What is fuse? What are the types of fuses?

FUSE

A fuse is a short piece of metal, inserted in the circuit, which melts when excessive current flows through it and thus breaks the circuit.

The fuse element is generally made of materials having low melting point, high conductivity and least deterioration due to oxidation e.g., silver, copper etc. It is inserted in series with the circuit to be protected. Under normal operating conditions, the fuse element is at a temperature below its melting point. Therefore, it carries the normal current without overheating. However, when a short-circuit or overload occurs, the current through the fuse increases beyond its rated value. This raises the temperature and fuse element melts (or blows out), disconnecting the circuit protected by it. In this way, a fuse protects the machines and equipment from damage due to excessive currents.

The time required to blow out the fuse depends upon the magnitude of excessive current. The greater the current, the smaller is the time taken by the fuse to blow out. In other words, a fuse has inverse time-current characteristics as shown in Fig. 20.1. Such a characteristic permits its use for overcurrent protection.

Advantages

(i) It is the cheapest form of protection available.

(ii) It requires no maintenance.

(iii) Its operation is inherently completely automatic unlike a circuit breaker which requires elaborate equipment for automatic action.

(iv) It can break heavy short-circuit currents without noise or smoke.

(v) The smaller sizes of fuse elements impose a current limiting effect under short-circuit conditions.

(vi) The inverse time-current characteristic of a fuse makes it suitable for overcurrent protection.

(vii) The minimum time of operation can be made much shorter than with the circuit breakers.

Disadvantages

(i) Considerable time is lost in rewiring or replacing a fuse after operation. 

(ii) On heavy short-circuits, "discrimination between fuses in series cannot be obtained unless there is sufficient difference in the sizes of the fuses concerned. 

(iii) The current-time characteristic of a fuse cannot always be co-related with that of the protected apparatus.

Desirable Characteristics of Fuse Element

The function of a fuse is to carry the normal current without overheating but when the current exceeds its normal value, it rapidly heats up to melting point and disconnects the circuit protected by it. In order that it may perform this function satisfactorily, the fuse element should have the following desirable characteristics :

(i) low melting point e.g., tin, lead.

(ii) high conductivity e.g., silver, copper.

(iii) free from deterioration due to oxidation e.g., silver.

(iv) low cost e.g., lead, tin, copper.

The above discussion reveals that no material possesses all the characteristics. For instance, lead has low melting point but it has high specific resistance and is liable to oxidation. Similarly, copper has high conductivity and low cost but oxidises rapidly. Therefore, a compromise is made in the selection of material for a fuse.

Fuse Element Materials

The most commonly used materials for fuse elements are lead, tin, copper, zine and silver. For small currents upto 10 A, tin or an alloy of lead and tin (lead 37%, tin 63%) is used for making the fuse element. For larger currents, copper or silver is employed. It is a usual practice to tin the copper to protect it from oxidation. Zinc (in strip form only) is good if a fuse with considerable time-lag is required i.e., one which does not melt very quickly with a small overload.

The present trend is to use silver despite its high cost due to the following reasons: 

(i) It is comparatively free from oxidation.

(ii) It does not deteriorate when used in dry air.

(iii) The coefficient of expansion of silver is so small that no critical fatigue occurs. Therefore, the fuse element can carry the rated current continuously for a long time. 

(iv) The conductivity of silver is very high. Therefore, for a given rating of fuse element, the mass of silver metal required is smaller than that of other materials. This minimises the problem of clearing the mass of vapourised material set free on fusion and thus permits fast operating speed.

(v) Due to comparatively low specific heat, silver fusible elements can be raised from normal temperature to vapourisation quicker than other fusible elements. Moreover, the resistance of silver increases abruptly as the melting temperature is reached, thus making the transition from melting to vapourisation is almost instantaneous. Consequently, operation becomes much faster at higher currents. 

(vi) Silver vapourises at a temperature much lower than the one at which its vapour will readily ionise. Therefore, when an arc is formed through the vapourised portion of the element, the arc path has high resistance. As a result, short-circuit current is quickly interrupted.

Important Terms

The following terms are much used in the analysis of fuses:

(i) Current rating of fuse element. It is the current which the fuse element can normally carry without overheating or melting. It depends upon the temperature rise of the contacts of the fuse holder, fuse material and the surroundings of the fuse.

(ii) Fusing current. It is the minimum current at which the fuse element melts and thus disconnects the circuit protected by it. Obviously, its value will be more than the current rating of the fuse element. For a round wire, the approximate relationship between fusing current I and diameter d of the wire is I = kd³^/²

The fusing current depends upon the various factors such as: 

(a) material of fuse element

(b) length - the smaller the length, the greater the current because a short fuse can easily conduct away all the heat

(c) diameter

(d) size and location of terminals

(e) previous history

(iii) Fusing factor. It is the ratio of minimum fusing current to the current rating of the fuse.

Its value is always more than one. The smaller the fusing factor, the greater is the difficulty in avoiding deterioration due to overheating and oxidation at rated carrying current. For a semi-en closed or rewirable fuse which employs copper wire as the fuse element, the fusing factor is usually 2. Lower values of fusing factor can be employed for enclosed type cartridge fuses using silver or bimetallic elements.

(iv) Prospective Current. Fig. shows how a.c. current is cut off by a fuse. The fault current would normally have a very large first loop, but it actually generates sufficient energy to melt the fuseable element well before the peak of this first loop is reached. The r.m.s. value of the first loop of fault current is known as prospective current. Therefore, prospective current can be defined as under:

It is the r.m.s. value of the first loop ordinary conductor of negligible resistance. the fault current obtained if the fuse is replaced by an 

(v) Cut-off current. It is the maximum value of fault current actually reached before the fuse melts.

On the occurrence of a fault, the fault current has a very large first loop due to a fair degree of asymmetry. The heat generated is sufficient to melt the fuse element well before the peak of the first loop is reached (point 'a' in Fig.). The current corresponding to point 'a' is the cut off current. The cut off value depends upon :

(a) current rating of fuse

(b) value of prospective current

(c) asymmetry of short-circuit current

It may be mentioned here that an outstanding feature of fuse action is the breaking of circuit before the fault current reaches its first peak. This gives the fuse a great advantage over a circuit breaker since the most severe thermal and electro-magnetic effects of short-circuit currents (which occur at the peak value of prospective current) are not experienced with fuses. Therefore, the circuits protected by fuses can be designed to withstand maximum current equal to the cut-off value. This consideration together with the relative cheapness of fuses allows much saving in cost. 

(vi) Pre-arcing time. It is the time between the commencement of fault and the instant when cut off occurs.

When a fault occurs, the fault current rises rapidly and generates heat in the fuse element. As the fault current reaches the cut off value, the fuse element melts and an arc is initiated. The time from the start of the fault to the instant the arc is initiated is known as pre-arcing time. The pre-arcing time is generally small: a typical value being 0.001second

(vii) Arcing time. This is the time between the end of pre-arcing time and the instant when the arc is extinguished.

(viii) Total operating time. It is the sum of pre-arcing and arcing times.

It may be noted that operating time of a fuse is generally quite low (say 0-002 sec.) as compared to a circuit breaker (say 0-2 sec or so). This is an added advantage of a fuse over a circuit breaker. A fuse in series with a circuit breaker of low-breaking capacity is a useful and economical arrangement to provide adequate short-circuit protection. It is because the fuse will blow under fault conditions before the circuit breaker has the time to operate.

(ix) Breaking capacity. It is the r.m.s. value of a.c. component of maximum prospective current that a fuse can deal with at rated service voltage.

TYPES OF FUSES

Fuse is the simplest current interrupting device for protection against excessive currents. Since the invention of the first fuse by Edison, several improvements have been made and now-a-days, a variety of fuses are available. Some fuses also incorporate means for extinguishing the arc that appears when the fuse element melts. In general, fuses may be classified into :

(i) Low Voltage Fuses

(ii) High Voltage Fuses

It is a usual practice to provide isolating switches in series with fuses where it is necessary to permit fuses to be replaced or rewired with safety. If such means of isolation are not available, the fuses must be so shielded as to protect the user against accidental contact with the live metal when the fuse carrier is being inserted or removed.

Low Voltage Fuses

Low voltage fuses can be subdivided into two classes viz. (i) semi-enclosed rewireable fuse (ii) high rupturing capacity (H.R.C.) cartridge fuse.

1. Semi-enclosed rewireable fuse. Rewireable fase (also known as kit-kat type) is used where low values of fault current are to be interrupted. It consists of (i) a base and (ii) a fuse carrier. The base is of porcelain and carries the fixed contacts to which the incoming and outgoing phase wires are connected. The fuse carrier is also of porcelain and holds the fuse element (tinned copper wire) between its terminals. The fuse carrier can be inserted in or taken out of the base when desired.

When a fault occurs, the fuse element is blown out and the circuit is interrupted. The fuse carrier is taken out and the blown out fuse element is replaced by the new one. The fuse carrier is then re-inserted in the base to restore the supply. This type of fuse has two advantages. Firstly, the detachable fuse carrier permits the replacement of fuse element without any danger of coming in contact with live parts. Secondly, the cost of replacement is negligible. 

Disadvantages

(i) There is a possibility of renewal by the fuse wire of wrong size or by improper material. 

(ii) This type of fuse has a low-breaking capacity and hence cannot be used in circuits of high fault level.

(iii) The fuse element is subjected to deterioration due to oxidation through the continuous heating up of the element. Therefore, after some time, the current rating of the fuse is decreased L.e., the fuse operates at a lower current than originally rated.

(iv) The protective capacity of such a fuse is uncertain as it is affected by the ambient conditions.

(v) Accurate calibration of the fuse wire is not possible because fusing current very much depends upon the length of the fuse element.

Semi-enclosed rewireable fuses are made upto 500 A rated current, but their breaking capacity is low e.g.. on 400 V service, the breaking capacity is about 4000 A. Therefore, the use of this type of fuses is limited to domestic and lighting loads.

2. High-Rupturing capacity (H.R.C.) cartridge fuse. The primary objection of low and uncertain breaking capacity of semi-enclosed rewireable fuses is overcome in H.R.C. cartridge fuse. It consists of a heat resisting ceramic body having metal end-caps to which is welded silver current-carrying element. The space within the body surrounding the element is completely packed with a filling powder. The filling material may be chalk, plaster of paris, quartz or marble dust and acts as an arc quenching and cooling medium.

Therefore, it carries the normal current without overheating. When a fault occurs, the current ind creases and the fuse element melts before the fault current reaches its first peak. The heat produced in the process vapourises the melted silver element. The chemical reaction between the silver vapour and the filling powder results in the formation of a high resistance substance which helps in quenching the arc.

Advantages

(i) They are capable of clearing high as well as low fault currents. 

(ii) They do not deteriorate with age.

(iii) They have high speed of operation.

(iv) They provide reliable discrimination.

(v) They require no maintenance.

(vi) They are cheaper than other circuit interrupting devices of equal breaking capacity.

(vii) They permit consistent performance. 

Disadvantages

(i) They have to be replaced after each operation.

(ii) Heat produced by the arc may affect the associated switches.

3. H.R.C. fuse with a tripping device. Sometimes, H.R.C. cartridge fuse is provided with a tripping device. When the fuse blows out under fault conditions, the tripping device causes the circuit breaker to operate. The body of the fuse is of ceramic material with a metallic cap rigidly fixed at each end. These are connected by a number of silver fuse elements. At one end is a plunger which under fault conditions hits the tripping mechanism of the circuit breaker and causes it to operate. The plunger is electrically connected through a fusible link, chemical charge and a tungsten wire to the other end of the cap.

When a fault occurs, the silver fuse elements are the first to be blown out and then current is transferred to the tungsten wire. The weak link in series with the tungsten wire gets fused and causes the chemical charge to be detonated. This forces the plunger outward to operate the circuit breaker. The travel of the plunger is so set that it is not ejected from the fuse body under fault conditions.

Advantages. H.R.C. fuse with a tripping device has the following advantages over a H.R.C. fuse without tripping device :

(i) In case of a single phase fault on a three-phase system, the plunger operates the tripping mechanism of circuit breaker to open all the three phases and thus prevents "single phasing".

(ii) The effects of full short circuit current need not be considered in the choice of circuit breaker. This permits the use of a relatively inexpensive circuit breaker.

(iii) The fuse-tripped circuit breaker is generally capable of dealing with fairly small fault currents itself. This avoids the necessity for replacing the fuse except after highest currents for which it is intended.

Low voltage H.R.C. fuses may be built with a breaking capacity of 16,000 A to 30,000 A at 440V. They are extensively used on low-voltage distribution system against over-load and short circuit conditions.

High Voltage Fuses

The low-voltage fuses discussed so far have low normal current rating and breaking capacity. Therefore, they cannot be successfully used on modern high voltage circuits. Intensive research by the manufacturers and supply engineers has led to the development of high voltage fases. Some of the high voltage fuses are:

(i)Cartridge type. This is similar in general construction to the low voltage cartridge type except that special design features are incorporated. Some designs employ fase elements wound in the form of a helix so as to avoid corona effects at higher voltages. On some designs, there are two fuse elements in parallel; one of low resistance (silver wire) and the other of high resistance (tungsten wire). Under normal load conditions, the low resistance element carries the normal current. When a fault occurs, the low-resistance element is blown out and the high resistance element reduces the short-circuit current and finally breaks the circuit.

High voltage cartridge fuses are used upto 33 kV with breaking capacity of about 8700 A at that voltage. Rating of the order of 200 A at 6-6 kV and 11 kV and 50 A at 33 kV are also available.

(ii) Liquid type. These fuses are filled with carbon tetrachloride and have the widest range of application to h.v. systems. They may be used for circuits upto about 100 A rated current on systems upto 132 kV and may have breaking capacities of the order of 6100 A.

Liquid fuse consists of a glass tube filled with carbon tetrachloride solution and sealed at both ends with brass caps. The fuse wire is sealed at one end of the tube and the other end of the wire is held by a strong phosphor bronze spiral spring fixed at the other end of the glass tube. When the current exceeds the prescribed limit, the fuse wire is blown out. As the fuse melts, the spring retracts part of it through a baffle (or liquid director) and draws it well into the liquid. The small quantity of gas generated at the point of fusion forces some part of liquid into the passage through the baffle and there effectively extinguishes the arc.

(iii) Metal clad fuses : Metal clad oil-immersed fuses have been developed with the object of providing a substitute for the oil circuit breaker. Such fuses can be used for very high voltage circuits and operate most satisfactorily under short-circuit conditions approaching their rated capacity.

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What is Circuit Breaker and it's Classification?

During the operation of a power system, it is often desirable and necessary to switch on or off the various circuits (e.g., transmission lines, distributors, generating plants etc.) under both normal and abnormal conditions. In earlier days, this function used to be performed by a switch and a fuse placed in series with the circuit. However, such a means of control presents two disadvantages. Firstly, when a fuse blows out, it takes quite some time to replace it and restore supply to the customers. Secondly, a fuse cannot successfully interrupt heavy fault currents that result from faults on modern high-voltage and large capacity circuits. Due to these disadvantages, the use of switches and fuses is limited to low-voltage and small capacity circuits where frequent operations are not expected e.g.. for switching and protection of distribution transformers, lighting circuits, branch circuits of distribution lines etc.

With the advancement of the power system, the lines and other equipment operate at very high voltages and carry large currents. The arrangement of switches along with fuses cannot serve the desired function of Switchgear in such high capacity circuits. This necessitates to employ a more dependable means of control such as is obtained by the use of circuit breakers. 

A circuit breaker can make or break a circuit either manually or automatically under all conditions, viz, no-load, full-load and short circuit conditions. This characteristic of the circuit breaker has made it a very useful equipment for switching and protection of various parts of the power system.

CIRCUIT BREAKERS

A circuit breaker is a piece of equipment which can 

(i)make or break a circuit either manually or by remote control under normal conditions 

(ii) break a circuit automatically under fault conditions 

(iii)make a circuit either manually or by remote control under fault conditions 

Thus a circuit breaker incorporates manual (or remote control) as well as automatic control for switching functions. The latter control employs relays and operates only under fault conditions.

Operating Principle : A circuit breaker essentially consists of fixed and moving contacts, called electrodes. Under normal operating conditions, these contacts remain closed and will not open automatically until and unless the system becomes faulty. Of course, the contacts can be opened manually or by remote control whenever desired. When a fault occurs on any part of the system, the trip coils of the circuit breaker get energised and the moving contacts are pulled apart by some mechanism, thus opening the circuit.

When the contacts of a circuit breaker are separated under fault conditions, an arc is struck between them. The current is thus able to continue until the discharge ceases. The production of arc not only delays the current interruption process but it also generates enormous heat which may cause damage to the system or to the circuit breaker itself. Therefore, the main problem in a circuit breaker is to extinguish the arc within the shortest possible time so that heat generated by it may not reach a dangerous value.

ARC PHENOMENON

When a short-circuit occurs, a heavy current flows through the contacts of the circuit breaker before they are opened by the protective system. At the instant when the contacts begin to separate, the contact area decreases rapidly and large fault current causes increased current density and hence rise in temperature. The heat produced in the medium between contacts (usually the medium is oil or air) is sufficient to ionise the air or vapourise and ionise the oil. The ionised air or vapour acts as conductor and an arc is struck between the contacts. The p.d. between the contacts is quite small and is just sufficient to maintain the arc. The arc provides a low resistance path and consequently the current in the circuit remains uninterrupted so long as the arc persists.

During the arcing period, the current flowing between the contacts depends upon the arc resistance. The greater the arc resistance, the smaller the current that flows between the contacts. The arc resistance depends upon the following factors: 

(i) Degree of ionisation – the arc resistance increases with the decrease in the number of ionised particles between the contacts. 

(ii) Length of the arc – the arc resistance increases with the length of the arc i.e., separation of contacts.

(iii) Cross Section of arc – the arc resistance increases with the decrease in area of X-section of the arc.

CLASSIFICATION OF CIRCUIT BREAKERS

There are several ways of classifying the circuit breakers. However, the most general way of classification is on the basis of the medium used for arc extinction. The medium used for arc extinction is usually oil, air, sulphur hexafluoride (SF6) or vacuum. Accordingly, circuit breakers may be classified into:

(i) Oil circuit breakers – which employ some insulating oil (e.g., transformer oil) for arc extinction. 

(ii) Air-blast circuit breakers – in which high pressure air-blast is used for extinguishing the arc. 

(iii) Sulphur hexafluroide circuit breakers – in which sulphur hexafluoride (SF6) gas is used for arc extinction.

(iv) Vacuum circuit breakers – in which vacuum is used for arc extinction. Each type of circuit breaker has its own advantages and disadvantages. In the following sections, we shall discuss the construction and working of these circuit breakers with special emphasis on the way the arc extinction is facilitated.

Oil Circuit Breakers

In such circuit breakers, some insulating oil (e.g., transformer oil) is used as an arc quenching medium. The contacts are opened under oil and an arc is struck between them. The heat of the arc evaporates the surrounding oil and dissociates it into a substantial volume of gaseous hydrogen at high pressure. The hydrogen gas occupies a volume about one thousand times that of the oil decomposed. The oil is, therefore, pushed away from the arc and an expanding hydrogen gas bubble surrounds the arc region and adjacent portions of the contacts. The arc extinction is facilitated mainly by two processes. Firstly, the hydrogen gas has high heat conductivity and cools the arc, thus aiding the de-ionisation of the medium between the contacts. Secondly, the gas sets up turbulence in the oil and forces it into the space between contacts, thus eliminating the arcing products from the arc path. The result is that the arc is extinguished and the circuit current is interrupted. 

Advantages : The advantages of oil as an arc quenching medium are:

(i) It absorbs the arc energy to decompose the oil into gases which have excellent cooling properties.

(ii) It acts as an insulator and permits smaller clearance between live conductors and earthed components.

(iii) The surrounding oil presents a cooling surface in close proximity to the arc. 

Disadvantages : The disadvantages of oil as an arc quenching medium are:

(i) It is inflammable and there is a risk of a fire.

(ii) It may form an explosive mixture with air

(iii) The arcing products (e.g., carbon) remains in the oil and its quality deteriorates with successive operations. This necessitates periodic checking and replacement of oil.

Types of Oil Circuit Breakers

The oil circuit breakers find extensive use in the power system. These can be classified into the following types:

(i) Bulk oil circuit breakers – which use a large quantity of oil. The oil has to serve two purposes. Firstly, it extinguishes the arc during opening of contacts and secondly, it insulates the current conducting parts from one another and from the earthed tank. Such circuit breakers may be classified into :

(a) Plain break oil circuit breakers.

(b) Arc control oil circuit breakers.

In the former type, no special means is available for controlling the arc and the contacts are directly exposed to the whole of the oil in the tank. However, in the latter type, special arc control devices are employed to get the beneficial action of the arc as efficiently as possible.

(ii) Low oil circuit breakers – which use a minimum amount of oil. In such circuit breakers, oil is used only for arc extinction; the current conducting parts are insulated by air or porcelain or organic insulating material.

Air-Blast Circuit Breakers

These breakers employ a high pressure air-blast as an arc quenching medium. The contacts are opened in a flow of air-blast established by the opening of the blast valve. The air-blast cools the arc and weeps away the arcing products to the atmosphere. This rapidly increases the dielectric strength of the medium between contacts and prevents re-establishing the arc. Consequently, the arc is extinguished and flow of current is interrupted.

Advantages : An air-blast circuit breaker has the following advantages over an oil circuit breaker:

(i)The risk of fire is eliminated.

(ii)The arcing products are completely removed by the blast whereas the oil deteriorates with successive operations; the expense of regular oil replacement is avoided. 

(iii) The growth of dielectric strength is so rapid that the final contact gap needed for arc extinction is very small. This reduces the size of the device.

(iv) The arcing time is very small due to the rapid build up of dielectric strength between contacts. Therefore, the arc energy is only a fraction of that in oil circuit breakers, thus resulting in less burning of contacts.

(v) Due to lesser energy, air-blast circuit breakers are very suitable for conditions where frequent operation is required.

(vi) The energy supplied for arc extinction is obtained from high pressure air and is independent of the current to be interrupted.

Disadvantages : The use of air as the quenching medium offers the following disadvantages: 

(i) The air has relatively inferior extinguishing properties.

(ii) The air-blast circuit breakers are very sensitive to the variations in the rate of rise of restriking voltage.

(iii) Considerable maintenance is required for the compressor plant which supplies the air-blast. The air blast circuit breakers are finding wide applications in high voltage installations. Majority of the circuit breakers for voltages beyond 110 kV are of this type.

Types of Air-Blast Circuit Breakers

Depending upon the direction of air-blast in relation to the arc, air-blast circuit breakers are classified into:

(i) Axial-blast type – in which the air-blast is directed along the arc path.

(ii) Cross-blast type – in which the air-blast is directed at right angles to the arc path.

(iii) Radial-blast type – in which the air-blast is directly radially.

Sulphur Hexaflouride (SF₆) Circuit Breakers

In such circuit breakers, sulphur hexaflouride (SF₆) gas is used as the arc quenching medium. The SF₆ is an electro-negative gas and has a strong tendency to absorb free electrons. The contacts of the breaker are opened in a high pressure flow of SF₆ gas and an arc is struck between them. The conducting free electrons in the arc are rapidly captured by the gas to form relatively immobile negative ions. This loss of conducting electrons in the arc quickly builds up enough insulation strength to extinguish the arc. The SF₆ circuit breakers have been found to be very effective for high power and high voltage service.

Construction : It consists of fixed and moving contacts enclosed in a chamber (called arc interruption chamber) containing SF₆ gas. The chamber is connected to the SF₆ gas reservoir. When the contacts of the breaker are opened, the valve mechanism permits a high pressure SF₆ gas from the reservoir to flow towards the arc interruption chamber. The fixed contact is a hollow cylindrical current carrying contact fitted with an arc horn. The moving contact is also a hollow cylinder with rectangular holes in the sides to permit the SF₆ gas to let out through these holes after flowing along and across the arc. The tips of fixed contact, moving contact and arcing horn are coated with copper-tungsten arc resistant material. Since SF₆ gas is costly, it is reconditioned and reclaimed by a suitable auxiliary system after each operation of the breaker.

Working : In the closed position of the breaker, the contacts remain surrounded by SF₆ gas at a pressure of about 2-8 kg/cm². When the breaker operates, the moving contact is pulled apart and an arc is struck between the contacts. The movement of the moving contact is synchronised with the opening of a valve which permits SF₆ gas at 14 kg/cm pressure from the reservoir to the arc interruption chamber. The high pressure flow of SF₆ rapidly absorbs the free electrons in the arc path to form immobile negative ions which are ineffective as charge carriers. The result is that the medium between the contacts quickly builds up high dielectric strength and causes the extinction of the arc. After the breaker operation (i.e., after arc extinction), the valve is closed by the action of a set of springs.

Advantages : Due to the superior arc quenching properties of SF₆ gas, the SF circuit breakers have many advantages over oil or air circuit breakers. Some of them are listed below:

(i) Due to the superior arc quenching property of SF₆ such circuit breakers have very short arcing time.

(ii) Since the dielectric strength of SF₆ gas is 2 to 3 times that of air, such breakers can interrupt much larger current.

(iii) The SF₆ circuit breaker gives noiseless operation due to its closed gas circuit and no exhaust to atmosphere unlike the air blast circuit breaker.

(iv) The closed gas enclosure keeps the interior dry so that there is no moisture problem.

(v) There is no risk of fire in such breakers because SF₆ gas is non-inflammable.

(vi) There are no carbon deposits so tracking and insulation problems are eliminated. 

(vii) The SF₆ breakers have low maintenance cost, light foundation requirements and minimum auxiliary equipment.

(viii) Since SF₆ breakers are totally enclosed and sealed from the atmosphere, they are particularly suitable where explosion hazards exist e.g., coal mines.

Disadvantages :

(i) SF₆ breakers are costly due to the high cost of SF₆.

(ii) Since SF₆ gas has to be reconditioned after every operation of the breaker, additional equipment is required for this purpose.

Applications : A typical SF₆ circuit breaker consists of interrupter units each capable of dealing with currents upto 60 kA and voltages in the range of 50-80 kV. A number of units are connected in series according to the system voltage. SF₆ circuit breakers have been developed for voltages 115 kV to 230 kV. power ratings 10 MVA to 20 MVA and interrupting time less than 3 cycles.

Vacuum Circuit Breakers (VCB)

In such breakers, vacuum (degree of vacuum being in the range from 10^-7 to 10^-5 torr) is used as the arc quenching medium. Since vacuum offers the highest insulating strength, it has far superior arc quenching properties than any other medium. For example, when contacts of a breaker are opened in vacuum, the interruption occurs at first current zero with dielectric strength between the contacts building up at a rate thousands of times higher than that obtained with other circuit breakers.

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 reader may note the salient feature of vacuum as an arc quenching medium. 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.

Construction : It consists of fixed contact, moving contact and arc shield mounted inside a vacuum chamber. The movable member is connected to the control mechanism by stainless steel bellows. This enables the permanent sealing of the vacuum chamber so as to eliminate the possibility of leak. A glass vessel or ceramic vessel is used as the outer insulating body. The arc shield prevents the deterioration of the internal dielectric strength by preventing metallic vapours falling on the inside surface of the outer insulating cover.

Working : 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 faces 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).

Advantages : Vacuum circuit breakers have the following advantages: 

(i) They are compact, reliable and have longer life.

(ii) There are no fire hazards.

(iii) There is no generation of gas during and after operation.

(iv) They can interrupt any fault current. The outstanding feature of a VCB is that it can break any heavy fault current perfectly just before the contacts reach the definite open position.

(v) They require little maintenance and are quiet in operation. 

(vi) They can successfully withstand lightning surges.

(vii) They have low arc energy.

(viii) They have low inertia and hence require smaller power for control mechanisms.

Applications : For a country like India, where distances are quite large and accessibility to remote areas difficult, the installation of such outdoor, maintenance free circuit breakers should prove a definite advantage. Vacuum circuit breakers are being employed for outdoor applications ranging from 22 kV to 66 kV. Even with limited ratings of say 60 to 100 MVA, they are suitable for a majority of applications in rural areas.

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