Re-settable Fuse is a passive electronic component used to protect against over-current faults in electronic circuits. They are actually non-linear thermistors and cycle back to a conductive state after the current is removed, acting more like circuit breakers, allowing the circuit to function again without opening the chassis or replacing anything. Every re-settable fuse has a current rating.
In normal functioning, it offers very small resistance to allow the current to flow through it. In case of over-current, it becomes hot and its resistance increases to several orders of magnitude thereby greatly reducing the current. These are generally used in conjunction with TVS, which redirects the current to fuse (in case of surge) and fuse provides second level of protection by either breaking down or providing strong resistance.
Re-settable fuses are used in power supply unit (at input), DI card (at field input) and AI card (at field input).