Firstly, once a fuse blows out, it takes quite it slow to exchange it and restore providers to the purchasers. Secondly, a fuse cannot with success interrupt significant fault currents that result from faults on trendy high-voltage and enormous capability circuits. because of these disadvantages, the utilization of switches and fuses is proscribed to low-tension and tiny capability circuits wherever frequent operations don’t seem to be expected e.g. for change and protection of distribution transformers, lighting circuits, branch circuits of distribution lines, etc.
With the advancement of the facility system, the lines and alternative instrumentality operate at terribly high voltages and carry giant currents. The arrangement of switches together with fuses cannot serve the required perform of switchgear in such high capability circuits. This necessitates using a lot of dependable suggests that management like is obtained by the utilization of circuit breakers. A fuse will build or break a circuit either manually or mechanically below all conditions viz., no-load, full-load and short-circuit conditions.
Operating principle. A fuse basically consists of fastened and moving contacts, referred to as electrodes. beneath traditional in operation conditions, these contacts stay closed and cannot open mechanically till and unless the system becomes faulty. Of course, the contacts are often opened manually or by the device whenever desired. once a fault happens on any a part of the system, the trip coils of the fuse get energized and also the moving contacts are forced apart by some mechanism, therefore gap the circuit.
When the contacts of a fuse ar separated beneath fault conditions, Associate in Nursing arc is smitten between them. the present is therefore ready to continue till the discharge ceases. the assembly of arc not solely delays the present interruption method however it additionally generates huge heat which can cause injury to the system or to the fuse itself.
- A circuit breaker is a piece of equipment which can
make or break a circuit either manually or by remote control under normal conditions.
- Break a circuit automatically under fault conditions.
- Make a circuit either manually or by remote control under fault conditions.
When a short-circuit happens, an important current flows through the contacts of the breaker before they’re opened by the protection system. At the moment once the contacts begin to separate, the contact space decreases apace and huge fault current causes raised the current density and therefore rise in temperature. the warmth made within the medium between contacts (usually the medium is oil or air) is sufficient to ionize the air or vaporize and ionize the oil. The ionizing air or vapor acts because the conductor Associate in Nursing an arc is stricken between the contacts.
During the arcing amount, the present flowing between the contacts depends upon the arc resistance. The bigger the arc resistance, the smaller the present that flows between the contacts. The arc resistance depends upon the subsequent factors.
- Degree of ionization,
- Length of the arc,
- Cross-section of arc.
Methods of Ar Methods of Arc Extinction:
There are two methods of extinguishing the arc in circuit breakers viz.
- High resistance method.
- Low resistance or current zero method
High resistance method:
In this methodology, arc resistance is created to extend with time so current is reduced to a worth skimpy to take care of the arc. Consequently, this is interrupted or the arc is destroyed. The principal disadvantage of this methodology is that big energy is dissipated within the arc. Therefore, it’s utilized solely in d.c. circuit breakers and low-capacity a.c. circuit breakers. The resistance of the arc could also be redoubled by :
- Prolongation the arc: The resistance of the arc is directly proportional to its length. The length of the arc is redoubled by increasing the gap between contacts.
- Cooling the arc: Cooling helps within the deionization of the medium between the contacts. This will increase the arc resistance. economical cooling could also be obtained by a gas blast directed on the arc.
- Reducing X-section of the arc: If the realm of X-section of the arc is reduced, the voltage necessary to take care of the arc is redoubled. In alternative words, the resistance of the arc path is redoubled. The crosswise of the arc is reduced by holding the arc submit to a slim gap or by having a smaller space of contacts.
- Cacophonic the arc: The resistance of the arc is redoubled by cacophonic the arc into a variety of smaller arcs asynchronous. every one of those arcs experiences the result of prolongation and cooling. The arc could also be split by introducing some conducting plates between the contacts.
Low resistance or current zero methods:
This methodology is utilized for arc extinction during a.c. circuits solely. during this methodology, arc resistance is unbroken low till the present is zero wherever the arc extinguishes naturally and is prevented from restriking in spite of the rising voltage across the contacts. All trendy high power a.c. circuit breakers use this methodology for arc extinction. In an a.c. the system, current drops to zero once each half-cycle. At each current zero, the arc extinguishes for a quick moment. currently, the medium between the contacts contains ions and electrons in order that it’s little nonconductor strength and may be simply lessened by the rising contact voltage called restriking voltage.
Therefore, the important downside during a.c. arc interruption is to speedily deionize the medium between contacts as shortly because the current becomes zero in order that the rising contact voltage or restriking voltage cannot breakdown the house between contacts. The de-ionization of the medium is achieved by:
- Protraction of the gap: The nonconductor strength of the medium is proportional to the length of the gap between contacts. Therefore, by the gap the contacts speedily, the upper nonconductor strength of the medium is achieved.
- Air mass: If the pressure within the neck of the woods of the arc is enhanced, the density of the particles constituting the discharge conjointly will increase. The enhanced density of particles causes a better rate of de-ionization and consequently, the nonconductor strength of the medium between contacts is enhanced.
- Cooling: The natural combination of ionized particles takes place earlier if they’re allowed to chill. Therefore, the nonconductor strength of the medium between the contacts is enhanced by cooling the arc.
- Blast impact: If the ionized particles between the contacts are caught in a frenzy and replaced by unionized particles, the nonconductor strength of the medium is enhanced significantly. this could be achieved by a gas blast directed in conjunction with the discharge or by forcing oil into the contact house.
There square measure many ways that of classifying circuit breakers. However, the foremost general approach of classification is on the premise of the medium used for arc extinction. The medium used for arc extinction is typically oil, air, greenhouse gas (SF6) or vacuum. consequently, circuit breakers could also be classified into :
- Oil circuit breakers
- Air-blast circuit breakers
- Sulfur hexafluoride circuit breakers(SF6)
- Vacuum circuit breakers