Impedance protection is a method for protecting fan motors from burning when a motor operating at the rated voltage locks up with the power still being supplied. It raises the impedance (AC resistance) of the motor windings to suppress increases in current and keep the motor temperature from rising above a preset level. Unlike thermal protection, this type of protection is a passive system. UL standards, which stipulate no burning and a winding temperature of 150°C maximum after the motor is operated in a locked condition for fifteen continuous days, are used as safety standards. Impedance protection is generally used on fans that have a diameter of 120 mm or smaller.