Explosion Protection Methods

Principle	How It Works	Methods
Energy limitation	Reduce electrical energy below ignition threshold	Ex i
Exclusion	Keep the explosive atmosphere away from ignition sources	Ex m, Ex o, Ex p, Ex t, Ex nR
Avoidance	Ensure no effective ignition source exists	Ex e, Ex nA
Dilution	Dilute explosive atmosphere below LEL	Ex p
Containment	Contain the explosion within the equipment	Ex d, Ex q

Protection Methods — Gas/Vapour

Ex d — Flameproof Enclosure (IEC 60079-1)

Principle: Containment. The enclosure is strong enough to withstand an internal explosion. Precisely machined gaps ("flame paths") cool escaping hot gases below the ignition temperature of the surrounding atmosphere.

How it works:

• Equipment is enclosed in a robust housing (cast iron, aluminium, stainless steel)
• Internal explosion can occur but is contained
• Flame paths (flanged joints, threaded entries) cool escaping gases to below ignition temperature
• Escaping gases can exceed 1,000°C but the contact time is too short to transfer sufficient energy

Sub-levels:

• Ex db — for EPL Gb (Zone 1)
• Ex dc — for EPL Gc (Zone 2)

Key design parameters:

• Wall thickness: volume-dependent (3–6mm minimum for cast iron)
• Flame path gap: 0.1–0.5mm depending on gas group
• Flame path length: 9.5mm (IIA) to 25mm (IIC)
• Internal pressure rating: up to 10 bar
• Clearance distances around enclosure: 10mm (IIA), 30mm (IIB), 40mm (IIC)

Critical requirements:

• Cable glands must be Ex d certified (never use Ex e glands on Ex d enclosures)
• Flame path surfaces must be clean and lightly greased, never painted
• Contents must not be modified without recertification
• Only fully certified enclosures may be used (not "U" component certificates alone)

Typical applications: Motors, switchgear, junction boxes, lighting fixtures, power distribution, transformers

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Ex e — Increased Safety (IEC 60079-7)

Principle: Avoidance. The equipment cannot produce sparks, arcs, or hot surfaces during normal operation or foreseeable malfunction. Enhanced design margins ensure no ignition source develops.

How it works:

• No spark-producing components allowed
• Enhanced clearances and creepage distances
• IP54 enclosure protection minimum
• Increased insulation margins
• Thermally protected terminals and connections

Sub-levels:

• Ex eb — for EPL Gb (Zone 1)
• Ex ec — for EPL Gc (Zone 2)

Key design parameters:

• Creepage and clearance distances exceed normal standards
• Terminal connections rated for specific fault current/time combinations
• IP54 or better ingress protection
• Thermal monitoring for motors and transformers

Critical requirements:

• All electrical connections must be properly tightened (loose connections = sparks)
• Seals must maintain IP rating
• Cable glands should be Ex e certified (Ex d glands also acceptable)
• Not suitable for equipment with normal sparking contacts

Typical applications: Terminal/junction boxes, motors (non-sparking type), control panels, light fittings, cable glands, busbars

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Ex i — Intrinsic Safety (IEC 60079-11)

Principle: Energy limitation. The electrical energy in the circuit is limited to a level that cannot ignite the explosive atmosphere — neither by sparking nor by surface heating.

How it works:

• Current and voltage are limited by safety barriers or galvanic isolators
• Total circuit energy (including stored energy in capacitance and inductance) stays below the minimum ignition energy (MIE) of the gas
• Even under fault conditions, insufficient energy exists to cause ignition

Sub-levels:

• Ex ia — for EPL Ga (Zone 0) — safe with 2 simultaneous faults + 1.5× safety factor
• Ex ib — for EPL Gb (Zone 1) — safe with 1 fault + 1.5× safety factor
• Ex ic — for EPL Gc (Zone 2) — safe under normal operation

Key design parameters:

• Maximum voltage, current, power defined per gas group
• Capacitance and inductance limits for cable and equipment
• Safety barriers: zener barriers (earth-referenced) or galvanic isolators
• Cable parameters (capacitance per metre, inductance per metre) must be included in assessment

Minimum ignition energies:

• IIA (methane): ~280 μJ
• IIB (ethylene): ~60 μJ
• IIC (hydrogen): ~17 μJ

Critical requirements:

• Intrinsically safe circuits must be segregated from non-IS circuits
• Blue-coloured cables/wiring for identification
• System documentation (entity parameters) must be verified
• Cable routing separate from power cables
• Earth connection for zener barriers must be reliable (<1Ω)

Typical applications: Sensors, transmitters, thermocouples, level switches, portable gas detectors, communication devices, HART instruments

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Ex p — Pressurization (IEC 60079-2)

Principle: Exclusion + Dilution. The enclosure is pressurized with clean air or inert gas to prevent explosive atmosphere from entering. Alternatively, the internal atmosphere is diluted below LEL.

Sub-levels:

• Ex px — for EPL Gb — reduces Zone 1 inside enclosure to non-hazardous
• Ex py — for EPL Gb — reduces Zone 1 inside to Zone 2
• Ex pz — for EPL Gc — reduces Zone 2 inside to non-hazardous

How it works:

• Pre-purge: enclosure volume swept with clean air/gas (typically 5× volume)
• Maintained positive pressure during operation (typically >50 Pa above ambient)
• Pressure/flow monitoring with alarm and automatic disconnection on loss

Typical applications: Large motors, control rooms, analyser houses, VFDs (variable frequency drives), MCC rooms, large distribution panels

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Ex n — Type "n" / Non-Sparking (IEC 60079-15)

Principle: Avoidance (primarily). Equipment does not produce arcs or sparks capable of ignition during normal operation. Less stringent than Ex e — designed for Zone 2 only.

Sub-types:

• Ex nA — non-sparking apparatus
• Ex nC — sparking apparatus with contacts in sealed or hermetically sealed enclosure
• Ex nR — restricted breathing enclosure (limits gas ingress)
• Ex nL — energy-limited apparatus (similar to Ex ic)

EPL: Gc only (Zone 2)

Typical applications: General-purpose instruments, solenoid valves, relay boxes, indicator lights in Zone 2 areas

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Ex m — Encapsulation / Moulding (IEC 60079-18)

Principle: Exclusion. Components are completely encased in a compound (resin, epoxy) that prevents contact with the explosive atmosphere.

Sub-levels:

• Ex ma — for EPL Ga (Zone 0)
• Ex mb — for EPL Gb (Zone 1)
• Ex mc — for EPL Gc (Zone 2)

Typical applications: Solenoid coils, LED modules, electronic PCBs, sensors, small actuators

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Ex o — Oil Immersion (IEC 60079-6)

Principle: Exclusion. Electrical contacts and arcing components are submerged in protective oil.

EPL: Gb (Zone 1)

Typical applications: Transformers, switchgear (mainly legacy installations)

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Ex q — Powder/Quartz Filling (IEC 60079-5)

Principle: Containment. Equipment is surrounded by fine quartz sand that quenches flame propagation and absorbs heat.

EPL: Gb (Zone 1)

Typical applications: Capacitors, fuses, small electronic modules

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Ex Op — Optical Radiation (IEC 60079-28)

Sub-types:

• Ex Op is — inherently safe optical radiation
• Ex Op pr — protected optical radiation (interlock shutdown)
• Ex Op sh — optical radiation with shielded enclosure

Typical applications: Fibre optic systems, optical sensors, laser-based instruments

Protection Methods — Dust

Ex t — Protection by Enclosure (IEC 60079-31)

Principle: Exclusion. A dust-tight enclosure prevents dust from reaching ignition sources and limits surface temperature.

Sub-levels:

• Ex ta — for EPL Da (Zone 20) — IP6X required
• Ex tb — for EPL Db (Zone 21) — IP6X required
• Ex tc — for EPL Dc (Zone 22) — IP5X minimum

The maximum surface temperature must account for both dust cloud and dust layer ignition temperatures — see Temperature Classes.

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Zone-to-Protection-Method Matrix