GAS TUBE SURGE ARRESTORS FOR ISOLATION 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.

Isolation Protection

Signal transformers can be protected using Gas Discharge Tubes. In the communication line example below, a 3-electrode GDT is used to protect the signal transformer from a common-mode voltage surge exceeding the transformers maximum isolation rating. A common-mode surge is an equipotential event where both lines rise in voltage with respect to ground. If this voltage surge exceeds the transformers maximum isolation rating it will breach the insulation barrier damaging the transformer. The transformer is protected when an overvoltage surge exceeds the GDTs DC sparkover voltage and switching it into is on-state. In its fully on-state, the GDT forms a virtual short circuit from each line to ground shunting the surge current and limiting the line voltage to the on-state arc voltage of the GDT which is typically around 20V.

Isolation Protection using GDT

Selecting the appropriate Gas Tube

Selecting a GDT for isolation protection requires determination of two key characteristics. Peak system voltage which is the maximum voltage the GDT could be expected to see during normal system operation and the isolation rating of the transformer. In the circuit example above, if you assume a 48V telco circuit with analog ringing you could expect the GDT to see a maximum peak voltage of 275V. By selecting a 350V GDT with 20% tolerance, the lowest expected GDT DC sparkover voltage is 280V which is above the stated 275V peak system voltage. The second factor is the Isolation rating of the transformer being protected. For example, if the transformer has a 1000 Vrms isolation rating, the peak voltage the transformer could be expect to safely isolate would be 1414 Vpk dc. In this case, the selected GDT must limit surge voltages to <1414V. By selecting the 350V GDT, the maximum sparkover voltage (protection level) expected under transient conditions is <800V. As selected, the 350V GDT provides a significant amount surge protection margin to the isolation rating of the transformer being protected while not interfering with normal circuit operation.

Recommended GDT

Part Number	DC Sparkover Voltage	Max Impulse Voltage	In	Link
BTR 350/20	350 +/-20%	<800	20kA
BTR 500/20	500+/-20%	<1050	20kA

References