an overcurrent protection system giving rapid response to relatively small overcurrents has a sensor element and an interruption element in series between a power source and a load . the sensor element is functionally linked to the circuit interruption element via a control element . when current exceeds a predetermined amount , the sensor element transfers heat to the control element which causes the circuit interruption element to change from a conducting state to a non - conducting fault state . the control element may include a ptc device .