening of the blast valve. The air-blast cools thearc and sweeps away the arcing products of theatmosphere. Consequently, the arc is extinguishedand flow of current is interrupted.

Whenever current at high voltages needs to beinterrupted, more breaking units are used, inseries. Dry and clean air supply is one of the mostessential requirements for the operation of the air-blast circuit breakers.

Air-Blast Circuit Breakers


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Advantages:

1. The risk of fire is eliminated in these circuitbreakers.

2. The arcing products are completely removed

by the blast whereas the oil deteriorates withsuccessive operations. So the expenditure of oilreplacement is avoided in air-blast circuitbreakers.

3. The size of these breakers is reduced, as thedielectric strength grows so rapidly that finalcontact gap for the arc extinction is very small.

4. Due to the rapid growth of the dielectric


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Disadvantages:

1. These circuit breakers are very sensitive to thevariation s in the rate of rise of restriking voltage.

2. The air-blast is supplied by the compressor plantthat needs considerable maintenance tages:


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In these circuit breakers, Sulphurhexafluoride gas (SF6) is used as the arcquenching medium. The SF6is anelectronegative gas and has a strong tendency

to absorb free electrons.

Applications

SF6 circuit breakers find use in systemswith voltages ranging in 115 kV to 230 kV,as they are designed for this range withpower ratings of 10 MVA to 20 MVA and

interrupting time less than 3 cycles.

Sulphur Hexafluoride (SF6)Circuit Breakers


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Advantages:

1. Because of the high conductivity of the arc in theSF6 gas, the arc energy is low. (Arc voltage isbetween 150 and 200V.)

2. Due to the low energy the contact erosion is small.3. The gaseous medium SF6 possesses excellentdielectric and arc quenching properties. After arcextinction, the dissociated gas molecules recombine

almost completely to reform SF6. This means thatpractically no loss/consumption of the quenchingmedium occurs.


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4. Due to the superior arc quenching property of theSF6 gas, such circuit breakers have very short arcing

time. Furthermore, they can interrupt much largercurrent.

5. These breakers give noiseless operation due to itsclosed gas circuit and no exhaust to atmosphere

unlike the air-blast circuit breaker.

6. The SF6 gas is not inflammable, so there is no riskof fire in SF6 breakers.

7. Since SF6 breakers are totally enclosed and sealedfrom the atmosphere, they are particularly suitable

where explosion hazards exist, i.e. in coal mines.


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Disadvantages:

1.These circuit breakers are expensive due tothe high cost of SF6 gas.

2. Since SF6 gas has to be reconditioned after

every operation of the breaker, additionalequipment is required for this purpose.

3.The SF6gas has been identified as a

greenhouse gas, and safety regulations arebeing introduced in many countries in orderto prevent its release into the atmosphere.


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In such circuit breakers, the vacuum is used as thearc quenching medium. The vacuum circuit breakertakes the advantage of non-sustainability ofelectric arc in vacuum, and employs the principleof contact separation under vacuum where there isno ionization due to medium. The initial arc causedby field and thermionic emissions during thecontacts separation, will die away soon, as there isno further ionization because of vacuum.

Vacuum Circuit Breakers


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Advantages:

1. The vacuum circuit breakers are compact insize and have longer lives.

2. Operating energy requirements are low,

because the mechanism must move onlyrelatively small masses at moderate speed,over very short distances.

3. Because of the very low voltage across themetal vapor arc, energy is very low. (Arc voltageis between 50 and 100V.)

4. Due to the very low arc energy, the rapid

movement of the arc root over the contact and-


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5. There is no generation of gases during andafter the circuit breaker operation.

6. The outstanding feature of these breakers isthat it can break any heavy fault current

perfectly just before the contacts reach adefinite open position.

7. They can successfully withstand lightningsurges.

8. It is now possible to produce cost-effective VI(vacuum interrupter) designs with electrical

lives that exceed the required mechanical life ofthe circuit breakers and that will even be able


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Air circuit breaker is defined as a circuit breaker, inwhich the contacts open and close in air atatmospheric pressure. In general, the use of thistype of circuit breakers is restricted to low voltageapplications or high security installations where therisk of an oil fire or oil contamination of theenvironment is too high to be tolerated. Countriesfollowing the American practice used air circuitbreakers almost exclusively for systems up to 15

kV until the advent of the new vacuum and SF6technologies.

Air Circuit Breakers

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