GAS TUBE SURGE ARRESTORS FOR VDSL OVER POTS & GR-1089

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.

VDSL equipment requiring GR-1089-CORE compliance

To assure equipment performance and operability in harsh electrical environments, GR-1089-CORE (Electromagnetic Compatibility and Electrical Safety) is used to evaluate VDSL equipment. GR-1089 Lightning and Power Fault requirements simulate equipment exposure to lightning surges, power contact and power induction. Equipment requirements in GR-1089 are typically defined by port type. Ports 1,3 and 5 typically define VDSL requirements and are specific to Inter-building requirements for Network, Customer Premise and Outside plant equipment as well as Cell sites. In the generic example below, a GDT is used as a primary overvoltage protector due to its low capacitance and high surge current carrying capabilities. An ECL (electronic current limiter) in this example provides over-current protection.

VDSL over POTS Circuit Protection Example

Selecting the appropriate Gas Tube

Key considerations for selection of a GDT for VDSL equipment include system voltage, package type, fail short capability and surge ratings. In the circuit protection example above, a single 3-electrode surface mountable gas tube is selected to provide common mode protection. When selecting a voltage rating, the low end of the GDTs DC sparkover tolerance must always be greater than peak voltage of the system being protected. GDTs with DC sparkovers of less than 800V and used as a primary protection element in GR-1089 compliant circuits must have a fail safe device that will short out the GDT during power cross and power induction testing. It should be noted that GDT are not designed to handle long duration RMS voltages/currents and can become a heat source or fire hazard. Upon activation of the GDT fail short, the two fuses in front of the GDT will operate disconnecting the equipment allowing it to safely pass GR-1089.

Recommended GDT

Part Number	Circuit	DC Sparkover Voltage	In	Link
BMSQ CMS FL (T&R) 420/20	T1-E1	420 +/-20%	10kA

References