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MCB s or Miniature Circuit Breakers are electromechanical devices which protectan electrical circuit from an overcurrent. The overcurrent, in an electrical circuit, mayresult from short circuit, overload or faulty design. An MCB is a better alternative to aFuse since it does not require replacement once an overload is detected. nli!efuse, an MCB can be easily reset and thus offers improved operational safety and

greater convenience without incurring large operating cost.

The principal of operation is simple. An MCB functions by interrupting the continuityof electrical flow through the circuit once a fault is detected. "n simple terms MCB is aswitch which automatically turns off when the current flowing through it passes thema#imum allowable limit. $enerally MCB are designed to protect against over current and over temperature faults %over heating&.

There are two contacts one is fi#ed and the other moveable. 'hen the currente#ceeds the predefined limit a solenoid forces the moveable contact to open %i.e.,disconnect from the fi#ed contact& and the MCB turns off thereby stopping thecurrent to flow in the circuit. "n order to restart the flow of current the MCB is

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manually turned on. This mechanism is used to protect from the faults arising due toover current or over load.

To protect against fault arising due to over heating or increase in temperature a bi(metallic strip is used. MCBs are generally designed to trip within ).* millisecondwhen an over current fault arises. "n case of temperature rise or over heating it mayta!e ) seconds to ) minutes for the MCB to trip.

All the parts of this single pole MCB have been shown together in the followingimage.

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APPLICATIONS

1. Home E lectrical Panels

2. Ground Fault Trip Mechanism

3. Lights

4. Industrial Applications

5. Heaters

MCB +electionThe first characteristic is the overload which is intended to prevent the accidental overloading of the

cable in a no fault situation. The speed of the MCB tripping will vary with the degree of the overload. This is

usually achieved by the use of a thermal device in the MCB.

The second characteristic is the magnetic fault protection, which is intended to operate when the fault

reaches a predetermined level and to trip the MCB within one tenth of a second. The level of this magnetic

trip gives the MCB its type characteristic as follows -

Type Tripping Current Operating Time

Type B / To * time full load current 0.01 To 2/ +ec

Type C * To 20 times full load current 0.01 To * +ecType 3 20 To )0 times full load current 0.01 To / +ec

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The third characteristic is the short circuit protection, which is intended to protect against heavy faults

maybe in thousands of amps caused by short circuit faults.

The capability of the MCB to operate under these conditions gives its short circuit rating in 4ilo amps

%4A&. "n general for consumer units a 54A fault level is adequate whereas for industrial boards 204A fault

capabilities or above may be required.

FUSE

The verb "fuse" means "to melt". A fuse is an overcurrent responsivesafety device used to protect an electrical circuit from the effects ofexcessive current. It is placed in series with the electrical circuit it isintended to protect. Its essential component is a strip of metal thatwill melt when the electric current in the circuit exceeds the fuse'srated value. The element (link) in the fuse melts and opens thecircuit.

!! #$% $&!% %%! T %$T! TI$

*hen a problem exists+ the fuse is called upon to open (melt its link).The openin, of a fuse si,nifies that somethin, is wron, with thecircuit and should be corrected before the current is turned back on.The problem can be an accident+ a defective component or a worn-outcomponent.

Types

The main components of a standard fuse unit consist of the following items

• Metal fuse element• Set of contacts• Support body

Rewireable Fuses

This !ind of fuse is most commonly used in the case of domestic wiring and small scale usage. Anothername for this type is the 4"T(4AT type fuse. The main composition is of a porcelain base which holds the

wires.

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The fuse element is located inside a carrier that is also made out of porcelain. "t is possible for you toremove the fuse carrier without any ris! of electrical shoc!. 6ormally what happens is that when the fuseblows, you can replace it without having to change the complete thing.The main metals or alloys used in ma!ing fuse wire include lead, tinned copper, aluminum or tin lead alloy.

'hen there is an over surge that causes the fuse element to blow off, you can replace it. A new fuse carrier is inserted in the base.The main advantage of this type of fuse is that it is easy to install and also replace without ris!ing anyelectrical in7ury. But there are certain shortcomings associated with it too. For instance, with this fuse youwould have an element of unreliability. There is a level of lac! of discrimination and a small time lag, whichmay hinder its functionality.'ith a slow speed of operation, you also get a low rupturing capacity.8ther types have current limiting features, and this one does not.

All this being said it is still a valuable fuse device for small scale usage.

Totally Enclosed or Cartridge Type

"n this type of fuse, we have a completely closed container and there are contacts %metal& on either side.The level of sub division in this case includes

• D type• Link Type

APPLICATIONS

!arth eaka,e ircuit /reaker or ! /If any current leaks from any electrical installation+ there must-beany insulation failure in the electrical circuit + it must be properlydetected and prevented otherwise there may be a hi,h chance ofelectrical shock if-anyone touches the installation. An earthleakage circuit breaker does it efficiently. 0eans it detects theearth leaka,e current and makes the power supply off by openin,the associated circuit breaker. There are two types of earth

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by these two current s is also 3ero-ideally. The relay coil isconnected with another third windin, wound on the T core assecondary. The terminals of this windin, are connected to a relaysystem. In normal operatin, condition there would not be anycurrent circulatin, in the third windin, as here is no flux in thecore due to e1ual phase and neutral current . *hen any earthleaka,e occurs in the e1uipment+ there may be part of phasecurrent passes to the earth+ throu,h the leaka,e path instead ofreturnin, via mental wire. 2ence the ma,nitude of the neutralcurrent passin, throu,h the % / is not e1ual to phase current passin, throu,h it.

This relay causes trippin, of the associated circuit breaker todisconnect the power supply to the e1uipment under protection.Residual current circuit breaker is sometimes also referred asresidual current device (% ) when we consider the device bydisassociatin, the circuit breaker attached to RCCB . That means+the entire parts of % / except circuit breaker are referred as% .

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