Is an IP65 enclosure acceptable in a dust Zone 21?

No, not on the strength of its IP rating. IP65 means the enclosure passed the dust-tight (IP6X) test of IEC 60529, and dust-tightness is one of the requirements for equipment in Zone 21. But Zone 21 needs equipment certified for EPL Db (ATEX category 2D), for example Ex tb, which adds a maximum surface temperature that suits the dust, enclosure tests after ageing and impact, and controlled seals, cable entries and documentation.

Short answer

• Zone 21 requires EPL Db or Da. In the EU, Directive 1999/92/EC allows category 1 or 2 equipment suitable for dust in Zone 21.
• For protection by enclosure level tb, IEC 60079-31 requires IP6X for conductive dust (IIIC) and non-conductive dust (IIIB), and IP5X for flyings (IIIA). IP65's first digit "6" meets that level of ingress protection.
• IP is only one of several requirements. The equipment also needs a maximum surface temperature in °C below the limits for the dust, and an enclosure that keeps its seal after the type tests of IEC 60079-0.
• An uncertified IP65 box with only passive terminals still contains connections that can heat up or loosen; a certified Ex tb or Ex eb/tb junction box is the normal choice.
• The same logic applies in Zone 22: a good IP rating helps, but the equipment must still be EPL Dc or better.

Why an IP rating is not an Ex rating

The IP code tells you how well an enclosure keeps out solid objects and water under the test conditions of IEC 60529. It says nothing about:

• Surface temperature. A dust cloud or a layer on top of a hot enclosure can ignite. Ex equipment for dust is marked with its maximum surface temperature, for example T135 °C, which the user compares with the dust's ignition temperatures.
• What happens over time. Ex t samples go through the enclosure tests of IEC 60079-0 (such as thermal endurance to heat and cold and impact) before the IP test. IEC 60079-31 also requires grease to be removed from joints before the IP test, so the seal cannot depend on grease.
• Faults. EPL Db means no ignition source in normal operation or in expected malfunctions. Internal components, terminals and their temperatures are assessed with that in mind.
• Static and materials. Plastic enclosures are checked for charging (surface resistance or other limits), and materials for UV resistance where relevant.
• Marking and instructions. Certified equipment has a certificate, conditions of use and an ambient range that inspectors can check.

The two numbers that decide a Zone 21 enclosure

Ingress protection, by level and dust group (IEC 60079-31:2022):

For level ta, IEC 60079-31:2022 raises the depression in the dust test to at least 4 kPa, held for at least 8 h. The normal IEC 60529 dust test uses a depression of at most 2 kPa and ends when 80 enclosure volumes of air have been drawn through or after 8 h.

Maximum surface temperature (IEC 60079-14): the marked temperature must not exceed two thirds of the dust cloud's minimum ignition temperature, and, for layers up to 5 mm, must stay at least 75 K below the minimum ignition temperature of a 5 mm layer. Thicker layers need a further reduction. The lower limit applies.

Worked example: junction box in a sugar packing room

A packing room is classified Zone 21 around the bag filling heads. The dust is powdered sugar, a non-conductive dust (IIIB). An electrician proposes a standard IP65 polycarbonate junction box.

• Not acceptable as it stands. It has no EPL Db assessment, no marked maximum surface temperature and no tested seal or material properties.
• Acceptable alternative: a box marked, for example, II 2 D Ex tb IIIC T85 °C Db, or a combined marking such as II 2 GD Ex eb IIC T6 Gb Ex tb IIIC T85 °C Db. IIIC covers IIIB, tb gives Db, and the temperature must be checked against the sugar's tested cloud and layer ignition temperatures.
• Cable entries must keep the IP rating of the box and be suitable for the type of protection, normally certified glands with the right seals; an ordinary gland can be the weak point of the whole assembly.

Use ignition temperatures from tests of the actual dust where possible; published values vary with particle size and moisture.

Common mistakes

• Equating IP66 with "explosion-proof". IP ratings test dust and water ingress only.
• Using IP5X equipment with conductive dust. IIIC needs IP6X at every level.
• Ignoring dust layers. A thick layer on top of equipment insulates it and lowers the safe surface temperature; good housekeeping is part of the protection.
• Adding holes or glands on site without keeping the certified IP rating and the certificate conditions.

Related on excodex

• Dust ignition protection by enclosure (Ex t): ta, tb and tc
• Dust zones 20, 21 and 22 explained with practical examples
• Surface temperature limits for dust: the two-thirds cloud rule and the 75 K layer margin
• Is an IP67 enclosure the same as an explosion-proof enclosure?
• Tools: Ex marking check; standards spaces IEC 60079-31, IEC 60079-14

Standards and further reading

• IEC 60079-31, equipment dust ignition protection by enclosure "t" (2022), including the IP table by level and dust group.
• IEC 60079-0, general requirements, including enclosure tests and Group III marking.
• IEC 60079-14, design, selection and installation, including dust temperature limits.
• IEC 60529, degrees of protection provided by enclosures (IP code).
• AS/NZS 60079.14:2009 (IEC 60079-14:2007 with national modifications), dust cloud and layer temperature limits: archive.org
• IEC 60079-31:2022 preview with Table 1: standards.iteh.ai
• SafeWork NSW TestSafe, dust ignition protection Ex t: nsw.gov.au
• Health and Safety Authority (Ireland), combustible dust, dust groups: hsa.ie
• Directive 1999/92/EC, Annex II, Part B, equipment categories for each zone: legislation.gov.uk

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.