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What is the basic working principle for this, Is it so that when current goes beyond the rated value its resistance increases , but whether this process is instantaneous or slow, and where it has got its best use.
yes,its resistance increase.As curnt thru the PTC (resettable fuse) reaches its rated threshold, the polymer material begins to heat causing the polymer to expand,expansion breaks the carbon chains to reduce the current through the circuit.when the overcurrent is removed,it will reset by itself.. The process is comparitively slow wrt other fuses.
its used mainly at places where replacing fuses each time an overcurrent condition occurs is unacceptable.computers,industrial controls,home security systems. etc...