1. Flour Dust Explosion Hazard
Flour dust, wheat dust, corn starch, and other grain-derived fine powders are classified as combustible dusts under EN 60079-10-2 and NFPA 61. When suspended in air at concentrations above the minimum explosive concentration (MEC), these dusts form an explosive cloud that can be ignited by any ignition source with sufficient energy — including motor surface temperatures above the minimum ignition temperature (MIT) of the dust layer.
| MEC (cloud) | 50–100 g/m³ |
| MIT cloud | 380°C |
| MIT layer (5 mm) | 190°C |
| MIE | 10–50 mJ |
| Kst | 56–180 bar·m/s |
| Dust class | St 1 |
| MEC (cloud) | 40–60 g/m³ |
| MIT cloud | 390°C |
| MIT layer (5 mm) | 260°C |
| MIE | 30–80 mJ |
| Kst | 148–202 bar·m/s |
| Dust class | St 1–St 2 |
| MEC (cloud) | 60–125 g/m³ |
| MIT cloud | 430°C |
| MIT layer (5 mm) | 220°C |
| MIE | 15–100 mJ |
| Kst | 43–112 bar·m/s |
| Dust class | St 1 |
The dust layer MIT is the critical value for motor selection. A standard motor frame in a milling room accumulates flour dust on its surface over time, and this dust layer can be ignited if the motor surface temperature exceeds the MIT of the specific flour dust in the area. For wheat flour, the layer MIT is approximately 190°C — and a standard motor at full load in a warm milling room can reach 120 to 150°C frame temperature without fault, approaching the layer MIT with a safety margin of only 40 to 70°C. An explosion-proof motor certified to T135°C provides a 55°C safety margin below the wheat flour layer MIT.
2. Zone 20, 21, and 22: Dust Hazard Classification
IEC 60079-10-2 classifies areas containing combustible dust into three zones based on the frequency and duration of the explosive dust cloud presence. For flour mills, the classification of each area within the facility must be determined by a competent person and documented in the dust explosion protection document (ATEX Directive 1999/92/EC, Article 7).
Inside dust handling equipment — bucket elevators, pneumatic conveying systems, cyclone interiors, bag filter interiors, and grain silos. No motors are installed inside Zone 20 equipment under normal design practice. Where a motor must be integrated into Zone 20 equipment (such as an agitator or conveyor drive inside a silo), it must be certified Ex ia IIIB T135 or equivalent — a standard YB2 Ex d IIB T4 gas motor is not suitable for Zone 20 dust environments.
Areas around dust handling equipment — the immediate vicinity of bucket elevator boot and head sections, areas near sifter discharge points, roller mill feed zones, and areas within 1 m of any dust source that releases dust during normal operation. This is the primary zone in active milling rooms. Motors in Zone 21 must be certified Ex t IIIB T135 (dust-protected motor with IP6X and T135 surface limit) or Ex d IIB T4 (gas-rated flameproof motor, which also meets dust zone requirements under IEC 60079 Part 31).
General milling room and grain store areas where dust is not normally suspended in explosive concentrations but where a dust release could create explosive conditions due to equipment malfunction, maintenance activity, or abnormal operating conditions. Most of the roller mill floor and sifter room area falls in Zone 22 in a well-maintained facility. Zone 22 requires motors certified Ex t IIIB T135 or equivalent dust-protected motor — standard IP54 motors are not acceptable even in Zone 22.
3. Dust Group IIIA, IIIB, IIIC: Which Applies to Flour Mills?
| Group | Dust Type | Examples in Food and Grain | YB2 Suitable? |
|---|---|---|---|
| IIIA | Flyings (coarse fibrous material) | Wood shavings, hessian fibres, crop residue > 500 μm | Yes (IIIB covers IIIA) |
| IIIB ★ | Non-conductive dusts | Wheat flour, corn starch, grain dust, sugar dust, dried milk powder, cocoa — the standard for food processing dust zones | Yes — YB2 rated IIIB |
| IIIC | Conductive dusts (resistivity < 10⁴ Ω·m) | Aluminium powder, magnesium powder, carbon black — not typical in flour mills | No (requires IIIC motor) |
Flour, starch, grain dust, sugar, dried milk, and cocoa are all Group IIIB (non-conductive) dusts. The Korea Ever-Power YB2 series with Ex t IIIB T135 or Ex d IIB T4 certification covers all standard flour mill and grain processing dust zone motor applications. Confirm the dust classification document for the specific facility to verify that no IIIC dusts (aluminium or magnesium powder) are handled in the same area before specifying the motor group.
4. T135°C: Surface Temperature Limit for Flour Dust
The T135°C surface temperature class limits the motor enclosure surface temperature to 135°C maximum under all operating conditions including maximum ambient temperature and continuous full-load operation. This provides a minimum safety factor of 55°C below the wheat flour layer MIT of 190°C (at 5 mm dust layer depth), and a much larger safety factor below the cloud ignition temperature of 380°C.
Frame temperature: 80–110°C at rated load
Flour dust layer accumulates on motor frame
Frame temperature at moderate overload: 120–150°C
Wheat flour MIT at 5 mm depth: 190°C
Safety margin: only 40–70°C — inadequate
Flour layer MIT: 190°C
Safety margin: 55°C
Overload protection: PTC thermistors cut motor before 135°C is reached
Dust exclusion: IP55 prevents dust from entering motor interior
Compliant with IEC 60079-31 and EN ATEX requirements
5. Motor Certification Options for Flour Dust Zones
Ex t (dust-protected enclosure) is the standard certification type for motors in dust zones. The Ex t motor has an IP6X enclosure (dust-tight) that prevents dust from entering the motor, and is tested and certified to limit its maximum surface temperature to the stated T class (T135 for flour mill applications). The Ex t approach is simpler and lighter than Ex d flameproof, and is the most common specification for new flour mill motor installations. The motor does not need to be explosion-proof against internal ignition because there is no flammable gas inside the motor — the hazard is entirely external dust, and the IP6X enclosure and T135 surface limit are the two controls that eliminate the ignition risk.
A gas-rated Ex d IIB T4 motor can also be used in dust Zone 21 and Zone 22 under IEC 60079-14, provided the T4 surface temperature limit (135°C) meets the requirement for the specific dust (it does for wheat flour: MIT 190°C, with 55°C margin). The Ex d motor has IP55 or IP65 which prevents dust entry, and the T4 surface temperature limit is identical to the T135 limit for dust. This dual certification means that in facilities that handle both flammable gases and combustible dusts — for example, a combined grain store and solvent extraction plant — a single motor type (Ex d IIB T4) can serve both the gas Zone 1/2 and the dust Zone 21/22 requirements. The Korea Ever-Power YB2 Ex d IIB T4 is accepted for installation in dust Zone 21 and Zone 22 as well as gas Zone 1 and Zone 2.
6. Korea Ever-Power YB2 for Flour Mill Applications
The Korea Ever-Power YB2 Ex d IIB T4 is the correct motor specification for flour mill roller rooms, sifter rooms, and grain intake areas classified as Zone 21 or Zone 22 under IEC 60079-10-2. The IP55 cast iron enclosure prevents flour dust from accumulating inside the motor (eliminating the internal insulation degradation mode) and the Ex d T4 certification limits the external surface temperature to 135°C (providing the minimum 55°C safety factor below the wheat flour layer MIT). The YB2 is certified to both ATEX and IECEx standards and is accepted by dust explosion competent authorities in the EU, UK, Australia, and export markets worldwide. The complete YB2 range is available in the explosion-proof motor product section.
| Certification | ATEX + IECEx: Ex d IIB T4 |
| Dust zone suitability | Zone 21 and Zone 22 |
| Surface temp limit | T4 ≡ T135°C max |
| Dust group | IIIA and IIIB (not IIIC) |
| Enclosure | IP55 cast iron |
| Power range | 0.55–200 kW |
| Flour dust MIT safety | 55°C below layer MIT |
| Thermal protection | PTC thermistors (recommended) |
7. Flour Mill and Grain Processing Application Areas
Wheat berry roller mill drive motors in the milling floor are Zone 21 within 1 m of the mill inlet and outlet. YB2 4-pole 11 to 55 kW, IP55, Ex d IIB T4. High starting torque required for loaded mill start. Soft-starter recommended for mills above 22 kW to limit starting current and belt tension transient.
Plansifter frame drive motor and sifter conveyor drives in the sifting room. YB2 4-pole 0.75 to 5.5 kW. The sifter frame vibrates during operation — motor mounting must use anti-vibration isolators to prevent motor fastener fatigue from the continuous vibration load of the sifting machine frame.
Bucket elevator drive head motors for grain and flour vertical conveyance. The boot section (bottom) of the bucket elevator is Zone 20 internally; the surrounding area is Zone 21. Motor typically mounted at the head (top) in Zone 21 or Zone 22. YB2 4-pole 3.0 to 30 kW. Anti-runback device on elevator drive mandatory.
Horizontal screw conveyor for flour and middlings transfer between mill sections. General milling floor area is Zone 22. YB2 4-pole 1.5 to 15 kW paired with NMRV or inline helical gearbox. YB2 Ex d T4 specification covers Zone 22 dust requirements.
8. Frequently Asked Questions
Edited by Cxm
