GAS TUBE SURGE ARRESTORS FOR xDSL CPE

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.

xDSL CPE Protection-FCC Part 68

Part 68 of the FCC rules (47 C.F.R. Part 68) governs the direct connection of Terminal Equipment/CPE (TE) to the Public Switched Telephone Network (PSTN). Also covered by Part 68 are TE that are connected to wireline facilities owned by wireline telecommunications providers and used to provide private line services. Under Part 68, wireline telecommunications carriers must allow all TE to be directly connected to their networks, provided that the TE meets certain technical criteria for preventing four prescribed defined harms. These harms are electrical hazards to operating company personnel, damage to network equipment, malfunction of billing equipment, and degradation of service to customers other than the user of the TE and that person's calling and called parties. Per the harms claus in FCC Part 68, all xDSL CPE TE must implement protection to migitgate electrical hazards caused by lighting and power cross. As CPE are connected to the network beyond the Network Interface and primary protector, a line to line (metallic) protection scheme is typically utilized. (see below) Gas tubes are a good protection choice due to their low capacitance which provide robust protection to high speed vDSL and G.Fast CPE without signal degradation.

Gas Tube Protecting Tip and Ring

Selecting the appropriate Gas Tube

Key considerations for selection of a GDT for xDSL CPE equipment include location, system voltages, package types and surge ratings. xDSL CPE exposed to POTs and Ringing require a GDT that has a minimum DC Sparkover voltage >275V. If not exposed to ringing or if isolated by a POTs Splitter, the CPE equipment can safely use a 90V Gas tube. As xDSL are located behind the Network Interface and primary protector, a 5kA rated 5mm surface mount GDT is typically chosen. This provides enough surge current handling to absorb any let-through from the primary protector while its compact surface mount design offers ease of assembly.

Recommended GDT

Part Number	Circuit	DC Sparkover Voltage	In	Link
BASQ CMS 350/20	PSTN POTS+Ringing	350 +/-20%	5kA
BASQ CMS 90/20	48V-Non-Ringing	90 +/-20%	5kA

References