Why should an earth ground be necessary? A gas discharge tube is merely two electrodes with specific spacing in a special gas mixture. When the potential difference between the two electrodes reaches a certain level the gas in the tube ionizes effectively creating a short. Before zener diodes were used in some high voltage regulators gas discharge tubes were used. Tubes or valves if you prefer like the VR150 for example would ionize at 150 VDC and glow. Another simple example of a gas discharge tube is the everyday common NE2 Neon Lamp. So a GDT really has nothing to do with earth ground unless of course you want to shunt a lightening strike to earth ground in which case I do not want to be anywhere around it. Anyway, when the potential difference between the two electrodes reaches a predetermined point the gas ionizes and the tube effectively becomes a dead short.I am testing a protection circuit that has only a gas discharge tube (GDT) it has no ground/earth connected and the circuit works, can you explain me how the gas discharge tube (GDT) protects without a ground/earth?