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Current rating of fuses and MCB's.

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Introduction

CURRENT RATING OF FUSES AND MCBS

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    

CALCULATIONS:

For the fuse  image00.png

For MCB image01.png

From graphs drawn,

Fusing current for 5A fuse        =        

image02.png

Tripping current for 2A MCB        =        

image03.png

RESULTS

Fusing Factor

Tripping Factor

DISCUSSION:

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

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Middle

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 when replacing.

High Rupture Capacity (HRC) fuses

These are used for high current protection. These have a ceramic outer barrel. The fuse wire is covered with sand in order to quench the arc produced when the fuse burns. The ceramic barrel can withstand the shock of the interruption of the high current.

Usually semi-enclosed rewireable fuses have about 1.7-2 times the rated current carrying capacity. In this practical the fusing factor was       . This value is rather high. However the value might not be the exact fusing factor of the fuse used due to the errors that are present in the practical.

MCBS

Circuit symbol:

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Conclusion

  • Rewireable fuses tend to weaken with time due to oxidization. So the fuse will not be able to withstand the rated current.

Other Protection Techniques

  1. Large Current Protection - Here simply breaking the circuit is not enough to stop the flow of current. The current in some circuits is strong enough to jump, or arc, across the gap in the circuit, even after the circuit breaker has been tripped. Circuit breakers that deal with high levels of current, especially direct current, have methods of getting rid of the energy in the arc and stopping the current.
  • In oil breakers, the design of the circuit breaker forces the arc of broken circuit through a sealed container of oil or gas. The arc heats the oil around it. The hot oil begins to circulate in the tank, carrying heat and energy away from the arc.
  • Air-blast circuit breakers send the arc through compressed air, which is immediately released to the outside, carrying the heat and energy of the arc with it.
  1. Fuses used in vehicles and motors - Some of these fuses are Ribbon type, Knife blade cartridge type, Plug type with ribbon tube, Copper fuse link, Fibre tube, Cartridge type with glass tube and wire fuse link.
  1. Sensitive Equipment - Fuses used in 13A plug tops as well as in some sensitive equipment for additional protection (voltage stabilizers, computer power supplies) – These are mostly cartridge type fuses.

REFERENCES

Electrical Engineering Handout

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