Gas Discharge Tube (GDT)

A gas discharge tube is a surge-protection component made of two or three metal electrodes with a small spark gap, sealed inside a ceramic or glass tube filled with an inert gas at low pressure. Normally it is an open circuit with a very high impedance, but when the voltage across it exceeds the sparkover voltage the gas ionises and the tube conducts a large surge current at a low arc voltage, diverting the surge safely to ground.

When the voltage across a GDT rises above its sparkover voltage, the inert gas between the electrodes ionises. The tube passes quickly through a glow discharge, around 100 volts, and then strikes a low-impedance arc at only about 15 to 30 volts. In the arc state the GDT behaves almost like a closed switch, so it crowbars the surge current to ground while holding only a low voltage across the protected line. When the surge passes and the current falls below the holding current, the arc extinguishes and the tube returns to its high-impedance state.

The sparkover voltage (or breakdown voltage) is the voltage at which the gas in the tube ionises and the GDT starts to conduct. There is a DC sparkover voltage measured with a slowly rising voltage and a higher impulse sparkover voltage measured with a fast surge, because a fast-rising surge reaches a higher voltage before the gas has time to ionise. Common GDTs have DC sparkover ratings from around 75 volts to several kilovolts.

Follow current is the power-frequency current that can keep flowing through the GDT after the surge has passed, because the arc is still conducting and the normal system voltage sustains it. Holdover voltage is the maximum voltage at which the tube will reliably extinguish and stop conducting. On AC mains a GDT is chosen or coordinated so that follow current cannot latch it on; this is one reason GDTs are often combined with other devices.

Once the gas ionises, more current makes the arc more conductive, so the voltage across the tube actually falls as the current rises. This falling voltage with rising current is a negative-resistance region, and it makes the GDT a crowbar device: instead of clamping at a set voltage like a varistor, it collapses to a very low arc voltage and short-circuits the surge to ground, which lets it handle very large surge currents with little heating.

A two-electrode GDT has a single spark gap and protects one line to ground. A three-electrode GDT has two gaps sharing one gas chamber, with line and neutral on the outer electrodes and a common ground in the centre. Because they share a chamber, when one gap sparks over the ionised gas helps the other fire almost immediately, giving balanced protection across line, neutral and ground, which is important for telecom and data pairs.

A GDT is a crowbar device that switches to a very low arc voltage and handles very high surge currents but responds relatively slowly and can carry follow current. A metal-oxide varistor (MOV) clamps the voltage at a set level, is faster than a GDT and handles medium energy, but degrades with repeated surges. A TVS diode clamps very fast and precisely with low let-through voltage but handles the least energy. They are complementary, so many surge protectors use them together.

In a coordinated or hybrid surge protective device the GDT takes the high-energy hit and diverts most of the surge current to ground, a decoupling inductor or resistor separates the stages, and then a faster MOV or TVS clamps the residual voltage that gets through before and during the GDT firing. This combines the GDT's high current capability with the fast, precise clamping of the MOV and TVS, giving both high energy handling and a low let-through voltage.

Advantages of a GDT are very high surge current capability, very low leakage and very low capacitance, high insulation resistance and a compact size. Disadvantages are a relatively slow response time, a sparkover voltage that overshoots on fast surges, the possibility of follow current on AC power lines, and a finite life after many high-energy strikes. These trade-offs are why GDTs are often used as the first stage alongside faster clamping devices.

A GDT has a very low capacitance, typically around 1 picofarad or less, because its electrodes are small and separated by gas. This very low capacitance is a major advantage on high-speed data and RF lines, where a higher-capacitance protector such as a large MOV or TVS would distort or attenuate the signal. It is a key reason GDTs are chosen for coaxial, antenna and high-frequency data protection.

Gas discharge tubes are used wherever high-energy surges from lightning or switching must be diverted: telephone and telecom line protection, xDSL and Ethernet data lines, coaxial and antenna feeds, CATV, base stations, AC mains surge protective devices, power supplies, measurement and instrumentation inputs, and railway and industrial equipment. They are usually the primary, high-energy stage of the protection.

A GDT is symmetrical and not polarity sensitive, so it works the same for either polarity and is suitable for both AC and DC circuits. On AC and DC power lines, however, follow current must be considered, because the sustained system voltage can keep the arc conducting after the surge; on such lines the GDT is chosen or coordinated with series impedance and other devices so that it clears reliably.