GAS TUBE SURGE ARRESTORS FOR COAXIAL LINE PROTECTION

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 . Today's electronics 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.

Coaxial Port Protection

Coaxial cables that extent 10m/30ft into an outside environment can be expected to be exposed to transients caused by lighting and power surge related events. Coaxial cables entering an equipment cabinet or enclosure must be protected while attached equipment such as Cameras or Antennas should also utilize some type of surge protection. Gas tubes due to their high surge current carrying capabilities, extremely low capacitance and high off-state impedance are ideal for high frequency applications requiring surge protection. As shown below, the GDT is typically connected between the center conductor of the coaxial cable and ground.

Coaxial Line Protection using a GDT

Selecting the appropriate Gas Tube

When selecting a GDT for coaxial applications there are two key considerations. Voltage on then center conductor and the expected exposure of the cable to surges. The general rule for selecting a gas tubes voltage is that its minimum DC Sparkover Voltage must be selected to be greater than the peak voltage rating of the line being protected. Secondly, Peak and Nominal current ratings should also be considered as GDT are available current ratings ranging from light duty to heavy duty depending on size. For a coaxial system operating a <65V a 90V +/- 20% GDT can be selected since the low end of its DC Breakdown tolerance is 72V which is greater than the 65V Vpk rating of the coaxial system.

Recommended GDT

Part Number	Peak System Voltage	Exposure Level	In	Link
BA CMS 90/20	<65V	Light	5kA
BB 90/20	<65V	Medium	10kA
BH 90/20	<65V	Heavy	20kA

References