Switchgear covers a wide range of equipment concerned with switching and interrupting currents under both normal and abnormal conditions. It includes switches, fuses, circuit breakers, relays and other equipment. A brief account of these devices is given below. However, the reader may find a detailed discussion of them in the subsequent chapters.
A switch is a device that is used to open or close an electrical circuit in a convenient way. It can be used under full-load or no-load conditions but it cannot interrupt the fault currents. When the contacts of a switch are opened, an arc is produced in the air between the contacts. This is particularly true for circuits of high voltage and large current capacity. The switches may be classified into (i) air switches and (ii) oil switches. The contacts of the former are opened in air and that of the latter is opened in oil.
(I) AIR-BREAK SWITCH
It is an air switch and is designed to open a circuit under load. In order to quench the arc that occurs on opening such a switch, special arcing horns are provided. Arcing horns are pieces of metals between which an arc is formed during the opening operation. As the switch opens, these horns are spread farther and farther apart. Consequently, the arc is lengthened, cooled, and interrupted. Air-break switches are generally used outdoors for circuits of medium capacity such as lines supplying an industrial load from a main transmission line or feeder.
(II) ISOLATOR OR DISCONNECTING SWITCH
It is essentially a knife switch and is designed to open a circuit under no load. Its main purpose is to isolate one portion of the circuit from the other and is not intended to be opened while the current is flowing in the line. Such switches are generally used on both sides of circuit breakers in order that repairs and replacement of circuit breakers can be made without any danger. They should never be opened until the circuit breaker in the same circuit has been opened and should always be closed before the circuit breaker is closed.
(III) OIL SWITCHES
As the name implies, the contacts of such switches are opened under oil, usually transformer oil. The effect of oil is to cool and quench the arc that tends to form when the circuit is opened. These switches are used for circuits of high voltage and large current carrying capacities.
A fuse is a short piece of wire or thin strip which melts when excessive current flows through it for sufficient time. 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 load current without overheating. However, when a short circuit or overload occurs, the current through the fuse element increases beyond its rated capacity. This raises the temperature and the 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. It is worthwhile to note that a fuse performs both detection and interruption functions.
3. CIRCUIT BREAKERS
A circuit breaker is an equipment that can open or close a circuit under all conditions viz. no load, full load, and fault conditions. It is so designed that it can be operated manually (or by remote control) under normal conditions and automatically under fault conditions.
For the latter operation, a relay circuit is used with a circuit breaker. Figure 2 (i) shows the parts of a typical oil circuit breaker whereas Figure 2 (ii) shows its control by a relay circuit. The circuit breaker essentially consists of moving and fixed contacts enclosed in a strong metal tank and immersed in oil, known as transformer oil.
Under normal operating conditions, the contacts remain closed and the circuit breaker carries the full-load current continuously. In this condition, the EMF in the secondary winding of the current transformer (CT) is insufficient to operate the trip coil of the breaker but the contacts can be opened (and hence the circuit can be opened) by manual or remote control. When a fault occurs, the resulting overcurrent in the CT primary winding increases the secondary EMF This energizes the trip coil of the breaker, and moving contacts are pulled down, thus opening the contacts and hence the circuit.
The arc produced during the opening operation is quenched by the oil. It is interesting to note that relay performs the function of detecting a fault whereas the circuit breaker does the actual circuit interruption.
|Figure2: (i) Parts of a typical oil circuit breaker (ii) oil circuit breaker control by a relay circuit|
A relay is a device which detects the fault and supplies information to the breaker for circuit interruption. Figure2 (ii) shows a typical relay circuit. It can be divided into three parts viz.
(i) The primary winding of a current transformer (CT) which is connected in series with the circuit to be protected. The primary winding often consists of the main conductor itself.
(ii) The second circuit is the secondary winding of CT connected to the relay operating coil.
(iii) The third circuit is the tripping circuit which consists of a source of supply, trip coil of circuit breaker and the relay stationary contacts.
Under normal load conditions, the EMF of the secondary winding of CT is small and the current flowing in the relay operating coil is insufficient to close the relay contacts. This keeps the trip coil of the circuit breaker unenergized. Consequently, the contacts of the circuit breaker remain closed and it carries the normal load current. When a fault occurs, a large current flows through the primary of CT. This increases the secondary EMF and hence the current through the relay operating coil. The relay contacts are closed and the trip coil of the circuit breaker is energized to open the contacts of the circuit breaker.
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