National Electrical Code (NEC) Division and Zone Classification Systems
The National Electric Code (NEC) or NFPA 70 defines area classification and installation principles. The principles of the NEC Division and Zone classification systems for hazardous locations are applied in countries around the globe.
Specifically, Article 500 describes the NEC Division classification system, while Articles 505 and 506 describe the NEC Zone classification system. The NEC Zone classification system was created to provide multinational companies with a system that could be harmonized with IEC classification system and therefore reduce the complexity of management.
NEC Division Classification System
Classes define the type of hazard in terms of whether it is a gas or vapour, a combustible or conductive dust or an ignitable fibre or flying.
Divisions define the probability of the presence of the hazard being present during normal or abnormal conditions.
Groups classify the exact type and nature of the hazardous substance.
| Class | Definition |
|---|---|
| I | A location in which a flammable gas or vapour is or may be present in sufficient quantity to cause an explosive atmosphere. |
| II | A location in which a conductive or combustible dust is or may be present in sufficient quantity to cause a fire or an explosive hazard. |
| III | A location in which easily ignitable fibres or flyings are present in sufficient quantity to present a serious risk of fire. |
| Division | Definition |
|---|---|
| 1 | The defined hazard is present during normal operational conditions. |
| 2 | The defined hazard is present only during abnormal conditions such as equipment failure. |
Gas and Dust Groups under NEC Division System:
| NEC Division System Gas & Dust Groups | ||
|---|---|---|
| Area | Group | Representative Materials |
| Class I, Division 1 & 2 |
A | Acetylene |
| B | Hydrogen | |
| C | Ethylene | |
| D | Propane | |
| Class II, Division 1 & 2 |
E (Division 1 only) | Metal dusts, such as magnesium (Division 1 only) |
| F | Carbonaceous dusts, such as carbon & charcoal | |
| G | Non-conductive dusts, such as flour, grain, wood & plastic | |
| Class III, Division 1 & 2 |
None | Ignitible fibers/flyings, such as cotton lint, flax & rayon |
NEC & IEC Zone Classification System
The type of hazard will be in the form of either a gas or vapour or a dust or fibre. The classification of these hazardous is primarily divided into two groups depending on whether it is in a mining or above surface industry.
Group I – electrical equipment for use in mines and underground installations susceptible to firedamp
Group II and Group III – electrical equipment for use in surface installations
Group II gases are grouped together based upon the amount of energy required to ignite the most explosive mixture of the gas with air.
Group III dusts are subdivided according to the nature of the explosive atmosphere for which it is intended.
| Mining | Surface Industry | |||
|---|---|---|---|---|
| Group I | Group II | Group III | ||
| Electrical equipment for mines susceptible to firedamp | Electrical equipment for places with an explosive gas atmosphere | Electrical equipment for places with an explosive dust atmosphere | ||
| Sub-Div | Ignition Enery | Sub-Div | Explosive Atmosphere | |
| II A | 260 µJ | III A | Combustion flyings | |
| II B | 95 µJ | III B | Non-conductive dust | |
| II C | 18 µJ | III C | Conductive dust | |
Gas and Dust Groups under NEC & IEC Zone System:
The below groups are formed in order of how explosive the material would be if it was ignited, with IIC being the most explosive Zone system gas group and IIA being the least. The groups also indicate how much energy is required to ignite the material by energy or thermal effects, with IIA requiring the most energy and IIC the least for Zone system gas groups.
| NEC & IEC Zone System Gas & Dust Groups | ||
|---|---|---|
| Area | Group | Representative Materials |
| Zone 0, 1 & 2 | II C | Acetylene & Hydrogen (equivalent to NEC Class I, Groups A and B) |
| II B + H2 | Hydrogen (equivalent to NEC Class I, Group B) |
|
| II B | Ethylene (equivalent to NEC Class I, Group C) |
|
| II A | Propane (equivalent to NEC Class I, Group D) |
|
| Zone 20, 21 & 22 | III C | Conductive dusts, such as magnesium (equivalent to NEC Glass II, Group E) |
| III B | Non-conductive dusts, such as flour, grain, wood & plastic (equivalent to NEC Class II, Groups F and G) |
|
| III A | Ignitible fibers/flyings, such as cotton lint, flax & rayon (equivalent to NEC Class III) |
|
| Mines susceptible to firedamp | I (IEC only) | Methane |
Group IIC is the most severe Zone system gas group. Equipment marked as suitable for Group IIC is also suitable for IIB and IIA. Equipment marked as suitable for IIB is also suitable for IIA but NOT for IIC. If equipment is marked, for example, Ex e II T4 then it is suitable for all subgroups IIA, IIB and IIC.
Temperature Class Rating for Group II Electrical Apparatus
The below table tells us that the surface temperature of a piece of electrical equipment with a temperature classification of T3 will not rise above 200 °C.
| USA °C | International (IEC) °C |
Germany °C Continuous – Short Time |
|
|---|---|---|---|
| T1 – 450 | T3A – 180 | T1 – 450 | G1: 360 – 400 |
| T2 – 300 | T3B – 165 | T2 – 300 | G2: 240 – 270 |
| T2A – 280 | T3C – 160 | T3 – 200 | G3: 160 – 180 |
| T2B – 260 | T4 – 135 | T4 – 135 | G4: 110 – 125 |
| T2C – 230 | T4A – 120 | T5 – 100 | G5: 80 – 90 |
| T2D – 215 | T5 – 100 | T6 – 85 | |
| T3 – 200 | T6 – 85 | ||