Fuses have always been a backbone in a complete electrical network, but current situation demands more concrete circuit protection, which circuit breaker is ready to deliver
A circuit breaker is a piece of equipment designed to protect an electrical apparatus from damage caused by overload or short circuit. The special feature that seperates circuit breaker from fuse is its ability to resume normal operation without any replacement.
Basically, Circuit breakers are often operated with a solenoid (electromagnet) whose strength increases as the current increases and accordingly, trips the circuit breaker. A bimetallic strip may be used instead of solenoid as it exhibits the required property of bending due to heating in case of increased current. However, Some circuit breakers are designed to incorporate both techniques, enabling them to react at required moment. This feature provides the flexibility to suit the application, with the electromagnet responding in case of abrupt short and large surges in current (short circuit) and the bimetallic strip reacting to overcurrent conditions (which hangs back for a long time). Circuit breakers for larger currents are usually arranged with pilot devices to detect a fault current and to operate the trip opening mechanism. To rephrase it, Circuit breakers incorporate features to divide and extinquish the arc.
Usually, in case of short-circuit conditions, excessive current may flow through the circuit causing heavy damages. When a circuit breaker tries to interrupt this unrequired current, an arc gets generated; thus allowing the flow of current to continue even though the contact of the circuit breaker is not closed. In air-insulated and miniature breakers, an arc chute structure made (often) of ceramic ridges or metal plates cools the arc, and blowout coils deflect the arc into the arc chute. The Larger circuit breakers used in electrical power distribution makes use of vacuum and an inert gas such as sulfur hexafluoride, or have contacts immersed in oil to suppress the arc. The maximum short-circuit current that a breaker can withstand is decided by testing; moreover, it is necessary to note that application of a breaker in a circuit with a higher prospective short-circuit current may lead in disruption of circuit breaker to safely interrupt a fault.
Initial and Preventative Maintenance Costs
Small circuit breakers are either installed directly in equipment, or are arranged in a breaker panel. Power circuit breakers are built into switchgear cabinets.The initial cost for an electrical distribution system employing fuses circuit breakers is up to 300% more compared to equivalent system employing fuses. The various initial interrupting capacities of Breakers are as follows: 10,000AIC, 22,000AIC, 42,000AIC, 64,000 AIC, etc. As a matter of fact, depending upon the interrupting capacity of circuit breaker, the price of the circuit breaker is fixed. To put in better way, as the interrupting capacity increases, the price increases accordingly. Also, breaker manufacturers require annual exercising and re-calibration of their products to insure proper operation; furthermore, they need to maintain the manufacturer`s warranty. This annual maintenance adds considerable expense in terms of parts and labour.
There are many types of circuit breakers manufactured by diferent industries and they do not always fall into distinct categories. The following types are common in domestic, commercial and light industrial applications for low voltage (less than 1000 V) use.
· MCB (Miniature Circuit Breaker) : It has rated current not more than 100 A and trip characteristics cannot be adjusted. Breakers explained above fall under this category.
· MCCB (Moulded Case Circuit Breaker) : it has rated current up to 1000 A. Thermal or thermal-magnetic operation. Trip current is adjustable.
· Air Circuit Breaker : It has a rated current up to 10,000 A with some models being microprocessor controlled. Trip characteristics are fully adjustable including configurable trip thresholds and delays. These kind of breaker finds its appplication at main power distribution in large industrial plant.
· Vacuum Circuit Breaker :This type of circuit breaker has rated current up to 3000 A; however, this could also be extended upto 30,000 V. They are basically used to interrupt the arc in a vacuum bottle. Vacuum breakers tend to have longer life expectancies between overhaul than do air circuit breakers.
In the near future, present interrupting technologies will be developed to withstand higher rated breaking currents (63 kA to 80 kA) with reduced usage of operating energy. Moreover, circuit breaker using hybrid components will be given more preference, so that this hybrid system compiles the advantages of various components such as electromechanical switches (low losses if closed), semiconductor switches (allows immediate triggering) and a pure wire resistor (PTC-effect, high energy absorbing capacity).
|Posted : 12/7/2005|