What is an electric FUSE , its function , What is PPTC

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Fuse is used in electric circuits as safety device . It protects circuits and appliances by stopping the flow of any unduly high electric current .  The fuse is placed in series with the device . it consists of a piece of wire made of a metal oran alloy of appropriate melting point , for example aluminium copper iron lead etc . If a current larger than the  specified value flow through the circuit , the temperature of the fuse wire increases. this melts the fuse wire and breaks the circuit. The fuse wire is usually encased in a heritage of porcelain or similar material with metal ends . 
It always connecting in series and made up of metal wire having low melting point .
Resistance of a wire depends on lenght of wire , it's crossectional area ,and temperature .
Resistance resistance is directly proportional to length of wire and inversely proportional to area of cross section it also so that is proportion to temperature .
Therefore if metal wire of fuse is so thick then its resistance flow and more current flow through it . And sustain more current . If in a circuit where needed to flow high current, its fuse wire is used as as a thick wire having high Ampere and similarly for  low current the fuse wire is used as thin metal wire therefore resistance is high and sustain low current and its rating in low Ampere value .

Rating 1A ,2A ,3A...etc

          image of a cylindrical fuse

It is an application of heating effect current . 

PPTC is also work as safety device of circuit . Its full form is positive polymeric temperature coefficient . Its rating is also like fuses , But it  restore itself .
It ialso 2A ,4 A etc.



It is a resettable fuse is used to protect against overcurrent faults in electronic circuit . It is also known as multi fuse aur poly switch aur polyfuse . They are similar in function to PTC thermistor in certain situation but operate on machanical changes . Instead of charge carrier effects in semiconductors. 
These devices were first discover by Gerald Pearson 1939 .

All these things are based on JOULE'S lAW Of HEATING EFFECT OF CURRENT .
According to heating effect of current H=I^2 RT , where I is current ,R is Resistance and t is time  

Nowdays , glass fuses are used in railway signalling circuit . Glass fuse means fuse wire enclosed in cylindrical glass case . 

 
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