Ice Radio Products

The Infamous Gas Tube

A gas tube is a small electronic device, typically made of glass or ceramic, filled with inert gases. Its breakdown voltage is determined by the gas mixture inside. When voltage surpasses the set threshold, the gases ignite, creating a near short-circuit until the voltage drops.

Initially, gas tubes were designed to protect telephone circuits from overvoltage damage. This application works well because telephone lines are thin and rarely experience prolonged high currents. Gas tubes are also used for circuit board and industrial control protection where fast response is required but surge duration and current are minimal.

The Misapplication in Telecommunications

Despite their limitations, gas tubes are now marketed for lightning protection in telecommunications. Some manufacturers claim these tiny devices can handle enormous lightning currents—a misconception. While gas tubes can withstand 50,000 amps, they can only do so for a few billionths of a second. Unfortunately, lightning surges last far longer.

Why Gas Tubes Fall Short:

  1. Designed for Low-Capacity Circuits

    • Gas tubes work well with small wires that limit current. When used with thicker transmission lines, lightning surges easily overpower them.

  2. Degradation Over Time

    • Each ignition compromises the gas mixture, making it difficult to assess the tube's remaining lifespan and effectiveness.

  3. Prolonged Surge Duration

    • As lightning travels down coaxial lines, it slows significantly (due to the line’s RL time constant), forcing the gas tube to stay in conduction much longer than designed. This often leads to burnout or rupture.

  4. High Voltage Thresholds

    • To avoid accidental activation during normal transmission, high-voltage gas tubes are required. This reduces protection because larger surges are needed to trigger the tube, leaving equipment vulnerable to smaller but damaging strikes.

Key Takeaway

Gas tubes are excellent for low-power applications, but not for handling direct lightning strikes in telecommunications. Use them where they belong, and don't rely on them as the ultimate lightning defense. Sensible grounding and multi-stage arrestors offer far better protection for sensitive radio equipment.