This glossary defines the key terms used in hazardous area classification, explosion-proof equipment certification, and industrial surveillance system design. Terms follow NEC (NFPA 70), IEC 60079 series, and ATEX Directive definitions.
A
ATEX
From the French “ATmosphères EXplosibles.” The EU regulatory framework governing equipment intended for use in explosive atmospheres, comprising Directive 2014/34/EU (equipment) and Directive 1999/92/EC (worker protection). ATEX certification is required for equipment sold in the European Union for use in classified areas.
Area Classification
The process of dividing an industrial facility into zones (ATEX/IEC) or divisions (NEC) based on the likelihood and duration of an explosive atmosphere being present. Area classification drawings define where certified equipment is required and what certification level applies.
C
Class I
NEC (National Electrical Code) classification for locations where flammable gases, vapors, or liquids are or may be present in sufficient quantities to produce an ignitable mixture. Class I includes Divisions 1 and 2, and Groups A through D.
Class II
NEC classification for locations where combustible dust is or may be present. Class II Groups: Group E (metal dusts), Group F (carbon black, coal dust), Group G (grain, flour, starch, wood dust).
Class I Division 1 (C1D1)
NEC classification for locations where ignitable gas or vapor concentrations exist continuously, intermittently, or periodically under normal operating conditions. Requires the highest level of protection — equivalent to ATEX Zone 0 and Zone 1 combined. Equipment must be certified for Class I Division 1 (e.g., UL listed or FM approved for C1D1).
Class I Division 2 (C1D2)
NEC classification for locations where flammable gases are handled in closed containers or systems that could only become hazardous in the event of accidental rupture, breakdown, or abnormal operation. Equivalent to ATEX Zone 2. Less restrictive than Division 1; some standard equipment with appropriate protection methods is permitted.
E
Ex d (Flameproof / Explosion-Proof)
IEC 60079-1 protection concept. Equipment enclosed in a housing that can withstand the pressure of an internal explosion and prevent ignition of the surrounding atmosphere. The housing has precision-machined flame paths (gaps) that cool any escaping combustion gases below ignition temperature. The predominant protection method for explosion-proof cameras.
Ex ia / Ex ib (Intrinsic Safety)
IEC 60079-11 protection concept. Limits electrical and thermal energy in a circuit to levels incapable of causing ignition under normal or fault conditions. Ex ia (two faults) is suitable for Zone 0; Ex ib (one fault) for Zone 1. Requires full system certification — camera, cable, zener barrier or galvanic isolator, and power supply must all be part of the certified assembly.
Explosion-Proof
North American term for equipment constructed to withstand an internal explosion and prevent ignition of surrounding flammable atmospheres. Corresponds to the IEC/ATEX “flameproof” (Ex d) protection concept. Explosion-proof equipment must be listed by a recognized testing laboratory (UL, FM, CSA) for the specific Class, Division, and Group of the installation area.
F
Flame Path
The precision-machined gap between mating surfaces (cover and body) of an explosion-proof enclosure. The flame path quenches any escaping combustion gases below the ignition temperature of the surrounding atmosphere before they can exit the enclosure. Minimum flame path dimensions are specified in IEC 60079-1 and UL 1203 based on the volume of the enclosure and the gas group.
G
Gas Group (NEC)
NEC grouping of flammable substances based on their MESG (Maximum Experimental Safe Gap) and MIC ratio, which determine the minimum flame path dimensions required. Groups A (acetylene), B (hydrogen), C (ethylene, ethyl ether), D (propane, methane, gasoline). Group A is most demanding; Group D is least. Equipment certified for a more demanding group is compliant in less demanding groups.
Gas Group (IEC/ATEX)
IEC grouping: Group IIC (hydrogen, acetylene — most demanding), Group IIB (ethylene, ethyl ether), Group IIA (propane, methane — least demanding). IIC equipment is compliant for IIB and IIA applications. Corresponds to NEC groups as follows: IIA ≈ Group D; IIB ≈ Group C; IIC ≈ Groups A and B.
I
IECEx
The IEC System for Certification to Standards Relating to Equipment for Use in Explosive Atmospheres. An international certification scheme based on IEC 60079 standards. IECEx certificates are accepted in over 40 countries and are often specified for international projects where equipment must be acceptable to multiple national regulatory regimes.
IP Rating (Ingress Protection)
IEC 60529 rating system for enclosure protection against solid particles and liquids. Expressed as IP followed by two digits: first digit = solids (0–6), second digit = liquids (0–9K). Common ratings for explosion-proof cameras: IP66 (dust-tight, high-pressure water jets), IP67 (dust-tight, temporary immersion), IP68 (dust-tight, continuous immersion), IP69K (dust-tight, high-pressure/temperature steam cleaning).
Intrinsically Safe
Protection method (Ex ia or Ex ib) that limits electrical energy within equipment and its interconnecting wiring to a level below what is required to ignite a specific hazardous atmosphere. Unlike explosion-proof equipment which contains an explosion, intrinsically safe equipment prevents ignition from occurring. Requires full system certification including barriers and cables.
M
MESG (Maximum Experimental Safe Gap)
The maximum gap between two parallel metal surfaces that prevents flame propagation through the gap when tested with a specific gas/air mixture. MESG determines the minimum acceptable flame path dimensions for explosion-proof enclosures and is the primary basis for gas group classification. Hydrogen has a very small MESG (0.08mm); propane has a much larger MESG (0.92mm).
MSHA (Mine Safety and Health Administration)
US federal agency that regulates health and safety in mines. For electrical equipment used in US underground mines, MSHA approval under 30 CFR Parts 18 and 22 is required — this is separate from and in addition to UL listing. An explosion-proof camera with UL Class I Division 1 certification does not automatically have MSHA approval for underground mine use.
N
NEC (National Electrical Code)
NFPA 70, the National Electrical Code. The US standard for electrical installation safety, including Article 500 (Hazardous Locations), which defines the Class/Division/Group classification system used in North America. Published by the National Fire Protection Association (NFPA) and adopted by most US jurisdictions.
T
T-Code (Temperature Class)
Maximum surface temperature rating for explosion-proof equipment, indicating the highest temperature the equipment’s external surfaces will reach under normal operating conditions. T1 (450°C) through T6 (85°C). The T-code must be lower than the Auto-Ignition Temperature (AIT) of the gases present. Most petroleum gases have AITs above 250°C, making T4 (135°C) or T5 (100°C) sufficient for most oil and gas applications.
Z
Zone 0
ATEX/IEC classification for areas where explosive gas atmospheres are present continuously or for long periods (more than 1,000 hours per year). Requires Category 1G (Ex ia or Ex ma) certified equipment. Typically limited to the interior of process vessels, tanks, and pipes — rarely an outdoor area where cameras are installed.
Zone 1
ATEX/IEC classification for areas where explosive gas atmospheres are likely to occur in normal operation (10–1,000 hours per year). Requires Category 2G (Ex d, Ex e, Ex p, or other approved protection concepts) certified equipment. Equivalent to NEC Class I Division 1. Most explosion-proof cameras are Zone 1 certified.
Zone 2
ATEX/IEC classification for areas where explosive gas atmospheres are unlikely to occur in normal operation, and if they do, only for short periods (less than 10 hours per year). Requires Category 3G (Ex nA, Ex nC, or Zone 1 equipment) certified equipment. Equivalent to NEC Class I Division 2. Less restrictive than Zone 1; more equipment options are available.