Gas Discharge Tube (GDT)

GDT is a sealed glass-enclosed device containing a special gas mixture trapped between two electrodes, which conduct electric current after becoming ionized by a high voltage spike. It is often considered a slow operating overvoltage protector, but these devices are more intended to handle impulse currents many times higher than faster technologies such as solid states (semiconductor) devices. GDTs can conduct more current for their size than other components. It has a finite life expectancy, and can handle a few very large transients or a greater number of smaller transients.

Working

When a voltage applied across GDT is below breakdown voltage, it remains in high impedance off state condition (i.e. poor conductor). But when the applied voltage exceeds the breakdown voltage, the electric voltage is strong enough to ionize the gas making it low impedance on state (i.e. good conductor) allowing current passing through it to the ground line until voltage reaches normal.  

Use

GDTs are commonly used to help protect sensitive telecom equipment such as power lines, communication lines, signal lines and data transmission lines from damage caused by transient

surge voltages that typically result from lightning strikes and equipment switching operations.

Some terminology related to GDT

Arc Mode: The mode when the GDT is in the low impedance conducting state.

Arc Voltage: The voltage drop across GDT during its low impedance conducting state.

Arc Current: The current that flows through the GDT after breakdown that exceeds the glow to arc transition current during its low impedance conducting state i.e. arc mode.

DC Breakdown Voltage or Sparkover voltage: The voltage at which GDT sparks over or it starts conducting. The DC breakdown voltage is highly dependent upon the time rate of change of applied voltage.

Glow Mode:  The transition state of an energized gas between the high impedance state and the arc mode, in this mode the gas is partially ionized.

Glow Voltage: The voltage across the GDT in the glow state.

Impulse Breakdown Voltage: It represents the device reaction to fast rising voltage transients.

Impulse reset: The maximum DC voltage across the GDT at which the gap clears a given voltage/current condition.