Is carbon black or coal dust classified as conductive dust group IIIC?

Not automatically. In the IEC and ATEX system a dust that is not a metal belongs to group IIIC only if its measured electrical resistivity is 10³ Ω·m or less (otherwise IIIB), so for carbon black and coal the resistivity of the actual dust decides: some carbon blacks, cokes and graphites qualify, coal varies, and without a test result the safe choice is IIIC. In the US Zone system (NEC Article 506) IIIC means metal dust only, so carbon and coal dusts are IIIB there.

Short answer

• IEC definition: conductive dust means combustible metal dusts plus any other combustible dust with resistivity of 1 × 10³ Ω·m or less (ISO/IEC 80079-20-2; IEV 426-02-19).
• Carbon black, graphite, coke: grades differ widely. Grades made to conduct electricity can be IIIC; others are IIIB.
• Coal: depends on the coal and its processing; classify by measurement.
• No data: select IIIC equipment, or have the resistivity measured by the method in ISO/IEC 80079-20-2 on the dust as it occurs in the plant.
• Explosibility is a separate question. Tested soot and hard coal samples are explosible (St 1); several graphite, coke and anthracite samples did not explode in the test at all.
• Underground coal mines are outside Group III: they use Group I equipment, and IEC 60079-10-2 does not apply to underground mining.

What makes a dust IIIC

Group III equipment is divided by the kind of dust it can tolerate: IIIA for combustible flyings, IIIB for non-conductive dust and IIIC for conductive dust. The split is about what the dust does to the equipment, not about how violently it explodes. A conductive dust that gets inside an enclosure can bridge live parts and cause leakage currents, tracking and sparks; a non-conductive deposit insulates.

ISO/IEC 80079-20-2 defines conductive dust as combustible metal dust and other combustible dust with an electrical resistivity of 1 × 10³ Ω·m or less, and states that metal dust is treated as conductive because surface oxidation cannot be relied on to keep its resistivity higher. The International Electrotechnical Vocabulary (IEV 426-02-19) defines conductive dust by the same resistivity limit. For everything that is not a metal, the classification therefore needs a number: the bulk resistivity measured in the cell described in that standard.

Carbon black, graphite and coke

Carbon-rich dusts sit close to, and on both sides of, the limit:

• Carbon black is made in many grades. Some are used as conductive or antistatic additives in plastics and in battery electrodes, and such grades can be expected to be conductive; other grades and soots with higher volatile content conduct much less. Only a measurement on the grade in use settles the group.
• Graphite and coke can be conductive, but many samples are hard to ignite: in the GESTIS-DUST-EX database several graphite, coke and anthracite samples were not explosible in the test, while other samples of the same materials were St 1. A dust that is not combustible needs no dust group at all, so explosibility is the first test.
• Mixtures follow the most demanding component in practice. A plastic compound or rubber premix with a significant carbon black content may be IIIC even if the polymer alone is IIIB.

GESTIS-DUST-EX gives combustion and explosion data but not resistivity, so databases do not answer the group question.

Coal dust

Coal ranges from lignite to anthracite, and its composition and degree of processing vary widely, so no general rule can be given for its group. Because conditions such as moisture and particle size affect the result, test the dust in the state in which it occurs in the plant, and for dusts close to the limit choose the more demanding group. In surface coal handling (power stations, coal mills, coke works) the group is a matter for the area classification and the dust data. In underground coal mines the equipment is Group I, designed for firedamp and coal dust together.

The North American difference

The NEC uses two systems. In Article 506 (Zones), group IIIC is defined as combustible metal dust, equivalent to Class II Group E, while IIIB covers combustible dust other than metal and is equivalent to Class II Groups F and G; carbonaceous dusts such as coal and carbon black are Group F. A conductive carbon black can therefore be IIIC under IEC rules but IIIB under NEC Article 506. Use the definitions of the system that applies to the installation.

Practical case. A compounding plant in Europe adds 20 % conductive carbon black to a polymer. The polymer powder alone is IIIB. The carbon black supplier gives no resistivity, and the compound fines collected in the filter have not been tested. Until a resistivity test on the fines shows more than 10³ Ω·m, the assessor specifies IIIC equipment, which for protection by enclosure means dust-tight IP6X at every level, and records the reason in the explosion protection document.

Common mistakes

• Calling every carbon dust "conductive" or every coal "non-conductive" from its name.
• Testing the raw material instead of the plant dust, such as filter fines or a mixture.
• Applying the NEC metal-dust definition in an IEC or ATEX installation, or the reverse.
• Skipping the explosibility test. Some graphite and coke dusts are not explosible, which changes the whole assessment.

Related on excodex

• Dust groups IIIA, IIIB and IIIC: combustible flyings, non-conductive and conductive dusts
• Can IIIC-marked equipment be used for IIIA and IIIB dusts?
• How do I know if my dust is combustible?
• Equipment groups I, II and III: mining, gas and dust
• Standards space: NEC 500 to 506

Standards and further reading

• ISO/IEC 80079-20-2:2016, combustible dusts test methods, including the definitions of conductive and non-conductive dust and the resistivity test (revision in progress): sample pages
• IEC 60050-426 (International Electrotechnical Vocabulary), conductive dust: electropedia.org
• IEC 60079-0, Group III subdivision; IEC 60079-31, protection by enclosure.
• NFPA 70 (NEC) Article 506, Zone 20, 21 and 22 locations (material groups IIIA, IIIB, IIIC), and Article 500 (Class II Groups E, F, G).
• US Coast Guard OCS National Center of Expertise, hazardous location material groups (Class II Groups E, F, G; Group III subdivision): dco.uscg.mil
• SafeWork NSW TestSafe, dust ignition protection and Group III subgroups: nsw.gov.au
• IFA, GESTIS-DUST-EX sample entries for soot and graphite

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.