(i) Fuses.
(ii) Circuit Breaker (i) Fuses. It. is a device consisting of a melt-able metal link inside a casing. This metal link is also referred to as the fuse’s element. Fuses are installed in such a away in circuit that they disconnect the circuit when faulty or overload current passing through it. This faulty current produced excessive heating and as a ‘result due to this heat the insulation of conductors is damaged. There f ore we can say that “Fuse is a device, which opened the circuit or fusing the element when the current in the circuit exceeds to its rated ‘certain value”.
A fuse contains all the parts which form the complete device. Such as holder, fusing element, contacts and contact base. The current rating f a fuse is that current which it will carry without maximum overheating.
The term fusing factor is used here which is as follows. ‘The minimum fusing current at which the fusing element or fuse link will melt divided by the fuse current rating is known as the fusing factor”.
For example a 20A fuse will blow at 36A. The fusing factor of this fuse is 36/20 (1.8). The types of fuse arc discussed in section (3.2).
The fuse wire which is blown and new wire which is used to the place of blown wire always have the same rating. We always use the alloy-copper wire for fuse element because its melting point is low compared to copper. Because the melting the melting point of alloy-copper wire is 180°c” and copper has 1100°c. Tinned copper wire are also used for fuse link or element.
C.B and fuse are provided good protection to electrical systems against high current levels. C.B and fuses are rated in amperes. The most common ratings are 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 10, 125, 150, 1, and 200A corresponding. (ii) Circuit Breaker Circuit breaker is an automatic switch which can interrupt the fault current. A circuit breaker is a device which can make and break the circuit under normal and abnormal conditions. A modem C.B is designed and rated to be capable of being safely closed in any current with in its momentary rating. It can safely open the circuit when the faulty current flows through it to excess of its normal rating. Circuit breakers are available that can be actuated thermally, magnetically or through electronic sensing means. Most C.B’s is the manual reset type. In manual reset type, if au over current flow through C.B’s and as a result C.B’s to trip, its switch contacts open and remain open. The contacts can be re-closed only by pushing the reset button. Some C.B’s are automatic reset type. This type of C.B’s is remain open for a certain fixed time after tripping and then automatically re-closes its contacts. The C.B’s used in domestic wiring are the thermo magnetic type. Some C.Bs contain both, a magnetic actuating and thermal actuating mechanism. On momentary over currents that last only a few second, when we starting a electric motor for instance, the magnetic force is not strong enough by itself to trip the C.B’s and the thermal device also cannot heat up immediately, therefore it can not provide any force to magnetic device. However if over current passes through C.B’s for a considerable length of time, then thermal device will heat up and begin exerting additional force to help the force created by the magnetic device, as a result these two forces trip the C.B’s. C.B’s are commonly used for ground fault sensing, alarm switch, short-trip mechanisms etc.
C.B’s cabinets are usually sold without the breaks. The cabinet contains a bus bar type of arrangement into which you plug C.B’s. The cabinet also contains the central bar with lugs for the incoming neutral and the ground wire.
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