Electrical equipment is one of many ignition sources in hazardous areas. This unit covers electrical sparks, hot surfaces, batteries, friction, static electricity, lightning, thermite and pyrophoric reactions, RF radiation, and optical radiation — and the control measures for each.
Learning Objectives
Identify the main sources of ignition in potentially explosive atmospheres
Explain how electrical sparks are generated and why IS circuits operate below 30V
Describe the thermite reaction and why aluminium tools are a hazard in hazardous areas
Explain pyrophoric reactions and the iron sulphide hazard in H₂S service
State the control measures for static electricity including streaming current
Identify RF radiation and optical radiation as potential ignition sources
Electrical Sparks and Hot Surfaces
Electrical sparks are produced when contacts open or close under current and when faults occur. Igniting hydrogen/air requires only 20μJ — a 0.1ms break in a 20mA, 10V circuit. Flammable gases ignite more readily at high voltages — IS circuits are rarely designed above 30V for this reason. Test instruments (meters, insulation testers) are potential ignition sources — only use under PTW and gas-free certificate.
Hot surfaces from overloaded motors, blocked cooling fans, or bearing failure can reach "blue heat" (~430°C) — sufficient to ignite most Group II gases. Wrong lamp wattage in Ex e luminaires is another common hot surface source.
Static Electricity
Static Electricity Sources
Synthetic (nylon) clothing; fluid flow through pipework (streaming current); high-pressure cleaning nozzles; plastic equipment surfaces. Two insulating materials rubbing together transfer electrons — the material losing electrons becomes positively charged and can hold the charge for extended periods. Discharge sparks can ignite flammable gases.
Streaming Current
Fluid flow through pipework generates electrostatic charge on the pipe walls and fluid. Accumulated charge at ungrounded flanges can discharge as incendive sparks. Control: bond all pipework flanges (<1Ω), earth the pipework system, limit flow rates, avoid turbulent flow.
Thermite and Pyrophoric Reactions
Thermite Reaction
Impact or friction between aluminium (or Mg, Ti) and rusty steel produces an exothermic reaction generating ~2500°C. Aluminium ladders and tools must not be used in hazardous areas where they could contact corroded ferrous surfaces. Aluminium paint requires caution.
Pyrophoric Reaction — Iron Sulphide
H₂S reacts with iron pipework to produce iron sulphide (FeS). On exposure to air, FeS oxidises spontaneously and self-heats to ignition temperature. Major hazard during vessel entry after H₂S service. Control: purge, gas-free, WET all FeS deposits before air contact.
RF Radiation and Lightning
RF Radiation: powerful transmitters (radar, radio, TV) and mobile phones induce currents in nearby metalwork that can discharge as incendive sparks. ATEX-certified IS radios and devices are required in Zone 1/2 where radio communication is needed.
Lightning: atmospheric static causing very high voltage discharge. Metal plant structures provide earth path — but gas/vapour present at the discharge point can be ignited. Lightning protection per IEC 62305 (50mm² copper conductors, max 10Ω earth resistance).
Battery and Friction Hazards
Batteries: short-circuit sparks from terminals; hydrogen evolution during lead-acid charging (ventilate at high level). Battery replacement in hazardous areas is prohibited — only in safe areas.
Friction/Impact: abrasive wheels, drilling — incendive sparks and hot metal particles. Power tools must not be used in hazardous areas without PTW, gas monitoring, and gas-free certification.
Quick Check — 5 Questions
Test key concepts from this unit before moving on.
Q1Summary
Minimum energy to ignite hydrogen/air mixture?
Hydrogen MIE ≈ 20μJ — a 0.1ms break in a 20mA, 10V circuit. This is why IIC equipment requires the most stringent energy limitation.Ref: IEC 60079-20
Q2Summary
Why must aluminium ladders not be used in hazardous areas?
Thermite reaction: aluminium + iron oxide (rust) → intense heat and molten iron. Far above the ignition temperature of any Group II gas.Ref: Unit 12 — Thermite Reaction
Q3Summary
What specific hazard is present when opening a vessel that previously processed H₂S sour gas?
FeS is pyrophoric — it oxidises spontaneously on air contact without any external ignition source. Deposits must be wetted with water before and throughout air exposure.Ref: Unit 12 — Pyrophoric FeS
Q4Summary
What gas is produced during lead-acid battery charging and what does this require?
Lead-acid charging produces hydrogen (ρ = 0.07, rises to ceiling) and oxygen. Battery rooms require high-level ventilation and gas detection at ceiling level.Ref: Unit 12 — Battery Charging
Q5Summary
Why do IS circuits typically operate below 30V?
Higher voltage arcs carry more energy per unit time, increasing ignition probability. IS circuits below 30V keep arc energy at levels where the circuit energy limitation provides adequate safety margin.Ref: Unit 12 — Voltage and Ignition
EX Academy — independent CompEx-style preparation. Not official CompEx course materials.
Unit 12 · Foundation · Full Manual
Sources of Ignition
Electrical equipment is one of many ignition sources in hazardous areas. This unit covers electrical sparks, hot surfaces, batteries, friction, static electricity, lightning, thermite and pyrophoric reactions, RF radiation, and optical radiation — and the control measures for each.
Learning Objectives
Identify the main sources of ignition in potentially explosive atmospheres
Explain how electrical sparks are generated and why IS circuits operate below 30V
Describe the thermite reaction and why aluminium tools are a hazard in hazardous areas
Explain pyrophoric reactions and the iron sulphide hazard in H₂S service
State the control measures for static electricity including streaming current
Identify RF radiation and optical radiation as potential ignition sources
12.2 Electrical Sparks
Electrical sparks are caused by opening and closing contacts (switches, contactors, relays) and by faults (short circuits, loose connections, earth faults). The minimum ignition energy for hydrogen is approximately 20μJ — the energy from a 0.1ms break in a 20mA, 10V circuit.
Flammable gases ignite more readily at high voltages — this is why IS circuits are rarely designed above 30V. Electrical test instruments are potential ignition sources in hazardous areas and must only be used under PTW with gas-free certification.
12.3 Hot Surfaces
Current through motor windings generates heat. If overload relay is incorrectly set, or cooling is blocked, the surface temperature can exceed the T-rating. Blocked cooling fan intake, damaged fan, or bearing failure can cause local "blue heat" (~430°C) — capable of igniting most Group II gases.
Other sources: process pipework at elevated temperature; combustion engine exhaust; wrong lamp wattage in Ex e luminaires.
12.4 Batteries
Terminal short-circuit sparks — automotive batteries can deliver >1000A; spark energy sufficient to ignite any Group II gas
Lead-acid charging: produces hydrogen (rises, ceiling accumulation) and oxygen — ventilate at high level
Battery replacement must only be done in safe areas
Portable instrument batteries: only use type specified in certificate
12.5 Friction and Impact
Abrasive grinding wheels and power drilling produce incendive sparks and hot surfaces. Power tools must not be used in hazardous areas without PTW, gas monitoring, and gas-free certificate. Sparking commutators in power tools are a direct ignition source.
12.6 Static Electricity
Generated when two materials rub together, transferring electrons. Insulating materials hold charge because they have no conductive return path. Sources: synthetic clothing; fluid flow (streaming current); high-pressure nozzles; plastic equipment surfaces.
Streaming current: fluid flow through pipework generates electrostatic charge on pipe walls. Accumulated charge at ungrounded flanges can discharge as incendive sparks. Control: bond all flanges (<1Ω bond resistance), earth the complete pipework system, limit flow rate, use plastic pipes with high carbon content.
Cleaning non-conductive Ex equipment: use a damp cloth — dry rubbing generates static. This may be a special condition of use (X suffix) on the equipment certificate.
12.7 Lightning
Lightning is atmospheric static electricity. Metal plant structure provides earth path but gas/vapour at the discharge point can ignite. Lightning protection per IEC 62305: down conductors every 20m of perimeter (≤20m height); 50mm² copper conductors; earth resistance ≤10Ω; at least two embedded earth electrodes.
12.8 Thermite Reaction
Impact or friction between aluminium (or magnesium, titanium) and rusty iron or steel produces a thermite reaction:
Fe₂O₃ + 2Al → Al₂O₃ + 2Fe + heat (~2500°C)
The reaction produces molten iron at ~2500°C — far above any Group II gas ignition temperature. Therefore:
Aluminium ladders must not be used in hazardous areas where rusty steel is present
Aluminium paint requires caution — can react if paint chips contact rusty ferrous substrate
Non-sparking tools (copper-beryllium, bronze) should be used instead of aluminium tools
12.9 Pyrophoric Reactions
H₂S reacts with iron in pipework and vessel walls to form iron sulphide (FeS). When FeS is exposed to air it oxidises spontaneously — it is pyrophoric, self-heating to ignition temperature without external ignition. Major hazard during vessel entry following H₂S service.
Control:
Purge and gas-free all vessels before entry
WET all FeS deposits with water before and throughout air exposure
Maintain wetness — dry FeS in air is an immediate fire and explosion hazard
Conduct formal pyrophoric hazard assessment before all sour service vessel openings
12.10 RF Radiation and Optical Sources
RF Radiation: radar, radio/TV transmitters, and mobile phones induce currents in nearby metalwork. Induced currents can discharge as incendive sparks. Mobile phones at petrol filling stations are considered a risk (Zone 1 around pump dispensers). ATEX-certified IS radios and mobile devices required in Zone 1/2.
Optical Radiation: high-energy lasers are potential ignition sources. IEC 60078-28 (2006) covers optical radiation systems. A broken fibreoptic cable could produce a localised optical ignition source.
Vibration: excessive vibration causes bearing wear, loose connections, and structural fatigue — creating secondary hot surface and arcing ignition sources. Equipment exposed to excessive vibration requires reduced inspection intervals.
Unit 12 Knowledge Check — 10 Questions
CompEx-style questions covering the full unit content.
Q1Unit {num}
What energy ignites the most easily ignited hydrogen/air mixture, and what circuit can produce it?
Hydrogen MIE ≈ 20μJ. Produced by a 0.1ms break in a 20mA, 10V circuit — an extremely small energy from a very modest circuit.Ref: IEC 60079-20
Q2Unit {num}
Nylon overalls are worn entering Zone 1. What ignition hazard?
Synthetic fibres including nylon generate static by triboelectric effect. Charge discharge can ignite flammable atmosphere. Anti-static cotton overalls or approved anti-static workwear must be used in Zone 0 and Zone 1.Ref: Unit 12 — Static from Clothing
Q3Unit {num}
Aluminium ladder stored in a Zone 1 area near rusty steel gratings — concern?
Aluminium + rust (iron oxide) → thermite reaction → ~2500°C heat. Sufficient to ignite any Group II gas. Non-sparking composite ladders should be used instead.Ref: Unit 12 — Thermite Reaction
Q4Unit {num}
What causes streaming current and why is it significant?
Fluid friction against pipe walls generates static charge. Charge accumulates on insulated sections and discharges at flange gaps or filling nozzles. Control: bond all flanges (<1Ω), earth the system.Ref: Unit 12
Q5Unit {num}
Iron sulphide found in sour gas vessel is wetted with water before air exposure. Why?
FeS is pyrophoric — it reacts spontaneously with atmospheric oxygen, generating enough heat to ignite residual hydrocarbons. Water prevents this by excluding oxygen from contact with the deposits.Ref: Unit 12 — Pyrophoric FeS
Q6Unit {num}
Why are IS circuits designed below 30V?
Higher voltage arcs release more energy per unit time. By designing IS circuits below 30V, the maximum possible arc energy is kept within safe bounds even if the IS energy limits are approached.Ref: Unit 12 — Voltage and Ignition
Q7Unit {num}
Recommended cleaning method for non-conductive (plastic) Ex equipment?
Use a damp cloth. Dry rubbing of non-conductive surfaces (plastic enclosures, GRP panels) generates static charge. This requirement may appear as a special condition of use (X suffix) on the equipment certificate.Ref: IEC 60079-17
Q8Unit {num}
Bearing failure in an Ex de motor causes local surface temperature of 320°C. Motor is rated T3 (200°C max). What ignition risk?
The motor has become an ignition source above its rated T-class. H₂S (270°C) and acetylene (305°C) are both below 320°C and would be at risk. This is why correct overload protection and vibration monitoring are safety-critical for Ex de motors.Ref: Unit 12 — Hot Surfaces from Mechanical Failure
Q9Unit {num}
During battery charging, what hazard exists in the battery room and what ventilation is needed?
Lead-acid charging produces hydrogen (ρ=0.07, rises to ceiling) and oxygen. High-level ventilation is essential. Gas detection must be positioned at ceiling level. Battery rooms are typically classified Zone 2 during charging.Ref: Unit 12 — Battery Charging Hazard
Q10Unit {num}
A mobile radio is found being used in a Zone 1 IIB area. The radio is not ATEX certified. What ignition source risk and action?
Non-ATEX-certified radios transmit RF energy that can induce currents in metalwork producing incendive sparks. In Zone 1 IIB, only ATEX-certified IS mobile radios and communication devices should be used. Remove the non-certified radio immediately from Zone 1.Ref: Unit 12 — RF Radiation