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

MCB (Miniature Circuit Breaker) :

CHARACTERISTICS

• Rated current not more than 100 A.
• Trip characteristics normally not adjustable.
• Thermal or thermal-magnetic operation.

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 controlled—some models are microprocessor controlled.
• Often used for main power distribution in large industrial plant, where the breakers are arranged in draw-out enclosures for ease of maintenance.

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 breakers tend to have longer life expectancies between overhaul than do air circuit breakers.

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

CHARACT ERISTICS

• Phase (line) and Neutral both 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 through phase 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 main incoming 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 protection

The 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)

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.

Limitation of RCCB

• Standard electromechanical RCCBs are designed to operate on normal supplywaveforms 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 don’t 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 short circuit, or an overload), the RCD won’t 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.

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 500Ω 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

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

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. That’s 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 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.

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 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. 

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 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 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.

Transformer interview Questions

1) Where is a Buchholz relay placed in the transformer ?

Ans: Relay is a gas actuated protection relay which is generally used in large oil immersed transformers of rating more than 500 kVA. It is used for the protection of a Transformer from the faults occurring inside the transformer.

Buchholz relay `can prevent the development of conditions leading to a fault in the transformer, such as the falling of the oil level due to leaks, or the penetration of air as a result of defects in the oil circulating system.

The adoption of other forms of protection does not therefore exclude the use of the gas-actuated Buchholz relay, as this device is the only means of detecting incipient faults, which if unnoticed, can cause heavy failures.

2)What is the use of breather in transformer?

Ans: As far as breathers are concerned ,silica gel breathers are used in transformers (power ,distribution transformers)!!

Oil immersed transformers find silica gel applications.In a transformer along with main tank there is another tank typically cylindrical in physical appearance(conservator tank) ,it contains a balloon like structure (used for understanding purpose only here).

Now ,

Transformer in loaded condition-temperature increases inside main tank,oil inside starts heating and expands the excessive oil starts moving to the conservator tank and thus the balloon structure inside compreses and with the help of breather air is sent out

Transformer in the opposite condition ,or say in extreme condition (lower temperature ),oil inside contracts and then air is taken inside to main the balloon like structure through silica gel breather .

You might end up why only silica gel ,the atmospheric air consists of moisture content (say 1%only),this may end up in detoriration of insulation (paper+water),and Hence to avoid this and breather acts as medium and provide dry air (moisture free). after all moisture is harmful why not dust ,yes dust in air is even to be avoided ,there is a oil in sealed cup and this helps in avoiding dust.

3) What are the Relays in transformer?

Ans: There are many types of protection relays used in transformers depend upon its construction, size, use etc. 
They are 
1. Buchholz relay for incipient faults like short ciruit, 
winding temparature protection, oil temparature protection, 
oil level protection, oil pressure protection, Pressure 
relief valve (for oil filled transformers)
2. Differential protection
3. Thermal Protection
4. Earth fault protection
5. Over current protection
6. Over excitation protection(where OLTC is used)  etc.

4) Why is the transformer placed in an oil tank?

Ans: Many people know that for the oil type transformer, it must be full of transformer oil. Transforme r oil is a kind of insulation oil, it has many functions and to protect the transformer.

Transformer oil serves following purpose in a transformer.

I. Primary: Electrical Insulation

The main purpose of oil in transformers is to provide an effective electrical insulation between (a) the winding and tank and (b) between the different winding. Insulation function.

II. Secondary: Cooling of transformer

The secondary purpose of oil is to mobilize heat from core and winding to cooling surfaces of the transformer tank. Thermal isolation

The basic qualities of transformer oil are required:

I. The oil shall be free from dirt, moisture, metal ions fibers etc.

II. It should be stable against oxidation at operating distinct temperatures

III. It should not be corrosive in nature

IV. It should not be soluble with respect to transformer material of construction

For the professional company, they can test the transformer oil after some time using transformer.

For the old transformer, firstly, we will choose the oil purifier for purify it, and then use it again. For considering the cost of the oil purifier is expensive.

5) Is Buchholz relay used for External Fault also?

Ans : Buccholz relay is a Mechanical relay used to Give two Commands: Alarm & Trip to avoi catastrophic breakdown of a transformer. Given below, is an image of how the bucholz relay assembly looks. On one end of the pipe is the conservator tank and on other end is the main tank. So Bucholz relay is connected between conservator tank and main tank.

Below is a photo of the internal assembly of the bucholz relay.

So how does Bucholz relay operate? See the video below for an interactive example.

Bucholz relay operates when Oil inside transformer breakdowns and creates various gases or pressure buildup due to faults. It is mechanically actuated. Whenever there will be a minor internal fault in the transformer such as an insulation faults between turns, break down of core of transformer, core heating, the transformer insulating oil will be decomposed in different hydrocarbon gases, CO2 and CO. The gases produced due to decomposition of transformer insulating oil will accumulate in the upper part the Buchholz container which causes fall of oil level in it. More severe types of faults, such as short circuit between phases or to earth and faults in the tap changing equipment, are accompanied by a surge of oil which strikes the baffle plate and causes the mercury switch of the lower element to close. This switch energized the trip circuit of the circuit breakers associated with the transformer and immediately isolate the faulty transformer from the rest of the electrical power system by inter tripping the circuit breakers associated with both LV and HV sides of the transformer. This is how Buchholz relay functions.

So, you will realize that the Bucholz relay protects only against internal faults that take place inside the transformer tank.

6) Why do transformer make noise while working ?

Ans : Transformers make noise while working is due to Magnetostriction. This noise is known as Humming.

Inside every element dipoles are there, so as inside the iron core of the transformer. When we apply 50hz AC to it, the dipoles vibrate at a frequency double of it's input frequency, that is at 100hz, which is audible to human ear. This phenomena is known as Magnetostriction. Due to this we hear a noise.

7) Why do you mean by KVA of Transformer ? 

Ans :  A volt ampere is used to measure apparent power (the vector sum of true power and reactive power. Transformers are rated in volt amperes, rather than in watts because to rate transformers in watts, it's necessary to know the power factor of the load, and transformer manufacturers have no way of knowing that.

A kilovolt ampere (kV.A) is one-thousand volt amperes. Distribution transformers are typically rated in kilovolt amperes. For example, pole-mounted distribution transformers are typically 5 kV.A, 50 kV.A, etc., and are used to reduce high-voltages to residential low voltages.

Power transformers, used on transmission systems, are usually rated in megavolt amperes.

8) Why is the transformer impedance given in percentage ?

Ans : Percentage impedance is a significant parameter of a transformer. Percentage impedance means -

1. The voltage drop occurring in the secondary winding of a transformer when the transformer is loaded fully. In your case, there will be a voltage drop of 12.4% from no-load secondary winding terminal voltage when the transformer supplies 55MVA. In this case, the percentage impedance is specified along with winding temperature since the resistance of the winding increases with winding temperature. As the load current increases, transformer winding temperature also increases and hence its impedance and thus the voltage drop.
2. The percentage of primary voltage when applied to the primary winding of the transformer will result in full load secondary winding current with the secondary winding terminals short-circuited.
3. The short circuit capacity of the transformer. The percentage impedance specifies the maximum short circuit current that can be fed by the transformer when its secondary winding is short circuited and rated voltage is applied to its primary winding. In this case, the percentage impedance is used to determine the available short circuit capacity or the fault level at the point in the electrical system where the transformer secondary is connected. Percentage impedance is also used to calculate the voltage drop that will occur in the system when a large motor connected to the system to which the transformer feeds power is started.

9) Why do we conduct an OC test on the LV side of Transformer ?

Ans : We can perform test in either sides.

As OC tests are done in rated voltage , and very high rating measuring instruments are not available ( or comes at very high cost) we prefer to do it at LV side.

Second point availability of voltage source ( i.e. rated voltage to be given for test ) low voltage is readily available but high voltages are not. ( not even college labs .. consider 33kv or 66kv etc).

10) Why CT not used as a Power Transformer?

Ans : The main purpose of Power transformer is to transfer power from one circuit to other by maintaining electrical isolation between the two circuits though the voltage level and current value in primary and secondary winding may different depending on application (step up or step down).

So in power transformer the whole supply voltage appears in the primary winding (neglecting cable losses) , hence it's windings need to be properly insulated depending upon the voltage level.

Whereas the main purpose of CT is to measure the current hence full current passes through its primary but a very small voltage drop across primary winding, because CTs are connected in series with the load hence by connecting CT voltage across load must not change noticeably that's why insulation level is not of that level as required in power transformer.

THANK YOU...