Superconducting fault-current limiters use the transition of superconductors from zero to finite resistance to limit the fault currents that result from short circuits in electric power systems. Such short circuits can be caused by aged or accidentally damaged insulation, by lightning striking an overhead line, or by other unforeseen faults. If not deliberately checked, the subsequent fault current is limited only by the impedance of the system between the location of the fault and the power sources. This current can reach as much as 100 times the nominal current of the system (the largest current in normal operation), and leads to high mechanical and thermal stresses, both of which are proportional to the square of the current's value.