Current rating of fuses and MCB's.

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  Name of Instructor: Miss. B.S.K. Subasinghe                        

                                                                Name                :         Perera K.N.S.

                                  Index No        :                  020287        

                                Field                :                 ENTC

                                  Date of Per.        :            10/12/2003

                                  Date of Sub.:           19/12/2003    


For the fuse  


From graphs drawn,

Fusing current for 5A fuse        =        

Tripping current for 2A MCB        =        



Fuses and Miniature Circuit Breakers (MCBs) are used to protect electrical equipment and for the safety of the users. The conditions that require circuit protection are direct shorts, excessive current and excessive heat. When one or more of these conditions occur the above protective devices (connected in series with the supply) are able to disconnect the circuit thus preventing the danger.


Circuit symbol:

This provides protection by melting due to the heat produced because of ohmic resistance, when a higher than a rated current flows through it. There are three main types of fuses,

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Re-wirable fuses

These have a thin wire that is held on a ceramic holder. The wire is designed to burn when a high current passes. An advantage is the reusability but it is vulnerable to mishandling where incorrect fusing elements can be fitted by inexperienced people, putting users in danger. Other Problems with this type are the oxidization (causes the cross section of the wire to reduce reducing the current carrying capacity) and longer fusing time.

Cartridge fuses

Fuse wire is enclosed in an inert environment. Once burnt, the fuse has to be replaced i.e. a repetitive cost ...

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