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Lets say I want to use a Gas Discharge tube for indirect lightning protection, and I have a tvs diode in parallel with GDT. Since this is indirect lightning and has a fast waveform, should the tvs diode be slightly above the Impulse Sparkover voltage? Or can I just have tvs diode above the DC sparkover voltage?

I know that Impulse Sparkover applies for fast surge waveform, and a DC sparkover applies for a slow waveform in second.

If I were to choose a tvs clamping voltage slightly above the Impulse Sparkover voltage, wouldn't that already destroy the equipment the I am protecting due to high voltage. Impulse sparkover voltage are usually around 200V-600V.

Also which is more dangerous or destructive - equipment destroyed from high voltage or equipment destroyed from high current?

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Since this is indirect lightning and has a fast waveform, should the tvs diode be slightly above the Impulse Sparkover voltage?

If the TVS breakdown voltage was above the GDTs impulse sparkover voltage then the TVS will never operate and would be redundant in your circuit.

Or can I just have tvs diode above the DC sparkover voltage?

If the TVS breakdown voltage was below the GDTs DC sparkover voltage then the GDT would never operate. So yes, the TVS voltage must be greater than the GDTs DC sparkover voltage and less than the GDTs impulse sparkover voltage.

Also which is more dangerous or destructive - equipment destroyed from high voltage or equipment destroyed from high current?

This last part is unanswerable; equipment destroyed is still equipment destroyed hence the "destruction" factor is the same. You need to consider a specific scenario.

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