In previous analyses, we have seen how combustible dusts represent a hazard for the formation of explosive atmospheres. In recent years, regulations have introduced a specific equipment marking system, reflecting what had already been established for potentially explosive gas atmospheres. These markings have now been in place for more than fifteen years, following the replacement of the EN/IEC 61241 standards by the EN/IEC 60079 series.

There are several protection methods that mitigate the risks associated with combustible dusts [1]. This document focuses on certain aspects of the Ex-t protection method, specifically developed for dust, as defined in EN/IEC 60079-31.

The protection method by enclosures “t” [2] is based on a simple yet effective concept: providing electrical equipment with an enclosure that prevents the ingress of combustible dust and employing means to limit the external surface temperature.

Figure 1: Ex-t enclosure concept

Three protection levels are defined: Ex-ta, Ex-tb, and Ex-tc. The corresponding Equipment Protection Levels (EPLs), which are related to the installation Zone, are reported in Table 1.

Table 1: Correspondence between protection mode and EPL of the device

The primary requirement of Ex-t protection lies in the effectiveness of the enclosure. To this end, in addition to the specific tests required by the general standards for equipment intended for explosive atmospheres [3], an additional protection level is required, expressed by the IP (Ingress Protection) rating.

The first digit of the IP rating specifically indicates the degree of protection against solid foreign objects and dust. The minimum required rating in this case is IP5x, although equipment is more commonly certified as IP6x, ensuring compliance with all dust groups and the highest EPL [4].

IP testing for protection in explosive atmospheres may differ in some respects from those defined in the general industrial standard (IEC 60529). However, the main critical aspect lies in the sequence of tests, which can be particularly demanding for non-metallic materials (both polymeric enclosures and elastomeric seals).

IP tests performed on aged materials that have previously undergone impact testing are significantly more difficult to pass and provide a clear indication of the quality and suitability of the materials used.

The second part of the Ex-t protection definition — “employing means to limit the external surface temperature” — refers to a set of requirements aimed at managing potential equipment faults and their consequences in terms of maximum surface temperature. In this regard, particular attention is given to protection level Da, corresponding to the Ex-ta method.

Temperature tests for the Ex-tb protection method are carried out under normal operating conditions or under defined fault conditions, consistent with the requirements for the Ex-db protection method. This is not surprising, as both protection methods rely on the integrity of the protective enclosure.

The Ex-t protection method, as described in EN/IEC 60079-31, represents one of the primary approaches for ensuring the safety of electrical equipment installed in environments where combustible dust is present.However, it should be noted that, in managing dust-hazardous environments, a fundamental part of protection is ensured by maintenance procedures, including cleaning activities aimed at preventing excessive dust accumulation on electrical equipment. This requirement is often specified in equipment operation and maintenance manuals [5].

Reference Standards and Bibliography

[1] Other protection methods used for combustible dusts include: pressurization (EN/IEC 60079-2) for panels and motor enclosures; intrinsic safety (EN/IEC 60079-11) for sensors; and encapsulation (EN/IEC 60079-18) for electrical components and solenoid valves.

[2] In English: dust ignition protection by enclosure “t” – CEI EN IEC 60079-31:2024, clause 3.1

[3] The general requirements standard is EN/IEC 60079-0; one of the tests includes ageing of non-metallic materials in a climatic chamber.

[4] Table 1, IEC 60079-31:2022

[5] CEI EN 60079-17:2015-03, clause 4.7