Heat Tracing in Dust Hazard Zones
Dust hazard zones are often treated as a footnote to gas-area classification — "it's ATEX, same as before". They are not the same. Dust zones bring different temperature rules, different enclosure requirements and a surface-temperature limit that directly constrains which cable you can use.
Zone 20 / 21 / 22 in one line each
| Zone | Where it applies | Practical meaning for tracing |
|---|---|---|
| Zone 20 | Inside equipment where dust clouds form continuously | Rarely traced; if so, require the highest category equipment |
| Zone 21 | Areas where dust clouds form occasionally in normal operation | Common around conveyors, hoppers, bagging points |
| Zone 22 | Areas where dust clouds form only in fault conditions | Wider envelope around the plant; many traced lines sit here |
What actually changes
1. Surface temperature is the governing limit
For dust, the risk is a hot surface igniting a dust layer or cloud. The relevant value is the maximum surface temperature of the cable, junction box and any exposed metal — not the process temperature. A cable whose sheath runs at 120 °C may be unacceptable even if the line is only meant to reach 60 °C.
2. Dust layer versus dust cloud
A 5 mm dust layer insulates the surface and raises its temperature. Design limits for dust layers are stricter than for clouds. If the site has accumulating dust, the equipment must be rated for the layer thickness actually expected.
3. Equipment categories differ from gas
Dust equipment categories (1D / 2D / 3D by zone) are not interchangeable with gas categories (1G / 2G / 3G). A cable certified for gas zone 1 does not automatically satisfy a dust zone 21 requirement.
4. Enclosures and ingress
Dust needs IP6X. Junction boxes with poor sealing fill with dust, and dust inside a box changes both the temperature and the ignition risk.
5. Static and dust layers on the cable
Dust settling on the tracing cable itself is common on conveyors and silos. Consider routing that does not create horizontal ledges, and fixings that allow cleaning.
Design checklist
| Item | Confirm |
|---|---|
| Dust zone | 20 / 21 / 22 per area, per piece of equipment |
| Dust type | Conductivity, minimum ignition temperature, layer behaviour |
| Surface limits | Max surface temp for cloud and for stated layer thickness |
| Equipment category | Dust category (1D/2D/3D), not the gas one |
| Enclosure | IP6X, verified sealing, cleaning access |
| Routing | No ledges that accumulate dust |
| Documentation | Zone drawing + cable/box certificates with dust marking |
Common mistakes
- Using a gas-area certificate and assuming it covers dust.
- Setting the design limit from process temperature instead of surface temperature.
- Ignoring dust layers on top of the enclosure or cable.
- Junction boxes rated IP65 in a dusty area that needs IP6X with proper sealing.
- Horizontal cable trays or brackets that collect dust and are never cleaned.
Dust hazard project?
Send the zone drawing and dust data. We check surface temperature limits and dust-category certification before quoting.
Request a Quote