GDT Telephone protection circuit without ground

Thread Starter

tzetze

Joined Oct 18, 2017
1
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?
 

Externet

Joined Nov 29, 2005
1,410
A single spark gap GDT can protect from electric spikes only for potentials between its two terminals. For a POT system there is no other electrode at user premises unless the central office equipment has other implementation on the lines, highly likely.

Two GDT bonded to ground, one at each balanced copper wire should be the correct way.
For newer telephone technologies, perhaps there is unbalanced circuitry where a single spark gap can function.

----> http://www.epanorama.net/documents/surge/telesurge.html
 
Last edited:

Reloadron

Joined Jan 15, 2015
4,508
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?
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.

Ron
 
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