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Protective Devices

            All electrical circuits must be protected against over current therefore

   a protective device has to be installed in order to isolate the fault 

    from the supply so as to protect the equipment and

   appliances from being damaged .

            Over current is caused by :

1) Short circuit 

 Two or more live conductors touching each other

 
2) Overload   Adding loads greater than  that of the rated value
  3) Earth fault

A phase conductor touching the protective conductor by means of direct or indirect contact

 

             

            There are two types of protective devices .  They are

   cartridge fuse and miniature circuit breaker (MCB) .

 

           

            Cartridge fuse  

            You can find the cartridge fuse in the plugs and most 

   MCBs .

            The advantages of a cartridge fuse are :   

1) Fuse element does not deteriorate after many years
2) Small in size
3) Easy and quick to replace 
4) Needs no maintenance

            The disadvantages of a cartridge fuse are :

1) Does not suit high fault current
2) Spare cartridge fuse must be available
3) Can be shorted out by the use of metal foil

 

          

            Miniature circuit breaker (MCB)

            You can find the miniature circuit breaker (MCB) in

    consumer units (CU) .

            The advantages of a miniature circuit breaker (MCB)

    are:   

1) Shorter tripping time 
2) Can be reused
3) Easy to reset 
4) Has a switch that can isolate the equipment

            The disadvantages of a miniature circuit breaker

    (MCB) are:    

1) The most expensive protection device for home use
2) Slow tripping time due to aging 
3) Surrounding temperature may affect the MCB