GAS TUBE SURGE ARRESTORS FOR TWISTED PAIR COMMUNICATION LINES

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OVERVIEW

The Insurance Institute for Business & Home Safety found that an estimated $26B dollars annually is lost due to non-lightning power surges. Additionally, the are an estimated 25 million lighting strikes in the US each year that cause between $650M to $1B in losses according to the Insurance Institute, State Farm . Equipment and critical communications infrastructure must be protected from threats due to lighting, power surges and power induction.

How Gas Tubes Operate

Gas Discharge Tube (GDT) Surge Arrestors operate on the principle of arc discharge. Operating as a voltage-dependent switch, an arc is formed within nano-seconds inside the hermetically sealed discharge chamber once a voltage exceeds the GDTs spark-over voltage. During its on-state, the gas tube essentially forms a short circuit allowing the entire surge current to flow and instantaneously eliminating the overvoltage transient. Upon dissipation of the overvoltage event, the GDT device extinguishes and the internal resistance returns to its high impedance off-state. GDT devices reliably limit over voltages to permissible levels, can handle large surge currents and are invisible to the system being protected due to low capacitance and very high insulation resistance.

Communication Line Protection

Delivery of voice, data and video services in many cases is still provided via the Public Switched Telephone Network (PSTN) through a hybrid Fiber/Copper network. Copper communication lines in the last mile must be protected against lightning, power cross and induced voltages. Gas Tube surge protectors have been used in 5-pin and station type protectors for many years. Gas tubes have very high surge current ratings, low capacitance which does not interfere with high speed vDSL services, and provide a long service life. GDT devices are typically installed where exposed copper cables enter a Network Cabinet or Network interface.

Gas Tube Protecting Tip and Ring

Selecting the appropriate Gas Tube

In the example above, a 3-electrode Gas Discharge Tube is selected to provide common and differential mode protection. Key considerations for selecting a GDT for PSTN lines are the system voltage (48V) plus any imposed ringing. Battery and ringing voltages on twisted pair PSTN is typically a maximum of 275 Vpk. This peak voltage requires selection of 350V GDT with a 20% tolerance. The lowest sparkover voltage of the 350/20 GDT is 280V which is above the peak battery and ringing voltage for the PSTN. A heavy duty 20kA rated GDT with failshort is also selected as requirements for primary protection require high surge current performance and safe failure modes during overcurrent safety tests specifed in UL497, GR-974-CORE and GR-1089-CORE.

Recommended GDT

Part Number	Circuit	DC Sparkover	In	Link
BTR C 350/20	PSTN POTS+Ringing	350V +/-20%	20kA

References