Lec 5 Dist Equip April 2011

8/7/2019 Lec 5 Dist Equip April 2011

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Distribution EquipmentDistribution Equipment

Lecture 5Lecture 5


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Definition of Overload

Z10;

V

220 V

I=220/10=22 A

When the load exceeds the circuit design value, insulation damage

occurs which leads to short circuit


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Definition of Short Circuit

Z10;

V

220 V

I=220/0.01=22000 A


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electrical

protection

isolation control

-overload currents-short-circuit currents

-insulation failure

-isolation clearlyindicated by an

authorized fail-proof

mechanical indicator

-a gap or interposed

insulating barrier

between the open

contacts, clearly

visible.

-functional switching-emergency switching

-emergency stopping

-switching off for

Mechanical maintenance

Basic Functions of LV Switchgear


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electrical protectionassures:protectionofcircuitelementsagainstthethermal

andmechanicalstressesofshort-circuitcurrents

protectionofpersonsintheeventofinsulation

failureprotectionofappliancesandapparatus being

supplied (e.gmotors,etc.)

Electrical Protection


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Elementary Switching Devices

1-FusesFuses break a circuit by controlled melting of the

fuse element when a current exceeds a given

value for a corresponding period of time; the

current/time relationship being presented in the

form of a performance curve for each type of

fuse.


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Thisswitchisamanually-operated,lockable,

two-positiondevice (open/closed) which

providessafeisolationofacircuitwhenlockedin

theopen position.

2-Disconnector (or isolator)



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3-Load breaking switch


Itisusedtocloseandopenloaded

circuitsundernormalunfaultedcircuit

conditions


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6-Contactor

The contactor is a solenoid-operated switching device

which is generally held closed by (a reduced) current

through the closing solenoid (although various

mechanically-latched types exist for specific duties).Contactors are designed to carry out numerous

close/open cycles and are commonly controlled

remotely by on-offpushbuttons.



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7-Circuit Breaker

Providesoverloadandshortcircuit protection

Miniature CB

Molded Case CB

Power CB


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8-Combined switchgear elements

Wheretheinstallationofacircuit breakeris

notappropriate (notably wheretheswitching

rateishigh,overextended periods)

- Example: Switch and fuse combinations



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FuseFuse linklink : It consists

traditionally of silver strip.

Recently, copper strip of lesssurface oxidation alloying

effects.

FuseFuse holderholder : The combination

of fuse base and fuse carrier.

Fuses

1-RewirableFuses


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2-Quartz sand-filled Cartridge HRCF

Bolted end

connection

Quartz sand

Fuse link

Galvanizedceramic cartridge


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Fusing factor

A fuse mustmust reliably carry full load

current and small overloads such

as transformer magnetizing inrush

currents, capacitor charging

currents and motor starting

currents for a short time.

The ratioratio between the rated

current and the minimum fusing

current is the fusingfusing factorfactor and is

normally 1.45 or as low as 1.25. At

such overloads the fuse will melt

in about 1 hour and at higher

currents will melt more quickly.


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HRCF for use in special semiconductor protection

According to IEC 269-4, especially very fast acting on short circuit, low

energy let through (I2t), controlled overvoltages and high current HRC

fuses are required to protect power semiconductor devices because of

the low thermal mass and very short times for the semiconductor

devices to achieve thermal runaway to destruction.


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Fusing zones

Inf: is the value of currentthat the fusible element

can carry for a specified

time without melting.

If : is the value of currentwhich will cause melting of

the fusible element before

the expiration of the

specified time.


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Low Voltage CB


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Trip mechanism actuating device

11--ThermalThermal release

thermally-operated bi-metal strip

detects an overload condition


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22--MagneticMagnetic release

An electromagnetic striker pin

operates at current levels

reached in short-circuit

conditions

33--ElectronicElectronic release

An electronic relay operatedfrom current transformers,

one of which is installed on

each phase.

Trip mechanism actuating device


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IrIr overload (thermal or short-delay) relay trip-current setting.

ImIm short-circuit (magnetic or long-delay) relay trip-current setting.

I short-circuit instantaneous relay trip-current setting.

Pdc breaking capacity.

Long-term overload threshold djustment

Adjustable tripping time tr

Short-circuit protection

threshold adjustment

Adjustable time delay

Performance Curves ofCB


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Characteristics: heavy-duty industrial circuitbreakers of large current ratings up to 1200A

and breaking short circuit capacity up to 75kA.

Type of prortective relay: thermal-magnetic or

electronic.

Overload & short-circuit protection mode ofoperation: fixed or adjustable.

Auxiliary: can be equipped with shunt trippingcoil to receive tripping signals from other

relaying devices.

Molded Case CB (MCCB)


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MCCB-type Type of protective

relay

Overloadprotection

Short-circuitprotection

DomesticDomestic thermal-magnetic

Ir = In fixed at

Im= 7 to 10In

IndustrialIndustrial

thermal-magnetic

fixed atIr = In

fixed atIm= 7 to 10In

adjustable

5IneIme10In

adjustable at

Im= 7 to 10In

electronic long delay

0.4IneIr < In

short delay

adjustable at

1.5IreIm < 10Ir

instantaneous fixed inrange 12 to 15 In



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ExampleExample:

A circuit breaker equipped with a 320A overcurrent trip relay, set at

0.9, will have a trip-current setting:

Ir = 320 x 0.9 = 288A



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heavy-duty industrial circuit breakers of large current ratings,

conforming toIE

C9

47

-2, have numerous built-incommunication and electronic functions

PowerCB


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Thefundamentalratingsofacircuit breakerare:

Itsratedvoltage Ur

ItsratedcurrentIn

Itstripping-current-leveladjustment

Rangesforoverload protection (Ir) andforshort-circuitprotection (Im)

Itsshort-circuitcurrent breakingrating (Icuforindustrial

CBs;Icnfordomestic-type CBs)

Fundamental Ratings ofCB


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Rated operational voltage (Ur)

This is the voltage at which the circuit breaker

has been designed to operate, in normal (undisturbed)

conditions.

Rated current (In)

This is the maximum value of current that a circuit breaker, fitted with

a specified overcurrent tripping relay, can carry indefinitely at an

ambient temperature stated by the manufacturer, without exceeding

the specified temperature limits of the current carrying parts.

Ratings ofCB


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Frame-size rating

A circuit breakerwhichcan befittedwith

overcurrenttrippingunitsofdifferentcurrentlevel-

settingranges,isassignedaratingwhich

correspondswiththatofthehighestcurrent-level-

settingtrippingunitthatcan befitted.

Ratings ofCB

Short Circuit Current Interrupting rating

Theshortcircuitcurrentthatthe breakercaninterrupt

withoutmalfunctioning


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Example


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CB Coordination

x x x x x F


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CB Coordination


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Lec 5 Dist Equip April 2011

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