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Do gas discharge tubes used e.g. for surge protection have a polarity, i.e., do they protect in both directions (i.e., depending which wire is hit, or if it is a positive/negative lightnings?

The symbol (see below, from Wikipedia) has a dot that suggests some asymmetry. Electrons should be able to jump in any direction, but how are the positive/negative electrodes engineered to facilitate/stand the discharges?

Enter image description here

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3 Answers 3

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Photo attribution:
By Ceinturion 17:47, 19 August 2007 (UTC) - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2587926By Ceinturion 17:47, 19 August 2007 (UTC) - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2587926
Small dot indicates "gas-filled". Likely at sub-atmosphere pressure. Its position within the schematic symbol doesn't suggest polarity.
These devices may include thoriated electrodes to reduce striking voltage.
So for this example: it is bipolar....strikes with either polarity.

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Surge arrestor bulbs for lightning protection are symetrical usually. There are ones with three electrodes but in those, the third electrode is a shared ground, not an ignition electrode as in your examples. The schematic symbol is different then. See e.g. the this TDK/Epcos datasheet (last page).

In contrary, gas bulbs used for lighting purposes are sometimes symetrical, sometimes unsymetrical. See e.g. this Osram datasheet. Such bulbs may have an additional ignition electrode, which is the third electrode in your example.

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Both symmetrical and asymmetrical GDTs exist, the former have a symmetrical symbol, such as those surge protectors from Littlefuse:

enter image description here

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  • \$\begingroup\$ Would this imply that the symbols (with a dot not in the middle) in the original question refer to asymmetric GTD? Which electrode is it that is closest to the dot? then? \$\endgroup\$
    – Martin
    Sep 2, 2019 at 17:59

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