IEC 60389-1: Thermistors for Telecom Equipment — Standardized Overcurrent Protection for Telephone Lines

Why Telephone Equipment Survives Lightning Strikes — PTC Thermistor Protection

IEC 60389-1:1972 specifies telecom equipment thermistor requirements. Telephone lines (48VDC feed) may suffer lightning-induced overvoltage and power-line cross (>220VAC). A PTC thermistor in series with the SLIC has very low resistance (<10Ω) at normal loop current (<100mA); when fault current exceeds 500mA, the PTC self-heats→resistance jumps to >10kΩ — limiting fault current to <1mA and protecting the SLIC and terminal equipment.

TNLab — The PTC is the telecom line “self-resetting fuse” — after the fault clears, it auto-cools and recovers without replacement.