Before selecting, installing, or inspecting any Ex equipment, you need to understand the science behind explosive atmospheres — what makes gases ignite, how areas are classified by risk, and how equipment is matched to those risks. This unit covers the foundation knowledge required for all subsequent units.
Learning Objectives
Describe the three elements of the fire triangle and explain how each Ex protection concept removes one element
State the LEL and UEL for common gases including propane, methane, ethylene, hydrogen and acetylene
Explain how gas density determines where detectors must be positioned and where vapours collect
Define ignition temperature and select the correct T-rating for a given gas
Describe the MIC and MESG test methods and link them to gas group subdivision IIA, IIB, IIC
State the three effects of oxygen enrichment on a flammable atmosphere
Define Zone 0, Zone 1 and Zone 2 in terms of frequency and duration of explosive atmosphere
Select equipment by gas group, T-rating and IP rating for a given area classification
The Fire Triangle
Three elements must co-exist for combustion: fuel (gas, vapour, mist or dust), oxygen (~21% in air), and an ignition source (arc, spark, hot surface). Each Ex protection concept removes or controls one side of this triangle.
Flammable Limits and Gas Properties
The LEL is the minimum gas concentration that forms an explosive mixture. The UEL is the maximum. Between these limits is the flammable range. Hydrogen (4–77%) is more dangerous than propane (1.7–10.9%) due to its wider range. Gases with ρ < 1 (methane 0.55, hydrogen 0.07) rise; gases with ρ > 1 (propane 1.56, H₂S 1.19) sink and collect in low-lying areas.
Key Rule — T-Rating
Equipment surface temperature (T-rating) must never reach the ignition temperature of the gas present. Carbon disulphide ignites at 95°C — only T6 (85°C max) is acceptable. Hydrogen ignites at 560°C — any T-rating is acceptable.
Gas Groups and Oxygen Enrichment
Group II gases are subdivided by how easily they ignite: IIA (MESG >0.9mm, propane), IIB (0.5–0.9mm, ethylene), IIC (≤0.5mm, hydrogen, acetylene). Equipment must be rated for the area gas group or higher. Oxygen enrichment lowers ignition temperature, widens the flammable range, and reduces the energy required for ignition.
Area Classification — Zones
0
Continuous
Atmosphere present continuously or for long periods
>1000 hrs/year
1
Occasional
Atmosphere likely to occur in normal operation
10–1000 hrs/year
2
Unlikely
Not expected in normal operation; brief if it occurs
0–10 hrs/year
Equipment Selection
Equipment is selected by: gas group (must equal or exceed area group), T-rating (max surface temp must be below gas ignition temp), and IP rating (ingress protection must meet environmental requirement). Equipment marked IIC may be used in IIA, IIB, or IIC areas; IIA-marked equipment is for IIA areas only.
Quick Check — 5 Questions
Test key concepts from this unit before moving on.
Q1Summary
A process area contains diethyl ether (ignition temperature 160°C). Which T-ratings are acceptable?
T4 allows 135°C max, T5 100°C, T6 85°C — all below 160°C. T3 allows 200°C which exceeds the ignition temperature. T1 (450°C) and T2 (300°C) are also too hot.Ref: IEC 60079-0 — Temperature Classification
Q2Summary
Hydrogen has a density (ρ) of 0.07. Where will hydrogen vapour collect in a building?
Hydrogen is much lighter than air (ρ = 0.07 vs air = 1.0). It rises rapidly and accumulates at ceiling level and in roof spaces. Gas detectors for hydrogen must be positioned at the highest point of the space.Ref: IEC 60079-20 — Gas Properties
Q3Summary
An area is classified Zone 1, IIB, T4. Equipment marked Ex d IIC T3 Gb is available. Is it suitable?
T3 allows 200°C maximum surface temperature. T4 requires equipment surface to stay below 135°C. T3 (200°C max) does NOT meet T4 requirement — the surface could reach 200°C which exceeds the T4 limit. IIC covers IIB, and Gb is Zone 1 suitable, but the T-class is wrong.Ref: IEC 60079-0 — T-Rating Selection
Q4Summary
What are the three effects of oxygen enrichment on a flammable atmosphere?
Oxygen enrichment: (1) lowers ignition temperature, (2) significantly raises the UEL widening the flammable range, (3) allows ignition with much lower electrical energy. Standard Ex equipment tested in normal air (21% O₂) may not provide adequate protection in oxygen-enriched environments.Ref: IEC 60079 Unit 1 — Oxygen Enrichment
Q5Summary
A Zone 0 location requires which Equipment Protection Level (EPL) as minimum?
Zone 0 requires EPL Ga — protection safe with two simultaneous independent faults. Only Ex ia provides EPL Ga. EPL Gb (Zone 1 minimum) and Gc (Zone 2 only) are insufficient for Zone 0.Ref: IEC 60079-0 — EPL and Zone Suitability
EX Academy — independent CompEx-style preparation. Not official CompEx course materials.
Unit 1 · Ex01–Ex04 · Full Manual
General Principles of Explosive Atmospheres
Before selecting, installing, or inspecting any Ex equipment, you need to understand the science behind explosive atmospheres — what makes gases ignite, how areas are classified by risk, and how equipment is matched to those risks. This unit covers the foundation knowledge required for all subsequent units.
Learning Objectives
Describe the three elements of the fire triangle and explain how each Ex protection concept removes one element
State the LEL and UEL for common gases including propane, methane, ethylene, hydrogen and acetylene
Explain how gas density determines where detectors must be positioned and where vapours collect
Define ignition temperature and select the correct T-rating for a given gas
Describe the MIC and MESG test methods and link them to gas group subdivision IIA, IIB, IIC
State the three effects of oxygen enrichment on a flammable atmosphere
Define Zone 0, Zone 1 and Zone 2 in terms of frequency and duration of explosive atmosphere
Select equipment by gas group, T-rating and IP rating for a given area classification
1.2 Gases and Vapours
1.2.1 Explosive Atmospheres
Flammable gases mixed with air burn fiercely when ignited. The resulting rapid expansion of hot gas develops into an explosion. IEC 60079 is the international standard series governing all electrical equipment in explosive atmospheres.
The research and development that underpins IEC 60079 covers the behaviour of gas/air combinations across manufacturing and process industries worldwide. This course covers explosive atmospheres where the fuel is gas or vapour.
1.2.2 The Fire Triangle
Combustion requires three conditions to be present simultaneously:
🔥
Fuel
Gas, vapour, mist or dust mixed with air within ignitable concentration limits
💨
Oxygen
Approximately 21% by volume in normal atmospheric air
⚡
Ignition Source
Arc, spark, naked flame, or hot surface with sufficient energy
If the mixture is within the flammable range and the ignition source has sufficient energy, combustion occurs. When the combustion is self-sustaining after ignition, the mixture is described as an explosive atmosphere.
Ignition and explosion can be prevented or controlled by:
Removing one or more elements — isolating the ignition source from the gas/air mixture
Keeping the energy of the ignition source below the minimum ignition energy of the gas
Allowing an explosion to occur inside an enclosure and containing it robustly (Ex d)
These three approaches correspond to the Ex protection concepts: Ex d (contain), Ex i (limit energy), Ex p (exclude fuel by pressurisation), Ex e (prevent ignition sources forming).
1.2.3 Characteristics of Gases and Vapours
IEC 60079-20-1 provides data on flammable gases and vapours for equipment selection. The key properties are: flammable limits (LEL and UEL), ignition temperature, ignition energy (MIE), density, and flash point.
Flammable (Explosive) Limits — LEL and UEL
A flammable gas mixed with air will only ignite when the gas concentration falls within specific limits. Outside this range the mixture either lacks sufficient fuel (below LEL) or sufficient air (above UEL).
Lower Explosive Limit (LEL)
The concentration of flammable gas, vapour or mist in air below which an explosive gas atmosphere will not be formed. Below the LEL the mixture is too lean to burn — insufficient fuel and/or too much air. Operational safety is maintained by keeping concentrations well below the LEL. Gas detection systems are calibrated as a percentage of LEL (alarm typically at 10–20% LEL, shutdown at 40–60% LEL).
Upper Explosive Limit (UEL)
The concentration of flammable gas, vapour or mist in air above which an explosive gas atmosphere will not be formed. Above the UEL the mixture is too rich to burn — insufficient air. Important: a mixture that is above the UEL and then becomes diluted as it disperses will pass back through the explosive range before becoming fully safe — it can ignite during that passage.
The wider the gap between LEL and UEL (the flammable range), the more dangerous the gas — it is flammable over a greater range of concentrations.
Material
LEL % vol
UEL % vol
Flammable Range
Gas Group
Propane
1.7
10.9
9.2%
IIA
Methane
4.4
17.0
12.6%
IIA
Ethylene
2.3
36.0
33.7%
IIB
Hydrogen Sulphide
4.0
45.5
41.5%
IIB
Hydrogen
4.0
77.0
73.0%
IIC
Acetylene
2.3
100
97.7%
IIC
Diethyl Ether
1.7
39.2
37.5%
IIB
Kerosene
0.7
5.0
4.3%
IIA
Carbon Disulphide
0.6
60.0
59.4%
IIC
Density
Whether a gas rises or falls determines where it collects and where detectors must be placed. Density is expressed relative to air (ρ = 1). Gases lighter than air (ρ < 1) rise; heavier gases (ρ > 1) sink to floor level and collect in low-lying areas including cable trenches, drains, and below-grade spaces.
Material
Relative Density ρ
Behaviour in Atmosphere
Hydrogen
0.07
Rises rapidly — accumulates at ceiling height, roof spaces
Methane
0.55
Rises — detectors at ceiling level
Acetylene
0.90
Near neutral — disperses at any level
Ethylene
0.97
Near neutral
Air
1.00
Reference
Hydrogen Sulphide
1.19
Sinks — collects in pits, trenches, drains
Propane
1.56
Sinks — drifts at ground level, enters low-lying areas
Diethyl Ether
2.55
Heavy sinker — accumulates in lowest points
Carbon Disulphide
2.64
Heavy sinker
⚠ Heavy Vapour Hazard in Cable Trenches and Drains
Gases heavier than air (ρ > 1) drift along floor level and can travel long distances before dispersing. They collect in cable trenches, drains, pits, and below-grade areas. These locations may need to be classified as hazardous zones even when they appear to be remote from any release source. Ventilation directed to remove vapours at low level is essential in areas where heavy vapours may collect.
Ignition Temperature
The ignition temperature is the minimum temperature at which a flammable material will spontaneously ignite without any spark or flame, when mixed with air at normal atmospheric pressure. It is also called the auto-ignition temperature.
Electrical equipment generates heat during operation. The equipment must be selected so that no surface temperature reaches or exceeds the ignition temperature of any gas that may be present. This is expressed through the T-rating (temperature class) on the equipment nameplate.
Material
Ignition Temperature °C
T-rating Required (minimum)
Carbon Disulphide
95°C
T6 only (85°C max surface)
Diethyl Ether
160°C
T4, T5, or T6
Kerosene
210°C
T3, T4, T5, or T6
Hydrogen Sulphide
270°C
T3 or above
Acetylene
305°C
T2 or above
Ethylene
425°C
T2 or above
Propane
470°C
Any T-rating (T1–T6)
Methane
537°C
Any T-rating (T1–T6)
Hydrogen
560°C
Any T-rating (T1–T6)
Ignition Energy
Ignition energy is the electrical spark energy (in joules) required to ignite the most easily ignited concentration of the test gas in air. Every flammable material has a Minimum Ignition Energy (MIE) — below this level, a spark cannot cause ignition regardless of gas concentration.
Material
MIE (μJ)
Gas Group
Hydrogen
19 μJ
IIC
Acetylene
28 μJ
IIC
Ethylene
85 μJ
IIB
Propane
260 μJ
IIA
Methanol
290 μJ
IIA
The lowest ignition energy for any gas typically occurs at a concentration approximately midway between the LEL and UEL. Ignition energy is directly linked to the Minimum Igniting Current (MIC) ratio, which is used to classify gases for intrinsically safe (Ex i) equipment design.
Flash Point
Flash point applies to flammable liquids, not gases. It is the lowest temperature at which a liquid gives off sufficient vapour to form an ignitable mixture at the liquid surface when a spark or flame is applied.
Flash Point
The lowest temperature at which sufficient vapour is given off a liquid to form a flammable mixture with air that can be ignited by an arc, spark or naked flame. A flammable liquid is defined as a liquid with a flash point below 38°C (100°F). The flash point indicates how readily a liquid will produce ignitable vapour at ambient temperature.
Fuel
Flash Point °C
Practical Significance
Gasoline
-46°C
Ignitable vapour at all normal temperatures — always hazardous
Carbon Disulphide
-30°C
Extreme hazard — vapour at all operational temperatures
Diethyl Ether
-45°C
Ignitable vapour at all normal temperatures
Benzene
-11°C
Hazardous even in cold weather
Acetone
-19°C
Hazardous below freezing
Kerosene
38°C
Safer at ambient — but hot surfaces or pressurised release reduce effective flash point
Pressure Release Effect on Flash Point
When a flammable liquid is discharged under pressure — from a spray nozzle, a pressurised leak, or a jet — its effective flash point is significantly reduced. Kerosene (flash point 38°C) discharged as a fine mist under pressure can form ignitable vapour at temperatures well below 38°C. Area classification assessments for pressurised liquid handling systems must account for this effect.
1.3 Gas Groups and Subdivisions
Gas Group II (surface industries) is subdivided into IIA, IIB, and IIC based on how easily gases can be ignited. This subdivision determines the required Ex equipment type — specifically the flamepath dimensions for Ex d equipment and the energy limits for Ex i equipment.
Gas Group I covers underground mining environments where the primary hazard is methane (firedamp). Gas Group II covers all surface industries. Gas Group III covers dust hazards. This course deals exclusively with Group II gases.
1.3.1 Test Methods for Gas Subdivision
MESG — Maximum Experimental Safe Gap (for Ex d equipment)
A sphere containing the test gas at its most explosive concentration is fitted with flanges whose gap can be varied. The gas inside the sphere is ignited. The MESG is the maximum gap width that prevents the ignition from propagating to the gas/air mixture outside the sphere. The more dangerous the gas, the smaller its MESG — requiring tighter flamepath dimensions in Ex d equipment. The testing apparatus uses an 8-litre sphere with 25mm flanges.
MIC Ratio — Minimum Igniting Current Ratio (for Ex i equipment)
An inductive spark discharge apparatus measures the minimum electrical current needed to ignite the test gas. This is compared to the MIC for laboratory methane to give the MIC ratio. The lower the MIC ratio, the less current is needed to cause ignition, and the more stringent the energy limitation required in IS circuits. Equipment Group IIA requires MIC ratio >80%; IIB requires 45–80%; IIC requires <45%.
Equipment Group
Representative Gas
MESG (mm)
Max Working Gap (mm)
MIE (μJ)
MIC Ratio
Group I
Methane (Firedamp)
1.14
0.5
260
—
Group IIA
Propane
0.92
0.4
160
0.82
Group IIB
Ethylene
0.65
0.2
95
0.53
Group IIC
Hydrogen
0.29
0.1
20
0.25
Group IIC
Acetylene
0.37
0.28
19
—
MESG subdivision boundaries for Ex d equipment:
IIA — MESG greater than 0.9mm; MIC ratio greater than 80%
IIB — MESG 0.5mm to 0.9mm; MIC ratio 45% to 80%
IIC — MESG 0.5mm or less; MIC ratio less than 45%
Gas Group Equipment Compatibility
Equipment marked II (no subdivision) — suitable for all IIA, IIB, IIC atmospheres. Equipment marked IIC — suitable for IIA, IIB, and IIC atmospheres. Equipment marked IIB — suitable for IIA and IIB atmospheres only. NOT for IIC. Equipment marked IIA — suitable for IIA atmospheres only. Equipment marked II (XXX) — approved for specific named gas only.
1.4 Oxygen Enrichment
Normal atmospheric oxygen content is approximately 20.95%. Locations where this is exceeded — gas manufacturing plants, hospital operating theatres, areas around oxy-acetylene welding equipment — are described as oxygen-enriched.
Oxygen enrichment creates three compounding effects that make a flammable atmosphere significantly more dangerous than normal:
#
Effect
Example Data
1
Lowers the ignition temperature of flammable materials
IS energy limits certified at normal 21% O₂ may be insufficient
⚠ Standard Ex Equipment Not Suitable for Oxygen-Enriched Atmospheres
All Ex-certified equipment is designed, tested, and certified under normal atmospheric conditions (approximately 21% oxygen). Equipment for use in oxygen-enriched atmospheres must be specially tested and specifically certified for that condition. Never assume that standard Ex equipment provides adequate protection in oxygen-enriched environments — specialist engineering assessment and dedicated certification are required.
1.5 Sources of Ignition
Sources of ignition in hazardous areas are numerous. As far as this course is concerned, the primary concern is electrical equipment — the Ex protection concepts are all designed to prevent electrical equipment from providing a sufficient ignition source.
Electrical Sources
Non-Electrical Sources
Electrical arcs and sparks (switching, short circuits)
Thermite action (iron oxide + aluminium)
Hot surfaces from equipment or cables
Frictional sparks (grinding, impact between ferrous materials)
Area classification identifies locations where explosive gas atmospheres may be present and ranks them by probability, frequency and duration of occurrence. The result is a zone map that determines what equipment may be installed in each area. The applicable standard is IEC 60079-10.
Hazardous Area
An area in which an explosive gas atmosphere is present, or may be expected to be present, in quantities such as to require special precautions for the construction, installation and use of equipment. (IEC 60079-10)
Non-Hazardous Area
An area in which an explosive gas atmosphere is not expected to be present in quantities requiring special precautions. Equipment rooms within a hazardous area may be rendered non-hazardous by maintaining them at slight positive pressure with clean air drawn from a non-hazardous source, with entry via an airlock.
1.6.1 Zones
Zone classification represents the probability, frequency and duration of an explosive gas atmosphere. Classification is based on three factors: grade of release (continuous, primary, or secondary), presence and type of ventilation, and the expected extent and volume of each release.
0
Continuous
Explosive atmosphere present continuously, for long periods, or frequently
>1000 hrs/year (API RP 505)
EPL Ga required · Ex ia only
Typical: vapour space above liquid inside a storage tank or mixing vessel
1
Occasional
Explosive atmosphere likely to occur occasionally during normal operation
10–1000 hrs/year (API RP 505)
EPL Gb minimum · Ex d/e/p/de
Typical: around pump seals, around regularly opened vessel lids, at regular vent outlets
2
Unlikely
Explosive atmosphere not expected in normal operation; brief if it does occur
0–10 hrs/year (API RP 505)
EPL Gc minimum · Ex n and above
Typical: around outdoor pump seals with robust containment, areas surrounding Zone 1
The API RP 505 annual hours are a useful guide. IEC 60079-10-1 does not itself specify time thresholds — it classifies by grade of release and ventilation. The IEC approach is based on principles; the hours are for context.
The hazardous area drawings for any plant show the zone boundaries, the required equipment gas group, temperature class, and EPL for each zone. A typical drawing zone designation reads:
Zone 1 · IIC · T6 · EPL G'b'
The method of indicating zones on drawings may vary between clients and standards. Always check the notes, symbols, and legend key on any specific drawing before assuming a standard representation.
Zone 2 is frequently shown as the area surrounding a Zone 1 region, acting as a buffer. Flanged joints in pipework are typically Zone 2 (possible release at flange); welded pipe is non-hazardous (no release point). Pump glands may be Zone 1 or Zone 2 depending on the type of seal and expected leak frequency.
1.7 Equipment Selection
Equipment selection for hazardous areas requires three parameters to be verified correctly: gas group, temperature class, and IP rating. All three must be suitable — an error in any one creates a safety risk.
1.7.1 By Equipment Group
Electrical equipment for potentially explosive atmospheres is divided into Groups based on the type of hazard:
Group
Application
Subdivision
Group I
Underground mining — methane (firedamp) and coal dust
None
Group II
Surface industries — wide range of flammable gases and vapours
Equipment group marking on the nameplate and its area application:
Equipment marked II (no subdivision) — suitable for all Group II atmospheres (IIA, IIB, and IIC)
Equipment marked IIA — IIA areas only
Equipment marked IIB — IIA and IIB areas
Equipment marked IIC — all Group II areas (IIA, IIB, IIC)
Equipment marked II (XXX) — approved only for the specific gas named by chemical formula or hazard name
1.7.2 By Temperature Classification (T-Rating)
Temperature classification divides equipment into six classes based on the maximum surface temperature of any relevant part during operation at maximum design rating and at maximum ambient temperature (normally 40°C in the UK unless otherwise marked).
Temperature Class
Maximum Surface Temperature
T1
450°C
T2
300°C
T3
200°C
T4
135°C
T5
100°C
T6
85°C
The UK ambient temperature range for Ex equipment is -20°C to +40°C unless otherwise marked. Equipment for hot climates (Middle East) requires ambient ratings above 40°C. Arctic equipment may need to be rated as low as -50°C — requiring special materials and extensive certification testing.
The T-rating is based on the maximum ambient for which the equipment is rated. Equipment operating in a hotter environment than its rated ambient may produce higher surface temperatures than the T-rating guarantees — the equipment's marked T-class is only valid within its rated ambient range.
Permitted T-ratings for each ignition temperature range:
Ignition Temperature of Gas in Area
Acceptable Equipment T-Ratings
Above 450°C (e.g. propane 470°C, hydrogen 560°C)
T1, T2, T3, T4, T5, or T6 — any class
300°C to 450°C (e.g. acetylene 305°C)
T2, T3, T4, T5, or T6
200°C to 300°C (e.g. hydrogen sulphide 270°C)
T3, T4, T5, or T6
135°C to 200°C (e.g. diethyl ether 160°C)
T4, T5, or T6
100°C to 135°C
T5 or T6
85°C to 100°C (e.g. carbon disulphide 95°C)
T6 only
1.7.3 By IP Rating (Ingress Protection)
Enclosures are classified by their resistance to ingress of solid objects and water using the IP code — two letters followed by two numbers (e.g. IP66).
First Digit — Solid Objects
0
No protection
1
Objects >50mm (e.g. back of hand)
2
Objects >12mm (e.g. finger)
3
Objects >2.5mm (tools)
4
Objects >1mm (thin wire)
5
Dust protected — limited ingress, no harmful deposit
6
Dust tight — complete exclusion of dust
Second Digit — Water
0
No protection
1
Dripping water — vertical
2
Dripping water — 15° tilt
3
Spraying water
4
Splashing water — any direction
5
Water jets — any direction
6
Powerful water jets
7
Temporary immersion 0.15–1m / 30 min
8
Continuous immersion — depth specified by manufacturer
The IP rating of Ex equipment must be maintained throughout its service life. Any damage to seals, gaskets, diffusers, or drain plugs that compromises the IP rating must be remedied. For offshore and outdoor installations IP55 is typically the minimum; IP66 is standard for most field equipment. For instruments installed at low level where flooding is possible, IP67 or IP68 may be required.
Unit 1 Knowledge Check — 10 Questions
CompEx-style questions covering the full unit content.
Q1Unit {num}
Which gas has the widest flammable range (largest difference between LEL and UEL)?
Acetylene has a flammable range of 97.7% (2.3% to 100%). This makes it the most dangerous in terms of concentration range — it is explosive in almost any concentration with air. Hydrogen (73%) is also extremely wide-ranging. Propane (9.2%) and methane (12.6%) are comparatively narrow.Ref: IEC 60079-20-1 — Gas Data
Q2Unit {num}
Carbon disulphide has an ignition temperature of 95°C. What is the ONLY acceptable T-class for equipment in a carbon disulphide atmosphere?
T6 allows a maximum surface temperature of 85°C, which is below carbon disulphide's ignition temperature of 95°C. T5 (100°C max) would be too hot — a T5-rated piece of equipment could reach 100°C surface temperature, which exceeds the 95°C ignition threshold. T6 is the only acceptable choice.Ref: IEC 60079-0 — Temperature Classification
Q3Unit {num}
Propane has a relative density of 1.56. Which of the following best describes its behaviour in a confined space?
Propane (ρ = 1.56) is significantly heavier than air (ρ = 1.0). It sinks, drifts along the floor, and can travel considerable distances before dispersing. It collects in cable trenches, drains, pits, and other below-grade spaces. Gas detectors for propane must be positioned at low level. Hydrogen (ρ = 0.07) is the contrast — it rises to ceiling level.Ref: IEC 60079-20 — Gas Density Data
Q4Unit {num}
The MESG test determines gas subdivision for which type of Ex protection?
MESG (Maximum Experimental Safe Gap) is used to classify gases for flameproof (Ex d) equipment design. The smaller the MESG, the tighter the flamepath required. IIC gases (hydrogen, MESG 0.29mm) require the tightest flamepaths. MIC ratio is the test method used for intrinsic safety (Ex i) equipment.Ref: IEC 60079-20 — MESG Test Method
Q5Unit {num}
Equipment marked IIB is installed in a Zone 1 area containing hydrogen. What is the deficiency?
Hydrogen is classified IIC (MESG ~0.29mm). IIB equipment has flamepath gaps up to 0.9mm — nearly three times wider than hydrogen's MESG. Hot gases from an internal explosion in an IIB enclosure can propagate through the IIB flamepath and ignite hydrogen in the surrounding atmosphere. This is a Category X deficiency requiring immediate withdrawal from service.Ref: IEC 60079-1 — Gas Group Selection
Q6Unit {num}
An area is classified Zone 1, IIA, T3. Which of the following is TRUE about the equipment T-rating selection?
T3 requires equipment max surface temperature ≤200°C. Any T-rating of T3 or above (T3, T4, T5, T6) provides a max surface temperature of 200°C or less — all are acceptable in T3 areas as long as the equipment surface stays below the gas ignition temperature. T4 (135°C), T5 (100°C) and T6 (85°C) are all more stringent than T3 and are acceptable. T1 (450°C) and T2 (300°C) would both exceed the T3 limit.Ref: IEC 60079-0 — T-Rating Table
Q7Unit {num}
What is the definition of the LEL?
The LEL is the minimum gas concentration below which an explosive atmosphere cannot form. Below the LEL there is insufficient fuel to sustain combustion. Above the LEL and below the UEL is the flammable range. Gas detection systems alarm at low percentages of LEL (typically 10–20%) to warn well before the explosive range is reached.Ref: IEC 60079-20 — Flammable Limits
Q8Unit {num}
In an oxygen-enriched atmosphere compared to normal air, what happens to the flammable range of methane?
Oxygen enrichment significantly raises the UEL of most gases. Methane UEL goes from 17% in normal air to approximately 79% in enriched oxygen — dramatically widening the flammable range. The LEL rises slightly. The combined effect is a much wider range of concentrations that can ignite.Ref: IEC 60079 Unit 1 — Oxygen Enrichment
Q9Unit {num}
Zone 0 is typically found in which location in a storage tank containing flammable liquid?
Zone 0 is found inside the tank in the vapour space above the liquid surface — where flammable vapour is continuously present at concentrations within or above the flammable range. The bunded area around the tank is typically Zone 1 or Zone 2. The filling connection area is typically Zone 1 (releases occur during filling operations).Ref: IEC 60079-10 — Zone Classification
Q10Unit {num}
Equipment marked "Ex d IIC T4 Gb" is installed in an area classified Zone 1, IIB, T4. Which of the following is correct?
IIC equipment covers all Group II gas groups including IIB. T4 (135°C max surface) exactly meets a T4 requirement. EPL Gb (High protection) is the correct level for Zone 1. All three parameters are satisfied. This is a compliant selection — IIC rated equipment in an IIB area is acceptable (over-specified for gas group but fully compliant).Ref: IEC 60079-0 — Equipment Selection