What do brackets mean in markings such as [Ex ia] or II (1) G?
Abstract
Brackets mark associated apparatus such as IS barriers. [Ex ia] means the device feeds intrinsically safe circuits that may enter the zone but is itself installed in the safe area. In ATEX, II (1) G says the same: category 1 applies to the connected circuits, not the device.
Brackets identify associated apparatus: equipment such as an intrinsically safe barrier or isolator that contains intrinsically safe circuits going into the hazardous area, while the device itself is not intrinsically safe. [Ex ia] means the circuits it supplies meet level ia, but the device must be installed outside the hazardous area. In the ATEX marking, the round brackets in II (1) G say the same thing: category 1 applies to the connected circuits, not to the place where the device sits.
Short answer
- Square brackets around "Ex ia", as in
[Ex ia Ga] IIC: associated apparatus for the safe area. No temperature class, because the device is not in the explosive atmosphere. - Brackets around the IS part only, as in
Ex db [ia Ga] IIC T4 Gb: the associated apparatus is itself protected by another type of protection (here flameproof) and may be installed in the zone that the outer marking allows (here Zone 1). - Round brackets in the ATEX category, as in
II (1) G: the category in brackets belongs to the equipment or circuits the device serves.II 3 (1) Gmeans the device may sit in Zone 2 and serves category 1 equipment. - The ATEX Guidelines use the same convention for safety devices outside the zone that protect equipment inside it, for example
II (2) G. - The bracketed level and group limit the field side: an
[Ex ib]interface cannot feed a device in Zone 0, and an IIB output cannot serve a IIC field device.
Where the brackets come from
IEC 60079-11 defines associated apparatus as electrical equipment containing both intrinsically safe and non-intrinsically safe circuits, built so that the non-IS circuits cannot affect the IS ones. Its note gives two cases: equipment that has another type of protection for use in the explosive atmosphere, and equipment without such protection, which therefore stays outside it, such as a recorder whose thermocouple input alone is intrinsically safe.
IEC 60079-0 turns that into marking rules. For associated apparatus that is not suitable for a hazardous area, the symbol Ex and the type of protection go inside the same square brackets and no temperature class is given. For associated apparatus that is suitable for a hazardous area, only the intrinsic safety symbols go in brackets, after the outer type of protection. Since the 2007 edition, which introduced EPLs, the EPL of the IS circuits goes inside the brackets too, which is why modern labels read [Ex ia Ga] IIC or Ex d [ia Ga] IIC T4 Gb rather than the older [Ex ia] IIC. Where the group of the IS circuits differs from that of the equipment, it also goes in the brackets, for example Ex d [ia IIC Ga] IIB T4 Gb.
The European Commission's ATEX Guidelines (6th edition, January 2026) give the matching category examples: II (1) G D for a device in the non-hazardous area with ia circuits that can be connected to category 1 equipment, and II 2(1) G for category 2 equipment containing a safety device for category 1 equipment. They also apply round brackets to safety, controlling and regulating devices, where the figure in brackets is the category of the equipment being influenced.
Reading common markings
| Marking | Meaning | Where the device may be installed |
|---|---|---|
II (1) G [Ex ia Ga] IIC | IS barrier or isolator with ia outputs for IIC | Safe area only |
II (2) G [Ex ib Gb] IIB | Interface with ib outputs for IIB | Safe area only; field devices up to Zone 1, IIA or IIB |
II 2 (1) G Ex db [ia Ga] IIC T4 Gb | Flameproof enclosure containing associated apparatus with ia outputs | Zone 1 (or 2), T4 applies to the enclosure |
Ex ib [ia IIC] IIB T6 | IS apparatus at level ib, with ia outputs rated for IIC (example from IEC 60079-11) | Per the outer marking: EPL Gb, IIB, T6 |
II (2) G alone on a control unit | Safety or control device outside the zone, influencing category 2 equipment | Safe area |
The labels of associated apparatus also carry the output parameters (Uo, Io, Po, Co, Lo) and Um, the highest voltage that may be applied to the non-IS terminals without spoiling the protection. Those values are what the loop check uses.
Example: An instrument engineer specifies a level transmitter in a tank vapour space (Zone 0) with marking Ex ia IIC T4 Ga. The isolator in the control room is marked II (1) G [Ex ia Ga] IIC: its outputs reach level ia and group IIC, so it may supply a Zone 0 device. A spare isolator marked [Ex ib Gb] IIC would be wrong for this loop, even though it looks almost the same, because ib circuits may not enter Zone 0. Both isolators stay in the safe area; the brackets say nothing about putting them in the field.
Common mistakes
- Mounting
[Ex ia]equipment in the hazardous area because "it is intrinsically safe". The brackets mean the device itself is not; only its outputs are. - Reading
II (1) Gas "category 1 equipment" that may go into Zone 0. - Ignoring the bracketed group and level, for example feeding a IIC field device from a IIB interface.
- Forgetting Um. Connecting the non-IS side to a supply above Um voids the protection.
Related on excodex
- How to read an Ex marking: decoding II 2 G Ex db IIC T4 Gb step by step
- Intrinsic safety (Ex i): ia, ib and ic and the energy limitation principle
- Zener barriers vs galvanic isolators: choosing intrinsic safety interfaces
- What does a marking like II 1/2 G or Ga/Gb mean?
- Tool: Ex marking check; standards space IEC 60079-11
Standards and further reading
- IEC 60079-11, intrinsic safety "i": definition of associated apparatus and marking examples (public copy of the 2011 edition; edition 7, 2023, is current): law.resource.org (PDF)
- IEC 60079-0, general requirements: square bracket marking rules (public copy of the 2004 edition; edition 8, 2026, is current): law.resource.org (PDF). Bracket rules with EPLs in the 2007 edition, as the Indian Standard IS/IEC 60079-0:2007: archive.org
- European Commission, ATEX 2014/34/EU Guidelines, 6th edition (January 2026), marking examples and safety devices: single-market-economy.ec.europa.eu (PDF)
- IEC 60079-25, intrinsically safe electrical systems, for loop verification.
- NSW TestSafe, intrinsic safety overview: nsw.gov.au
This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.