Don't Zone Out On Area Classifications

Electrical ratings must start with analysis of chemicals present.

By Dirk Willard, Contributing Editor

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Determining whether a chemical is a hazard is the most difficult challenge in EAC work. Here're some general guidelines for classification: 1) only consider NFPA-30 Class I liquids -- Class II and Class III liquids don't produce enough vapor, unless heated; 2) treat a fire danger of at least 3 in an NFPA diamond (refer to NFPA-704) as Class I; 3) categorize mixtures with >30% H2 by volume as Group B (API-500, 5.5.5) and mixtures with >25% H2S as Group C (API-500, 5.5.4) -- NFPA-497 Annex B provides a rule for estimating the NEC group for mixtures of vapors or gases but it doesn't work for H2 and H2S; 4) when in doubt about the chemistry use the worst case in evaluating a hazard; and 5) if you can't measure the molecular weight (MW) of a complex organic, you can get a good estimate for n-alkanes with MWs from 80 to 1,400 via the equation in "Select the Right Hydrocarbon Molecular Weight Correlation" by Donald Schneider of Stratus Engineering: MW = 3.3955×10-15Tbf 6 - 1.2416×10-11Tbf5 + 1.8256×10-8Tbf4 - 1.3234×10-5Tbf3 + 5.2285×10-3Tbf2 – 0.741692Tbf + 116.19, where Tbf is the boiling point.

EAC is based on the auto-ignition temperature (AIT) -- the minimum temperature at which a combustible material will burst into flame without an external ignition source. AITs reported in MSDSs often aren't tested for the particular mixture but reflect testing of a pure compound. There are no mixing rules for the AIT, which severely handicaps studies. Use the following simplification if no other data are available: for compositions containing compounds with MW exceeding 50 use AIT = 280°C; for others, use 200°C.

In later columns, I'll discuss enclosures, preparation of EAC drawings, defining envelopes and remediation.

DIRK WILLARD is a Chemical Processing Contributing Editor. You can e-mail him at

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  • <p>Class II is for combustible dusts and Class III is for combustible fibres and flyings, NOT liquids as stated in this article.</p>


  • <p>Never mind that's for NFPA-30 classes of liquids not NEC area classes.</p>


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