What does [Ex ia] mean, and can associated apparatus be installed in Zone 1?

Square brackets mark associated apparatus: equipment such as a zener barrier, galvanic isolator, IS power supply or controller input card that provides an intrinsically safe circuit to the hazardous area but is not itself intrinsically safe. [Ex ia Ga] IIC means its output circuits are suitable for level ia (Zone 0 field devices) in IIC. With only the bracketed part, the equipment must be installed in the non-hazardous area. It may go into Zone 1 only when it is also protected by another type of protection suitable for EPL Gb, shown outside the brackets, for example Ex db [ia Ga] IIC T4 Gb.

Short answer

• Inside the brackets is the protection the equipment gives to the circuits it feeds, not the protection of the equipment itself.
• [Ex ia Ga] IIC alone: install in the safe area. It has no temperature class, because it is not in the explosive atmosphere.
• Ex db [ia Ga] IIC T4 Gb: a flameproof enclosure containing associated apparatus, suitable for Zone 1, with ia circuits that may run to Zone 0.
• For Zone 2, associated apparatus needs a protection for EPL Gc or better outside the brackets, for example level ec.
• The ATEX category is shown the same way: II (1) G for safe-area equipment with ia outputs, II 2(1) G for a category 2 product containing the safety device for category 1 equipment.

Reading the marking

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 adversely affect the IS ones. Mains supplies, relay contacts and system bus connections are on the non-IS side; the energy-limiting stage and the field terminals are on the IS side.

Older markings show the same idea without the EPL, for example [EEx ia] IIC or Ex d [ia] IIB T6, an example given in IEC 60079-11 itself for associated apparatus protected by a flameproof enclosure.

The EU ATEX Guidelines use round brackets for the category in the same sense. Their marking examples include II (1) GD 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.

Why associated apparatus cannot simply go into Zone 1

Associated apparatus contains ordinary circuits: a mains or 24 V supply, relays, microprocessors and heat-producing components. These are not intrinsically safe and are not built to the rules of any other protection concept. IEC 60079-11 therefore states that associated apparatus placed in an explosive atmosphere must be protected by an appropriate type of protection listed in IEC 60079-0, whose requirements then also apply.

In practice that means one of:

• a certified product that already combines the concepts, such as a flameproof field-mounted power supply or a transmitter head marked Ex db [ia Ga];
• associated apparatus mounted inside a pressurised (Ex p) cabinet or a flameproof enclosure, with the assembly covered by the certificate of that enclosure or panel;
• for Zone 2 only, associated apparatus designed and certified with a Zone 2 concept such as ec, or installed in a suitably certified Zone 2 enclosure.

The interior of such an enclosure is not intrinsically safe, so it is opened only under the rules for its outer protection (isolation, gas-free permit), even though the field circuits remain IS.

Installation points that come with brackets

• Um. Associated apparatus is marked with Um, the highest voltage that may be applied to its non-IS terminals, commonly 250 V. Where Um is below 250 V, IEC 60079-14 (2013 wording) restricts the supply to an SELV or PELV system (only where Um is at most 50 V a.c. or 120 V d.c.), a safety isolating transformer, equipment built to the IT or laboratory equipment safety standards, or cells and batteries.
• Loop check. The bracketed level and group set the maximum level of the loop: an [Ex ib] interface cannot feed a Zone 0 device, whatever the field device's own marking.
• Earthing. Shunt diode barriers need an IS earth; isolators do not.
• Temperature class. The temperature class of the field circuit comes from the field device, not from the associated apparatus.

Common mistakes

• Mounting a DIN-rail isolator marked only [Ex ia Ga] IIC in a field junction box in Zone 1. Without an outer protection concept, this is an uncertified ignition source in the zone.
• Reading [Ex ia] as "suitable for Zone 0". Only the outputs are; the box itself belongs in the safe area.
• Ignoring Um. A barrier with Um 250 V is not protected against a higher fault voltage on its supply or signal side.

Related on excodex

• Zener barriers vs galvanic isolators: choosing intrinsic safety interfaces
• Intrinsic safety (Ex i): ia, ib and ic and the energy limitation principle
• How to read an Ex marking: decoding II 2 G Ex db IIC T4 Gb step by step
• What do brackets mean in Ex markings such as II (1) G?
• Tools: Ex marking check, standards space IEC 60079-11

Standards and further reading

• IEC 60079-11, intrinsic safety "i" (edition 7, 2023), definition and marking of associated apparatus.
• IEC 60079-0, general requirements, including marking (edition 8, 2026).
• IEC 60079-14, electrical installations (edition 6, 2024), selection and installation of associated apparatus.
• IEC 60079-11:2011 text (edition 6, superseded), scope, definition and marking examples, with the 2016 interpretation sheet quoting the Um supply rules of IEC 60079-14:2013: law.resource.org
• European Commission, ATEX 2014/34/EU Guidelines, 6th edition (January 2026), marking examples in Tables 6 and 7: single-market-economy.ec.europa.eu

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.