You've already met the fast surge suppressors — MOVs clamp in nanoseconds, TVS diodes in picoseconds. But both share a weakness: they dissipate surge energy inside a small semiconductor or metal-oxide body. Hit them with a lightning strike carrying thousands of joules and they vaporize. That's where the Gas Discharge Tube comes in — a two- or three-electrode sealed capsule filled with a low-pressure noble gas (typically argon or neon) that acts as a switch, not a clamp.

How it works: Under normal voltage, the gas is an insulator and the GDT looks like an open circuit (leakage < 1 nA, capacitance often < 1 pF). When voltage exceeds the DC sparkover voltage — typically 75 V to 600 V depending on the part — the gas ionizes and the GDT collapses to an arc mode with a burning voltage of only 10–35 V. It's essentially a short circuit that dumps the surge current to ground.

The tradeoffs versus MOVs and TVS:

• Energy handling: A single GDT can shunt 10–20 kA of 8/20 μs surge current — orders of magnitude more than an MOV of the same size.
• Response time: Slow. GDTs take 100 ns to 1 μs to fire, versus < 1 ns for TVS diodes. Fast transients get through before the tube ionizes.
• Capacitance: Extremely low, which is why GDTs dominate on RF and antenna lines where an MOV's 100–1000 pF would detune the circuit.
• Follow current problem: Once ionized on an AC line, the GDT stays conducting at ~20 V until the current drops below its holding value. On a mains circuit, this can trip the breaker or destroy the tube. Solution: series with a fuse, PTC, or use a specially-rated AC version.

Real-world example: Every cable modem, DSL splitter, and cell tower antenna feed has GDTs on the incoming line. Cable TV F-connectors typically use a 90 V, 5 kA GDT because the RF signal needs the low capacitance and lightning induced onto a 30-meter coax run demands the surge capacity. TVS diodes alone would explode on the first strike.

Rule of thumb — hybrid protection: Serious surge protection uses a coordinated cascade. GDT at the entry point handles the bulk energy (say, 10 kA). A series inductor or resistor (a few μH or ohms) delays the wavefront. Downstream, an MOV clamps at a lower voltage (say, 275 V) for the medium-speed transients. Finally, a TVS diode nails the fast residual to safe silicon-level voltages. Each stage handles what the previous stage was too fast or too weak to catch.