What is a GDT? In essence, it’s a sealed version of the automotive spark plug, as a spark-gap device in a sealed, gas-filled glass or ceramic enclosure, which offers near-infinite resistance across its two terminals until some threshold breakdown or ignition voltage is reached. Once that voltage is exceeded, the gas conducts (explained by Townsend discharge ) and the GDT becomes a low-resistance path.
The gas in the GDT can be hydrogen, a noble gas such as helium, a metallic gas such as mercury vapor, among others, and the conducting voltage and timing is a function of the gas type, its pressure, and the distance between electrodes; early devices from the mid-1800s used air, which obviously has variability in humidity and thus performance.
In this day of semiconductor solutions to almost everything, it's reasonable to assume that GDTs are past their prime, no new ones are being announced, and the only ones in production are for already designed-in units, until they are declared obsolete. You also might think their form factor is large, clunky, and almost "retro", evoking vacuum tubes in their glory days.
But those are wrong assumptions, and GDTs are alive, well, and irreplaceable for some levels and classes of protection – and new ones being announced. For example, Littelfuse just released two related series of miniature GDTs with ultra-low capacitance (<0.3 pF), suitable for high-speed data links and connections, such as broadband, satellite and CATV, Ethernet (up to 10Gbps), ADSL equipment, and set-top boxes. The smaller of these surface-count devices has a diameter of just 3.5 mm; one series is rated at 3 kA (8/20 µs transient rise/fall) surge withstand capability (10 shots) while the other is rated at 1 kA with the same conditions. (Among the many other vendors of GDTs are Bourns, TE Connectivity , Epcos/TDK, NIC Components , and Phoenix Contact ).
GDT applications span a wide range, such as fluorescent bulbs