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MCB (Miniature Circuit Breaker)

Characteristics

Rated current not more than 100 A.

Trip characteristics normally 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.

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protection (hundred of amps)

A 300/500 mA RCCB may be used where only fire protection is required. eg., on lighting circuits,

where the risk of electric shock is small.

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Limitation of RCCB

Standard electromechanical RCCBs are designed to operate on normal supply

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 ac and pulsating dc.

RCDs dont offer protection against current overloads: RCDs detect an imbalance in the

live and neutral currents. A current overload, however large, cannot be detected. It is a

frequent cause of problems with novices to replace 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 shortcircuit, 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 an RCBO (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 a small, brief

current flow to earth, especially in old appliances. RCDs are very sensitive and operate very

quickly; they may well trip when the motor of an old freezer switches off. Some equipment is

notoriously `leaky, that is, 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 live and neutral

terminals the wrong way round.

RCD will not protect against the overheating that results when conductors are not properly

screwed into their terminals.

RCD will not protect against live-neutral shocks, because the current in the 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:

The safest limit of Current which Human Body can withstand is 30ma sec.

Suppose Human Body Resistance is 500 and Voltage to ground is 230 Volt.

The Body current will be 500/230=460mA.

Hence ELCB must be operated in 30maSec/460mA = 0.65msec

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RCBO (Residual Circuit Breaker with OverLoad)

It is possible to get a combined MCB and RCCB in one device (Residual Current Breaker with

Overload RCBO), the principals are the same, but more styles of disconnection are fitted into

one package

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Difference between ELCB and RCCB

ELCB is the old name and often refers to voltage operated devices that are no 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 the new name to

distinguish from voltage operated).

The new RCCB is best because it will detect any earth fault. The voltage type 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 earth wire 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 when the both currents are deferent and it withstand

up to both the currents are same. Both the neutral and phase currents are different that means

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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 in equipment without an earth of its

own.

This means that an RCD will continue to give shock protection in equipment that 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.

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MCB Selection

The 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 ofthe 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 3 To 5 time full load current 0.04 To 13 Sec

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

Type D 10 To 20 times full load current 0.04 To 3 Sec

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

Kilo amps (KA). In general for consumer units a 6KA fault level is adequate whereas for

industrial boards 10KA fault capabilities or above may be required.

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Fuse and MCB characteristics

Fuses and MCBs are rated in amps. The amp rating given on the fuse or MCB 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 device will 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, have very different

properties.

For example, For 32Amp MCB and 30 Amp Fuse, to be sure of tripping in 0.1 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 how much 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 fuse will trip whencarrying 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

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obvious reason

If the fuse is marked 30 amps, but it will actually stand 40 amps for over an hour, how can

we justify calling it a 30 amp fuse? The answer is that the overload 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.