# Symmetric gas discharge tube

Canonical URL: https://eloragrid.ai/symbols/iec-symmetric-gas-discharge-tube
Publisher: Elora Grid
Updated: 2026-10-28
Lookup label: F
Drawing use: Schematic
Reference families: IEC, Manufacturer

## Symbol convention

Convention: IEC
Form: Protective Gas Discharge Tube, Symmetric
Autodesk IEC-60617 Symbol Preview: Miscellaneous; vendor family reference, not certified IEC geometry.
Related representation: [Gas-discharge protection tube](https://eloragrid.ai/symbols/gas-discharge-protection-tube)

A protective gas discharge tube drawn with identical electrodes either side, listed as a distinct symmetric form.

## In plain English

### What does Symmetric gas discharge tube mean?

The publisher's IEC-oriented library lists a symmetric protective gas discharge tube separately from the standard form. The symmetric drawing presents both electrodes identically, emphasising that the device behaves the same for either polarity of surge, as is typical for AC and signal line protection.

### A simple example

The symbol is a drawing variant and does not establish electrical ratings. Breakdown voltage, impulse current and follow current behaviour come from the product data. Readers should not infer three-terminal construction from this form; three-electrode tubes need their own representation.

### What the drawings help you do

Read ratings from the product data, not the symbol. Check whether a three-electrode tube is intended instead. The symmetric form draws identical electrodes; the standard gas discharge tube form may show the electrodes differently.

### This particular reference

A protective gas discharge tube drawn with identical electrodes either side, listed as a distinct symmetric form. The symmetric form draws identical electrodes; the standard gas discharge tube form may show the electrodes differently.

### Terms

- AC: Alternating current: electricity whose direction changes periodically.
- Terminal: An identified point where a conductor connects.
- Signal: Electrical information passed between devices, rather than necessarily the main power supply.

Concept diagram: Identify the device and form → Read the connections and state → Check the equipment reference
A guide to reading this reference. The original symbol illustration and its drawing notes appear below; these boxes are not a wiring diagram.

- [AutoCAD Electrical: Earthing, sources and surge protection](https://help.autodesk.com/cloudhelp/2021/ENU/AutoCAD-Electrical/files/GUID-1E3FDEAE-B4B2-417C-8075-108AA2C6E509.htm)
- [IEC-60617 symbol preview directory](https://help.autodesk.com/cloudhelp/2021/ENU/AutoCAD-Electrical/files/GUID-7871E6EF-24D5-467E-9B74-321FEDC9DFDA.htm)


## What it represents

The publisher's IEC-oriented library lists a symmetric protective gas discharge tube separately from the standard form. The symmetric drawing presents both electrodes identically, emphasising that the device behaves the same for either polarity of surge, as is typical for AC and signal line protection.
The symbol is a drawing variant and does not establish electrical ratings. Breakdown voltage, impulse current and follow current behaviour come from the product data. Readers should not infer three-terminal construction from this form; three-electrode tubes need their own representation.

## How to read it

- Read ratings from the product data, not the symbol.
- Check whether a three-electrode tube is intended instead.

## Information to show or reference

- State sparkover voltage and impulse rating.
- Show the earth or return side.
- Keep the form consistent with the legend.

## Important distinction

The symmetric form draws identical electrodes; the standard gas discharge tube form may show the electrodes differently.

## Facts

- Graphic cue: Sealed envelope with matching electrodes either side
- Publisher form: Protective Gas Discharge Tube, Symmetric (HFU1_S00375 Autodesk identifier)
- Preview: Original educational illustration; consult the publisher and applicable standard for the exact graphic

## Public references

- [AutoCAD Electrical: Earthing, sources and surge protection](https://help.autodesk.com/cloudhelp/2021/ENU/AutoCAD-Electrical/files/GUID-1E3FDEAE-B4B2-417C-8075-108AA2C6E509.htm) — Autodesk. Public IEC-oriented preview listing the symmetric protective gas discharge tube form.
- [IEC-60617 symbol preview directory](https://help.autodesk.com/cloudhelp/2021/ENU/AutoCAD-Electrical/files/GUID-7871E6EF-24D5-467E-9B74-321FEDC9DFDA.htm) — Autodesk. Public directory of IEC-oriented symbol families.

## Graphic scope

Independent original educational concept illustration by Elora Grid. Not a certified standards graphic or manufacturer footprint. Unitless; not to scale. Original artwork licence: https://creativecommons.org/licenses/by/4.0/. Publisher material is linked only.