RAIL WITH GAS DISCHARGE TUBE

In an overvoltage ‘fault’ condition, the GDT (Gas Discharge Tube) presents itself as a virtual short circuit. This effectively diverts the resultant surge current to the referenced ground plane.

The voltage time curve illustrates the key operating areas of the GDT, with the prospective current that will flow through the device largely dependant on the source impedance. With the voltage increasing across the GDT terminals, at a critical point it will begin to conduct. It has entered the Glow Region. Here the gas within the device begins to rapidly ionise, lowering the internal impedance. Surge ‘fault’ current will now flow, limiting the voltage imposed on downstream equipment.

Typically, the voltage now across the device, the Arc Voltage, can be a few 10’s of volts depending on the rating. When the energy within the fault condition falls to a level that cannot maintain the Arc condition, the LVL will recover to its original ‘no fault’ condition. That is a high impedance, non-conducting state.