A GDT is an overvoltage protection device used in a range of applications that require protection against dangerous and potentially costly power surges and transient voltages. In normal operating conditions the gas inside the device acts like an insulator and the GDT does not conduct current. When an overvoltage condition occurs gas inside the tube breaks down and conducts current. When the overvoltage condition exceeds the parameters of the sparkover voltage rating the GDT turns on and discharges, destroying the damaging energy.