ATEX 95 and IECEx 02 both describe the requirements for equipment intended for use in (potentially) hazardous areas.
The difference between ATEX 95 and IECEx 02 is initially that ATEX 95 is only valid in the EU and IECEx 02 aims for global acceptance. ATEX 95 is legally required for all electrical and non-electrical equipment in hazardous environments. IECEx 02 is only intended for electrical equipment in hazardous environments.
Main difference between them is that ATEX 95 is law-driven and IECEx 02 is standard-driven. For ATEX, compliance with the standards is not mandatory, but according to IECEx the standards are mandatory. For ATEX, the manufacturer is fully responsible for collection of the modules for examination and production control. For IECEx, a third (certification) party is responsible for bringing together all aspects of design and manufacturing assessment for the issuance of a public certificate. In practice this means that IECEx 02 is experienced more stringent than ATEX 95 on evidence handling during the certification process and assurance of the quality system by the manufacturer.
ATEX and IECEx both use the same standards (eg EN-IEC 60079-0), so in terms of technical content, there is basically no difference. Only visible difference is in many cases the marking on the device.
For a complete overview of all differences between ATEX and IECEx, please see this document.
ATEX 95 directive is leading in the EU, which contains general requirements for products in the EU. To meet these requirements the underlying standards (eg EN-IEC 60079-0) can be used. The fact that the directive provides information on general requirements and standards may be used as aid means that there is more room for interpretations and deviation from the requirements of the standards is possible.
For IECEx the standards are leading, which means that products must meet the relevant requirements in the standards. Within IECEx 02 there is little room for interpretations and deviations from the standards are almost impossible. IECEx Test Reports (ExTR) to which apply more stringent requirements are an important information medium. All IECEx accredited Notified Bodies must accept each others IECEx test reports. So when evaluations and tests are carried out by Notified Body X, these should in principle be accepted by Notified Body Y (if both are accredited for IECEx 02). Information can easily be exchanged between different Notified Bodies. It also appears that in practice IECEx test reports are often accepted as basis for other local verification systems.
ATEX and IECEx both have a quality assurance system. Besides the product certification, in both cases a quality system must be set up by the manufacturer and will periodically be checked by a Notified Body. For both systems, the quality system is mainly based on ISO 9001 with additional requirements for the design, manufacture and testing of explosion proof products.