The fundamental reason for choosing gas discharge tubes (GDTs) as the primary surge protection component in signal lines is the need to prioritize the integrity of signal transmission while simultaneously providing effective surge protection. This choice is based on the superior characteristics of GDTs in terms of parasitic capacitance, balance maintenance, and tolerance to high currents, which are precisely the inherent shortcomings of MOVs (metal oxide varistors).

Firstly, the most critical limiting factor is parasitic capacitance. All semiconductor-based MOVs have junction capacitance ranging from tens to thousands of picofarads, which is connected in parallel with the high-speed signal line. For high-frequency signals (such as 100/1000 Gigabit Ethernet, video signals, and RF circuits), such a large capacitance creates a low-impedance path, severely attenuating the high-frequency signal and leading to signal waveform distortion, data loss, communication interruptions, or a drastic reduction in transmission distance. In contrast, the inter-electrode capacitance of GDTs is typically only between 1 and 3 picofarads, making them almost "invisible" to signals of most frequencies. The introduced insertion loss and return loss are negligible, thus perfectly preserving signal integrity.

Secondly, GDTs (Gas Discharge Tubes) exhibit excellent signal balance maintenance capabilities when discharging common-mode surges. Signal transmission, especially differential signals (such as RS485 and Ethernet), relies on precise voltage balance between wire pairs. When a common-mode surge (i.e., overvoltage simultaneously appearing between all signal lines and ground) occurs, an ideal GDT will simultaneously break down between all wire pairs and ground, forming symmetrical discharge paths. Due to the extremely high consistency of GDT components, this symmetry helps prevent the conversion of common-mode interference into differential-mode interference, thus protecting the differential voltage of the signal from damage. In contrast, the inherent parameter variations in MOV (Metal Oxide Varistor) manufacturing may lead to slight differences in the operating voltage and dynamic resistance of MOVs on different lines. This imbalance is amplified when discharging large currents, easily converting common-mode noise into harmful differential-mode noise, directly interfering with the signal itself.

Furthermore, from a purely protection performance perspective, GDTs have a natural advantage in current handling capacity. Their discharge principle, based on gas ionization, allows them to withstand very large transient currents (such as the 10/350μs direct lightning strike waveform), while maintaining a relatively compact size. This makes GDTs ideal for use as the first stage (coarse protection) in signal lines, effectively diverting most of the lightning current energy induced by outdoor cables.

However, it must be noted that GDTs also have drawbacks, including relatively slow response speed and the presence of an arc voltage after breakdown. Therefore, in the highest-level signal protection designs, a composite circuit of "GDT + decoupling/voltage limiting element + fine protection element (such as a TVS diode)" is typically used. In this architecture, the GDT acts as the "initial protector," responsible for withstanding and dissipating most of the surge energy; the subsequent TVS diode, with its extremely fast response speed and precise clamping voltage, further clamps the "blind spot voltage" before the GDT conducts and the arc voltage after conduction to a level that is absolutely safe for the equipment. This cascaded solution fully utilizes the advantages of both GDTs and TVS diodes, providing top-level protection while minimizing the impact on signal quality.