PTC thermistor for telecom equipment Exchange and main distribution frame (MDF)

General

Advanced developments in telephony equipment in recent years have radically altered the protection requirements for both exchange and subscriber equipment. PTC thermistor protect telephone line against overcurrent, which mainly caused by the following examples:
Surges due to lighting strikes on or near to the line plant.
Short-term induction of alternating voltage from adjacent power lines or railway systems, usually when these lines or systems occur fault.
Direct contact between telephone lines and power lines.
Main parameters

Rated zero power resistance
Ambient temperature: 25±2℃,within the allowable tolerance.
Non-trip characteristic
Ambient temperature: 40±2℃in still air for 30 minutes, power voltage 60VDC, maximum non-trip current ON for 60 minutes. （R-Rn）/Rn≤50%
Overcurrent trip characteristic
Relation between trip current to trip time of PTC thermistor.
Over voltage withstanding
Power voltage: 230 Vrms /250 Vrms, maximum current ON for 30 minutes. 4 hours after restoring in room temperature, retest zero power resistance, R/Rn≤20%.
Over current withstanding
Power voltage 220Vrms, maximum current 60S ON/600S OFF, cycles: 20, 4 hours after restoring in room temperature, retest zero power resistance, R/Rn≤20%.
Surge current withstanding
Short circuit wave: 10/1000s, minimum opening circuit voltage: 1.0KV, peak current：25A, interval time: 3 minutes, cycles: 30,4 hours after restoring in room temperature, retest zero power resistance, R/Rn≤20%.
Induction voltage withstanding (Only for maximum voltage over 600V)
Power voltage: 600 Vrms /650Vrms,initial current 1.1A, 2S ON/600S OFF, cycles: 10, 4 hours after restoring in room temperature, retest zero power resistance, R/Rn≤20%.
Fail model
Ⅰ. Power voltage: 250Vrmsinitial current: 10A.
. Power voltage: 600Vrms initial current: 15A
. Power voltage: 650Vrmsinitial current: 10A
Energized time 30 minutes, high resistance state or open circuit is allowable. Low resistance or firing is not allowed.
Restore time
The time necessary for PTC thermistor returning to twice Rn. Lower than 60 seconds is required.

Operating principle
A PTC thermistor is connected in series with each telephone line, usually as secondary protection. Please refer to fig. 1, however, even with primary line protection(usually a gas discharge tube), the PTC thermistor must fulfill severe requirements. Surge pulses up to 2KV can occur and in order to withstand short term power induction, the PTC thermistor must withstand high voltages. If the line has primary protection, a 200V ~300V PTC thermistor is adequate. Without primary protection, 600V PTC device is necessary. Ampron manufactures a series of PTC thermistor fulfilling both requirements.