NTC (Negative Temperature Coefficient) thermistor devices having base electrical resistances in the range of about 400 to 2200 ohms can be shunted each light. A string set of series-connected, low voltage incandescent lamps can be configured in which each of the lamp filaments in the set is provided with a shunt circuit that includes one of the temperature sensitive NTC thermistor devices which operates as a filament shunt. The thermistor devices may have relatively higher resistance than string lamps during normal operation of the string set. The devices, however, may become fully conductive in response to an increase in the voltage that exceeds the device's normal voltage drop. Remaining lamps in the string can continue to receive nearly rated current and nearly rated voltage and further continue to be illuminated at nearly the same illumination even though other lamps in the string are inoperative due to burning out, being loose in their sockets or being missing from their respective sockets all together. It is desirable for the NTC thermistor devices to use at most, a base resistance that would allow initial operation at or near the point of maximum power dissipation by the device upon failure of its shunted lamp. It is also desirable that the base resistances be high enough to limit "leakage" current so there is no more than a 5% increase in string current compared to the same string without shunts. With shunted circuit thermistor, when one lamp doesn’t work, others won’t become more incandescent and their lives won’t be influenced.
Our MF58 of 500ohm series, with the characteristics of good stability, small size, light weight, strong frame, and capability of operating in bad environment, have been widely applied in such string lights(Christmas lights,party lights and etc.)