What is the Difference Between MCB

7/31/2019 What is the Difference Between MCB

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What is the difference between MCB, MCCB, ELCB, and

RCCB

MCB (Miniature Circuit Breaker)

Characteristics

Rated current not more than 100 A. Trip characteristicsnormally not adjustable. Thermal or thermal-magnetic operation.
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MCCB (Moulded Case Circuit Breaker)

Characteristics

Rated current up to 1000 A. Trip current may be adjustable.

Thermal or thermal-magnetic operation.

Air Circuit Breaker

Characteristics

Rated current up to 10,000 A. Trip characteristics often fully adjustable including configurable trip

thresholds and delays.

Usually electronically controlledsome models are microprocessorcontrolled.

Often used for main power distribution in large industrial plant, where thebreakers are arranged in draw-out enclosures for ease of maintenance.


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Vacuum Circuit Breaker

Characteristics

With rated current up to 3000 A, These breakers interrupt the arc in a vacuum bottle. These can also be applied at up to 35,000 V. Vacuum circuit breakerstend

to have longer life expectancies between overhaul than do air circuit

breakers.

RCD (Residual Current Device / RCCB(Residual Current Circuit Breaker)

Characteristics

Phase (line) and Neutralboth wires connected through RCD.

It trips the circuit when there is earth fault current. The amount of current flows through the phase (line) should return through

neutral .

It detects by RCD. any mismatch between two currents flowing throughphase and neutral detect by -RCD and trip the circuit within 30Miliseconed.

If a house has an earth system connected to an earth rod and not the mainincoming cable, then it must have all circuits protected by an RCD (because

u mite not be able to get enough fault current to trip a MCB)

RCDs are an extremely effective form of shock protectionThe most widely used are 30 mA (milliamp) and 100 mA devices. A current flow

of 30 mA (or 0.03 amps) is sufficiently small that it makes it very difficult to

receive a dangerous shock. Even 100 mA is a relatively small figure when

compared to the current that may flow in an earth fault without such protection(hundred of amps)
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A 300/500 mA RCCB may be used where only fire protection is required. eg., onlighting circuits, where the risk of electric shock is small.

Limitation of RCCB

Standard electromechanical RCCBs are designed to operate on normalsupply waveforms and cannot be guaranteed to operate where none

standard waveforms are generated by loads. The most common is the half

wave rectified waveform sometimes called pulsating dc generated by speed

control devices, semi conductors, computers and even dimmers.

Specially modified RCCBs are available which will operate on normal acand pulsating dc.

RCDs dont offer protection against current overloads: RCDs detect animbalance in the live and neutral currents. A current overload, however

large, cannot be detected. It is a frequent cause of problems with novices toreplace an MCB in a fuse box with an RCD. This may be done in an attempt

to increase shock protection. If a live-neutral fault occurs (a short circuit, or

an overload), the RCD wont trip, and may be damaged. In practice, the

main MCB for the premises will probably trip, or the service fuse, so the

situation is unlikely to lead to catastrophe; but it may be inconvenient.

It is now possible to get an MCB and and RCD in a single unit, called anRCBO (see below). Replacing an MCB with an RCBO of the same rating is

generally safe.

Nuisance tripping of RCCB: Sudden changes in electrical load can cause asmall, brief current flow to earth, especially in old appliances. RCDs arevery sensitive and operate very quickly; they may well trip when the motor

of an old freezer switches off. Some equipment is notoriously `leaky, thatis, generate a small, constant current flow to earth. Some types of computer

equipment, and large television sets, are widely reported to cause problems.

RCD will not protect against a socket outlet being wired with its liveand neutral terminals the wrong way round.

RCD will not protect against the overheating that results when conductorsare not properly screwed into their terminals.

RCD will not protect against live-neutral shocks, because the current inthe live and neutral is balanced. So if you touch live and neutral conductors

at the same time (e.g., both terminals of a light fitting), you may still get a

nasty shock.


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ELCB (Earth Leakage Circuit Breaker)

Characteristics

Phase (line), Neutral and Earth wire connected through ELCB. ELCB is working based on Earth leakage current. Operating Time of ELCB:

o The safest limit of Current which Human Body can withstand is 30masec.

o Suppose Human Body Resistance is 500 and Voltage to ground is230 Volt.

o The Body current will be 500/230=460mA.o Hence ELCB must be operated in 30maSec/460mA = 0.65msec

RCBO (Residual Circuit Breaker with OverLoad)

It is possible to get a combined MCB and RCCB in one device (ResidualCurrent Breaker with Overload RCBO), the principals are the same, but

more styles of disconnection are fitted into one package

Difference between ELCB and RCCB

ELCB is the old name and often refers to voltage operated devices that areno longer available and it is advised you replace them if you find one.

RCCB or RCD is the new name that specifies current operated (hence thenew name to distinguish from voltage operated).


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The new RCCB is best because it will detect any earth fault. The voltagetype only detects earth faults that flow back through the main earth wire so

this is why they stopped being used.

The easy way to tell an old voltage operated trip is to look for the main earthwire connected through it.

RCCB will only have the line and neutral connections. ELCB is working based on Earth leakage current. But RCCB is not having

sensing or connectivity of Earth, because fundamentally Phase current is

equal to the neutral current in single phase. Thats why RCCB can trip whenthe both currents are deferent and it withstand up to both the currents are

same. Both the neutral and phase currents are different that means current is

flowing through the Earth.

Finally both are working for same, but the thing is connectivity is difference. RCD does not necessarily require an earth connection itself (it monitors only

the live and neutral).In addition it detects current flows to earth even inequipment without an earth of its own.

This means that an RCD will continue to give shock protection in equipmentthat has a faulty earth. It is these properties that have made the RCD more

popular than its rivals. For example, earth-leakage circuit breakers (ELCBs)

were widely used about ten years ago. These devices measured the voltage

on the earth conductor; if this voltage was not zero this indicated a current

leakage to earth. The problem is that ELCBs need a sound earth connection,

as does the equipment it protects. As a result, the use of ELCBs is no longer

recommended.

MCB Selection

The first characteristic is the overload which is intended to prevent theaccidental 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 intendedto 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:


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Type Tripping Current Operating Time

Type B3 To 5 time full load current 0.04 To 13 Sec

Type C5 To 10 times full load current 0.04 To 5 Sec

Type D10 To 20 times full load current 0.04 To 3 Sec

The third characteristic is the short circuit protection, which is intended toprotect 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 shortcircuit rating in Kilo amps (KA). In general for consumer units a 6KA fault

level is adequate whereas for industrial boards 10KA fault capabilities orabove may be required.

Fuse and MCB characteristics

Fuses and MCBs are rated in amps. The amp rating given on the fuse orMCB body is the amount of current it will pass continuously. This is

normally called the rated current or nominal current.

Many people think that if the current exceeds the nominal current, the devicewill trip, instantly. So if the rating is 30 amps, a current of 30.00001 amps

will trip it, right? This is not true.

The fuse and the MCB, even though their nominal currents are similar, havevery different properties.

For example, For 32Amp MCB and 30 Amp Fuse, to be sure of tripping in0.1 seconds, the MCB requires a current of 128 amps, while the fuse

requires 300 amps.

The fuse clearly requires more current to blow it in that time, but notice howmuch bigger both these currents are than the 30 amps marked current

rating.

There is a small likelihood that in the course of, say, a month, a 30-amp fusewill trip when carrying 30 amps. If the fuse has had a couple of overloads

before (which may not even have been noticed) this is much more likely.

This explains why fuses can sometimes blow for no obvious reason

If the fuse is marked 30 amps, but it will actually stand 40 amps for overan hour, how can we justify calling it a 30 amp fuse? The answer is that theoverload characteristics of fuses are designed to match the properties of

modern cables. For example, a modern PVC-insulated cable will stand a

50% overload for an hour, so it seems reasonable that the fuse should as

well.

What is the Difference Between MCB

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