Class II dust environments—like grain mills, silos, elevators, and conveyors—need egress equipment that won’t ignite suspended or layered dust and still delivers UL 924 emergency performance. This plain‑English guide helps facility managers and electrical contractors specify hazardous‑location emergency lights, explosion‑proof exit signs, and combo units for combustible dust (Groups E/F/G), with practical placement, sealing, testing, and documentation tips that hold up at AHJ walk‑throughs.
Last updated: October 2025
Why Class II Is Different (Combustible Dust)
- Hazard types: Ignitable dust clouds and surface layers (e.g., grain, flour, starch) can flash when heated or sparked.
- Classification: Class II areas divide into Division 1 (normal presence) and Division 2 (abnormal/occasional). Dust groups: E (metals), F (carbon/coal), G (grain/wood/flour).
- Design implications: For egress fixtures, verify UL 844 (haz‑loc) and UL 924 (emergency) on the same device. For dust, the nameplate typically shows a maximum surface temperature (°C) rather than a lettered gas T‑code—keep it below your dust cloud/layer ignition limits.
Temperature ratings explained: T‑Codes for Explosion‑Proof Fixtures
Codes, Listings & Nameplates (UL 844 / UL 924)
- UL 844 (hazardous‑location): Nameplate must show Class/Division/Group for Class II and the applicable surface temperature limit (°C).
- UL 924 (egress): Required for emergency lights/EXIT signs—automatic transfer and ~90‑minute runtime.
- NEC 500–516: Follow area classification and wiring methods; use listed sealing fittings/compounds for classified runs.
- Documentation: Photograph nameplates and include cut sheets in the commissioning packet for AHJ review.
Compliance steps: Hazardous‑Location Code Checklist
Fixture Types: Emergency Lights • Exit Signs • Combos
- Hazardous‑Location Emergency Lights: Dual LED heads for egress; specify Class II Div 1/2 per zone; consider narrow‑beam heads for long throws.
- Explosion‑Proof Exit Signs: LED EXIT legends in sealed housings; verify sightlines across conveyors, bins, and catwalks.
- Combos (Exit + Lights): Efficient at doorways/junctions; add separate hazardous‑location lights to cover long runs and high bays.
Environmental Hardening (NEMA/IP, Corrosion, Temperature)
- NEMA/IP: Use NEMA 4X / IP66 where washdown, dust ingress, or outdoor exposure exists—in addition to UL 844.
- Materials/finish: Copper‑free aluminum or FRP enclosures; sealed optics; stainless hardware for corrosive dust or salt air.
- Ambient limits: Verify the device’s marked max surface temperature applies at your Ta (ambient). Consider cold‑weather battery kits for exterior catwalks and silo tops.
Classification primer: Class, Division & Group Primer
Quick Reference: Dust Groups E/F/G
| Group | Typical Materials | Industries/Areas | Design Watch‑outs |
|---|---|---|---|
| E (metal dust) | Aluminum, magnesium, other metals | Powder handling, finishing rooms | Conductive dust; ensure effective sealing and surface temp control |
| F (carbon/coal) | Carbon black, coal, coke dust | Feed prep, mills, transfer points | Sooting on lenses; maintain frequent cleaning schedule |
| G (grain/wood/flour) | Grain, flour, starch, sugar, wood | Mills, silos, elevators, conveyors | Layer thickness raises surface temps; keep fixtures clean |
Placement & Photometrics in Mills & Silos
- Doors & stairs: Mount combos above exits; aim heads to eliminate shadows at treads/landings.
- Conveyors & catwalks: Space fixtures every ~40–60 ft to maintain minimums along long, narrow paths; verify photometrics for the full 90 minutes.
- Bins & process floors: Add hazardous‑location lights to clear obstructions from bins, ducts, and racks; verify coverage behind silos and tall equipment.
- High bays: Use narrow‑beam heads for throw; supplement with fills to maintain target foot‑candles at end‑of‑life battery voltage.
Installation: Installing Explosion‑Proof Lighting
Installation Notes (Seals, Hubs, Dust Control)
- Sealing fittings: Use listed explosion‑proof seals/compounds at required distances; no ordinary hubs in classified conduit.
- Flame paths: Keep mating surfaces pristine; torque cover bolts to spec; avoid debris during service; re‑torque after maintenance.
- Housekeeping: Dust layers act like insulation—raise surface temperature and reduce output. Clean lenses and housings on a schedule.
Inspection prep: What Inspectors Check
Testing, Logs & AHJ Documentation
- Monthly: 30‑second functional test; visual for gaskets, seals, corrosion, and dust buildup.
- Annual: Full 90‑minute discharge; confirm floor illumination throughout; re‑aim heads as needed.
- Records: Maintain logs plus cut sheets (UL 844/UL 924), as‑builts, and nameplate photos for AHJ.
Common Pitfalls & How to Avoid Them
- Specifying NEMA/IP‑only gear in a classified dust area without UL 844 Class II listing.
- Ignoring surface temperature (°C) vs. dust cloud/layer ignition limits; overlooking ambient (Ta) effects.
- Skipping sealing fittings; contaminating flame‑path joints during service.
- Dirty lenses and un‑re‑aimed heads after process/equipment changes.
Printable Spec & Commissioning Checklist
- Area map confirms Class/Division/Group and required max surface temperature
- Fixtures labeled UL 844 (haz‑loc) and UL 924 (egress) as applicable
- NEMA 4X/IP66 where washdown/dust ingress/outdoor exposure exists
- Sealing fittings installed/compounded per NEC; flame‑paths clean/torqued
- Photometrics verified for the full 90‑minute test at doors/stairs/turns
- Commissioning packet: cut sheets, nameplates, as‑builts, test logs ready for AHJ
This guide is informational and not a substitute for professional engineering judgment or the authority of your AHJ.