Circuit Breakers

ما الفرق بين القاطع الغازى و الزيتى و المفرغ من الهواء ؟

      

الفرق الرئيسى بين القواطع هو نوع المادة العازلة المستخدمة فى إطفاء الشرارة الكهربية اثناء فصل نقط التلامس الرئيسية للقاطع.

1- القاطع المفرغ من الهواء ( Vacuum Circuit Breaker):

هذا النوع من القواطع تكون غرفة أطفاء الشرارة مفرغة تماما من الهواء بدرجة عالية جدا جدا تصل الى

1000000000 / 1 Torr تحت الضغط الجوى و لذلك لا يمكن عمل صيانة داخلية للملامسات الرئيسية للقاطع و هذا يعتبر من عيوب هذا النوع من القواطع وعند اجراء الأختبارات على هذا النوع من القواطع و قياس المقاومة الداخلية للملامسات و وجد ان قيمتها غير سليمة يتم استبدال غرفة أطفاء الشرارة بالكامل مما يزيد من تكاليف الصيانة و هذا النوع يستخدم فى الجهود حتى 36 كيلو فولت.

2- القاطع الزيتي (Oil Circuit Breaker):

هذا النوع من اقدم انواع القواطع و مازال يستخدم حتى الآن و تكون غرفة أطفاء الشرارة مملؤة بزيت عازل يساعد على أطفاء الشرارة بين الملامسات الرئيسية و لكن يجب ملاحظة انه يجب عمل اختبارات دورية للزيت بعد عدة عمليات فصل للقصر و يتم تغيرة اذا لزم الأمر و يستخدم فى الجهود المنخفضة و المتوسطة و من عيوبة ان حجمة كبير جدا فى حالة استخدامة فى الجهد العالى.

3- القاطع الغازي (SF6 Circuit Breaker):

هذا النوع من القواطع اخذ فى الأنتشار فى الأونة الأخيرة لم له من مزايا كثيرة و متعددة و يستخدم فى جميع مستويات الجهود المختلفة حتى 1100 كيلوفولت.و فى هذا النوع يستخدم غاز سادس فلوريد الكبريت SF6 كوسط عازل داخل غرفة أطفاء الشرارة.

سداسي فلوريد الكبريت هو مركب غير عضوي صيغته SF₆ وهو عديم اللون، عديم الرائحة وغير سام، وهو غاز غير قابل للاشتعال (في الأحوال العادية). جزيء SF₆ له ثمانية سطوح ويتكون من ست ذرات فلور مرتبطة بذرة الكبريت الوسطية بشكل متناظر. وهو غاز ثقيل وينقل عموما كغاز مميع.

ومن خصائصه سداسي فلوريد الكبريت أنه غاز من صنع الإنسان (غير متواجد في الطبيعة) وهو معروف بخصائصه الفيزيائية المميزة

تفصيل أكثر.......

What is Circuit Breaker?

Is a switching device which can be operated manually as well as automatically for controlling and protection of electrical power system respectively.

Types of Circuit Breaker

According different criteria there are different types of circuit breaker. According to their arc quenching media the circuit breaker can be divided as:

  1.  Oil circuit breaker.

  2.  Air circuit breaker.

  3.  SF₆ circuit breaker.

  4.  Vacuum circuit breaker.

According to their services the circuit breaker can be divided as:

   1.  Outdoor circuit breaker.

   2.  Indoor breaker.

According to the operating mechanism of circuit breaker they can be divided as:

   1.  Spring operated circuit breaker.

   2.  Pneumatic circuit breaker.

   3.  Hydraulic circuit breaker.

According to the voltage level of installation types of circuit breaker are referred as:

   1.  High voltage circuit breaker.

   2.  Medium voltage circuit breaker.

   3.  Low voltage circuit breaker.

Oil circuit breaker

Working principle: 

(i) The hydrogen gas bubble generated around the arc cools the arc column and aids the desalinization of the medium between the contacts.

 (ii) The gas sets up turbulence in the oil and helps in eliminating the arcing products from the arc path.
 (iii) As the arc lengthens due to the separating contacts, the dielectric strength of the medium is increased.

Vacuum Circuit BreakerWorking principle:

The production of arc in a vacuum circuit breaker and its extinction can be explained as follows : 

• When the contacts of the breaker are opened in vacuum (10^-7 to 10^-5 torr), an arc is produced between the contacts by the ionisation of metal vapours of contacts. However, the arc is quickly extinguished because the metallic vapours, electrons and ions produced during arc rapidly condense on the surfaces of the circuit breaker contacts, resulting in quick recovery of dielectric strength. 
• The salient feature of vacuum as an arc quenching medium is that as soon as the arc is produced in vacuum, it is quickly extinguished due to the fast rate of recovery of dielectric strength in vacuum.

The working of Vacuum circuit breakers is briefly explained below:

• When the breaker operates, the moving contact separates from the fixed contact and an arc is struck between the contacts. The production of arc is due to the ionisation of metal ions and depends very much upon the material of contacts. 
• The arc is quickly extinguished because the metallic vapours, electrons and ions produced during arc are diffused in a short time and seized by the surfaces of moving and fixed members and shields. 
• Since vacuum has very fast rate of recovery of dielectric strength, the arc extinction in a vacuum breaker occurs with a short contact separation (say 0.625 cm). 

Vacuum circuit breakers are employed for outdoor applications ranging from 22kV to 66kV. Even with limited rating say 60 to 100MVA, they are suitable for majority of applications in rural areas.

SF6 Circuit Breacker :

Working principle:

At this point we are aware that the medium in which arc extinction of the circuit breaker takes place greatly influences the  important characteristics and life of the circuit breaker. In the last article the working of a vacuum circuit breaker was illustrated. We already know that the use of vacuum circuit breaker is mainly restricted to  system voltage below 38 kV. The characteristics of vacuum as medium and cost of the vacuum CB does not makes it suitable for voltage exceeding 38 kV. In the past for higher transmission voltage Oil Circuit Breaker (OCB) and Air Blast Circuit Breaker (ABCB) were used. These days for higher transmission voltage levels  SF₆  Circuit Breakers are largely used. OCB and ABCB have almost become obsolete.  In fact in many installations SF₆  CB is used for lower voltages  like 11 kV, 6 kV etc.. 

Sulphur Hexafluoride symbolically written as SF₆  is a gas which satisfy the requirements of an ideal arc interrupting medium. So SF₆  is extensively used these days as an arc interrupting medium in circuit breakers ranging from 3 kv  upto 765 kv class. In addition to this SF₆ is used in many electrical equipments for insulation. Here first we discuss in brief, some of the essential properties of  SF₆ which is the reason of it's extensive use in circuit breakers

• SF₆ gas has high dielectric strength which is the most important quality of a material for use in electrical equipments and in particular for breaker it is one of the most desired properties. Moreover it has high Rate of Rise of dielectric strength after arc extinction. This characteristics is very much sought for a circuit breaker to avoid restriking.
• SF₆ is colour less, odour less and non toxic gas.
• SF₆  is an inert gas. So in normal operating condition the metallic parts in contact with the gas are not corroded. This ensures the life of the breaker and reduces the need for maintenance.
• SF₆ has high thermal conductivity which means the heat dissipation capacity is more. This implies greater current carrying capacity when surrounded by SF₆ .
• The gas is quite stable. However it disintegrates to other fluorides of Sulphur in the presence of arc. but after the extinction of the arc the SF₆  gas is reformed from the decomposition.
• SF₆ being non-flammable so there is no risk of fire hazard and explosion. 

The construction and working principles of SF₆ circuit breaker varies from manufacturer to manufacturer. In the past double pressure type of SF₆ breakers were used. Now these are obsolete. Another type of SF₆ breaker design is the self blast type, which is usually used for medium transmission voltage. The Puffer type SF₆ breakers of single pressure type are the most favoured types prevalent in power industry.  Here the working principle of Puffer type breaker is illustrated .

Synchronous Generator

Analysis of the Synchronous Generator

Over the years, synchronous generators, also called alternators, have been used as a reliable ac electric power source. Therefore, it is of great importance to have a basic

knowledge of this kind of ac machinery, its construction. principles of operation and performance characteristics.

Physical Construction

A typical synchronous generator consists of the following essential elements:

The Stator

The stator of a synchronous generator is identical to that for an induction machine. It is built of thin iron laminations of highly permeable steel core. The laminated core is to reduce the magnetizing losses such as the eddy current and hysteresis losses. The stator accommodates the three-phase armature windings which are the distributed stator windings. The distributed windings are embedded in the slots inside the stator core 120 electrical degrees apart in the space to minimize the space harmonics in the resultant air gap flux waveform. They are also made short-pitch in order to produce a smooth sinusoidal voltage waveform at the stator terminals. The reason behind having the armature windings fixed on the stator is that they need to be well-insulated due to the high voltages and transient currents they may experience during different operation modes. A cross-sectional sketch of a stator of a synchronous machine is shown below in Figure.

Basic Stator Scheme for a 2-pole 3-phase synchronous generator

The Rotor

The rotor of the synchronous generator hosts windings which carry the dc field current and are connected to the excitation source of the generator via brushes and the slip rings assembly. The synchronous generator has two different rotor configurations based on its speed. Turbo- generators used for high speed operation have a round rotor structure.

These generators are commonly referred to as non-salient pole or cylindrical-rotor synchronous generators. They have a uniform air-gap and normally have either two or four field poles, depending on the required speed of operation. Hydro-generators used for low speed operation have rotors with salient poles structure. These generators are known as salient pole synchronous generators. Because of the rotor saliency, a non-uniform air gap is formed between the rotor and stator inner surface. The salient-pole rotor has a comparatively larger number of poles. A common practice is to have damper or amortisseur windings equipped to the rotor of this type in order 10 damp out the speed oscillation in the rotor. Damper windings are copper or brass bars embedded in the salient 32 pole faces with both their ends shorted-circuited by means of shorting rings to form a cage structure similar to that for a squirrel cage rotor of an induction machine. These windings play a major role in retaining the generator synchronism during the dynamic transient or hunting. The cross-sectional view of two rotor structures of the synchronous generator are shown in Figure.

Elementary rotor structure of2-pole alternator: (a) cylindrical rotor, (b) salient-pole rotor

MCB and MCCB

How to Read MCB Nameplate Rating?

1.     Model Number.: All reputed manufacturer has a particular code of each device type. It will be very easy to communicate with seller or manufacturer, if you quote the model no., in case of any service complaint.

2.     MCB Current and Curve Rating: As shown in example, it is mentioned C20 (and in the below image, it is B25). First letter is showing the characteristics curve. There are three characteristics curves (In common use) available- B C & D. B curve indicates that short circuit rating of device is in range of 3-5 times of standard rated current (Which means, TIME for Trip initiation i.e. the less rating of the the time will be Fast acting, like for protecting sensitive Electronics devices and equipment). C curve indicates it to be 5-10 times and D curve indicates it to be 10-20 times. Be very careful while selecting this. On a resistive load (say heater, normal lighting load) it will B Curve, for inductive load (Like pump, Motors etc.) it will be C curve and for highly inductive or capacitive load it will be D curve. The numeral part indicates rating of MCB in Ampere. In the given example it is 20A. MCB rating is very important and be very precise about it.

3.     Operating Voltage: It is in Volts and is the operational voltage for which current rating is said. In three phase it is usually 400V or 415 V. For single phase it is 230V or 240V. Choose as per your application only.

4.     MCB Breaking Capacity: Breaking capacity can be defined as the maximum level of fault current which can be safely cleared. It is written as in numerals like in in example it is 10000. It means it 10000A = 10kA. Choose breaking capacity as per your fault level possible. Since it is the parameter which may increase or decrease the cost, so it should be properly decided. Breaking capacity should be higher than the possible fault level. For domestic application where fault level cannot be calculated easily, it advisable that go for a standard breaking capacity of 10kA which is easily available. Please note that this rating is mentioned as per testing made on basis of IS 60898. If it is for IEC60947-2 then it need to be mentioned separately.

5.     Energy Class: MCB normally work on current limiting feature. It means that it does not allow fault to get it’s peak and trip before that. But since there is some time consumed in tripping, fault current will create some energy which will exist in system. This energy is termed as let through energy. For efficient MCB operation it should be limited. On basis of amount of energy it is classified in class 1, class 2 and class 3. Here Class 3 is best which allows maximum 1.5L joule/second. This is being tested as per IS 60898.

6.     Status Indicator: It shows the ON-Off Indication while in operation. Never buy an MCB which don’t have clear status indicator because serious damage may be occurred with ON-OFF confusion of the device.

7.     Operation Symbol: This is always printed by any good manufacturer. This shows operation mechanism of MCB.

8.     Additional Relevant Information : Information like Impulse voltage, ISI marking etc are usually printed on side of MCB. However there are many parameters on which quality of MCB should be judged but aim of this blog is to make you aware of printed information on MCB.

9.     Catalog No: Most of the MCB manufactures put the catalog number of the MCB products. This code provide the overall information on the manufacture website such as MCB specification and Datasheet ect.

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:

• 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:

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

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

• 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)
• 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
• RCDs are an extremely effective form of shock protection

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 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):

• 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. 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.

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.