Introductoin to circuit breaker
During the operation of the power systems, 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 faulty conditions. Previously this function was performed by a switch and a fuse in series with the circuit. However, such a means of control presents two disadvantages. Firstly, when fuse blows out, it takes quite sometime to replace it and restore the supply to the customers. Secondly, a fuse can not successfully interrupt the heavy fault currents that occur on the modern high voltage power systems and large capacity circuits. Therefore, with the
During the operation of the power systems, 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 faulty conditions. Previously this function was performed by a switch and a fuse in series with the circuit. However, such a means of control presents two disadvantages. Firstly, when fuse blows out, it takes quite sometime to replace it and restore the supply to the customers. Secondly, a fuse can not successfully interrupt the heavy fault currents that occur on the modern high voltage power systems and large capacity circuits. Therefore, with the
advancement in the power systems, there was a need to develop a more reliable means of control. The circuit breaker was developed to switch on and off the various circuits of a power system.
Devices used for circuit breaking (or making) :
1- Fuse and ironclad switches :
Fuse is an over current switch in the sense that when the current exceeds a preassigned value in a circuit or device, it melts and causes the current interruption. The supply is restored only when a healthy fuse replaces the damaged (melted) one in the line. To permit this without any danger of shock to the operator, fuses are connected on the load side of an ironclad switch.
2- Isolators :
An isolator is a switch connected after a circuit breaker. When a circuit or a busbar is taken out of service by tripping the circuit breaker, the isolator is then open circuited and the isolated line is earthed through earth switch so that the trapped line charges are safely conducted to ground.
3- Circuit breakers :
A circuit breaker is a complex circuit-breaking device with the following duties:
- Makes or breaks both normal and abnormal currents
- Appropriately manages the high-energy arc associated with current interruption. The problem has become more acute due to interconnection of power stations resulting in very high fault levels.
- Effects current interruption only when it is called upon to do so by the relay circuits. In fact they are required to trip for a minimum of the internal fault current and remain inoperative for a maximum of through fault current
- Rapid and successive automatic breaking and making to aid stable system
operation
- Three pole (3-pole) and single pole (1-pole) auto-reclosing arrangement
- Appropriately manages the high-energy arc associated with current interruption. The problem has become more acute due to interconnection of power stations resulting in very high fault levels.
- Effects current interruption only when it is called upon to do so by the relay circuits. In fact they are required to trip for a minimum of the internal fault current and remain inoperative for a maximum of through fault current
- Rapid and successive automatic breaking and making to aid stable system
operation
- Three pole (3-pole) and single pole (1-pole) auto-reclosing arrangement
In addition to the making and breaking capabilities, a circuit breaker is required to do following tasks under the following typical conditions:
- Short-circuit interruption
- Interruption of small inductive currents
- Capacitor switching
- Interruption of short-line fault
- Asynchronous switching
- Interruption of small inductive currents
- Capacitor switching
- Interruption of short-line fault
- Asynchronous switching
A circuit breaker is a switching i.e. current interrupting or making device in switchgear. In more proper words, a circuit breaker is defined as a piece of equipment which can do any one of the following tasks:
- Makes or breaks a circuit either manually or by remote control under normal conditions
- Breaks a circuit automatically under fault conditions
- Makes a circuit either manually or by remote control under fault conditions
- Makes a circuit either manually or by remote control under fault conditions
Figure 1: “A Circuit Breaker with Internal Structure”
for the switching function. It is easy to percept what manual operation means for the circuit breakers. On the other hand, the automatic control of the circuit breaker is incorporated with the help of relays and is only done in case of fault conditions. The main advantage associated with the use of circuit breaker is that, unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.
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