Flameproof protection is the most widely used Ex concept on offshore platforms and in heavy process industries. Unlike all other Ex protection methods, an internal explosion is permitted — the enclosure must survive and contain it. This unit covers the flamepath principle, construction requirements, cable entry rules, and inspection requirements in full detail.
Learning Objectives
State the IEC 60079-1 definition of a flameproof enclosure and explain the flamepath principle
Describe the four main flamepath joint types: flanged, spigot, cylindrical, and threaded
State the minimum thread engagement for metric threaded cable entries (5 full threads)
Explain flamepath gap dimensions, why they differ by gas group, and what pressure piling is
State minimum separation distances between flamepath openings and obstructions for IIA, IIB and IIC
Distinguish between direct and indirect (Ex de) cable entry and explain when a barrier gland is required
List unauthorised modifications that invalidate Ex d certification
State the inspection items and deficiency categories for Ex d equipment to IEC 60079-17
The Flameproof Principle
An Ex d enclosure is designed to contain an internal explosion rather than prevent it. The enclosure must: withstand the internal explosion pressure (up to 13 bar for IIC hydrogen); cool any escaping gases via the flamepath before they reach the external atmosphere; and not transmit ignition to the surrounding hazardous area.
This is the only Ex protection concept in which an explosion is permitted to occur.
Flamepath
Any joint, gap, or thread through which burning gases must travel after an internal explosion. The gap length and width cool the gases below the external atmosphere's ignition temperature before they escape. No sealant — only petroleum jelly for corrosion protection is permitted on flamepath faces.
Flamepath Joint Types and Gap Dimensions
Four main joint types in IEC 60079-1:
Joint Type
Typical Use
Key Rule
Flanged
Cover joints — JBs, panels
No intentional gap; gap measured with feeler gauges
Spigot
Cover joints — JBs, motor endshields
Cylindrical portion must engage to specified depth
Cylindrical
Motor shaft glands
Length and gap dimensions per gas group
Threaded
Cover joints, cable entries, conduit
Minimum 5 full threads, minimum 8mm axial length
Gap dimensions are tighter for IIC gases. IIC flanged joints require maximum gap of 0.10mm for volumes ≤100cm³ vs 0.30mm for IIA/IIB at the same volume.
Cable Entry and Glands
Direct Entry — Barrier Gland Required
Single Ex d chamber containing arcing components. Cable gland must be Ex d certified barrier type — epoxy resin compound seals cable cores and provides pressure-tight seal against internal explosion pressure.
Indirect Entry (Ex de) — Ex e Gland Acceptable
Two chambers: Ex e terminal box (outer) + Ex d mechanism (inner). Cable gland in the Ex e terminal chamber: Ex e or dual-certified Ex d/Ex e gland is acceptable.
⚠ Stopping Plugs — Certified Ex d Only
All unused cable entries must be sealed with certified Ex d stopping plugs. At least 5 full threads of engagement. Plastic stoppers, rubber bungs, and standard bolts are prohibited. One stopping plug per entry — thread adaptors and stopping plugs cannot be combined.
Unauthorised Modifications
Certification freezes the enclosure design. Any of the following invalidates the certificate:
Replacing components with wrong size or type
Adding components (increases explosion pressure)
Removing components (increases free volume)
Drilling additional entries without manufacturer authority
Using hardening sealants or tape on flamepath faces
Wrong stopping plug size or type
Attaching cable brackets to cover fixing bolts
Inspection of Ex d Equipment
Key inspection checks (IEC 60079-17):
Nameplate data matches area classification (zone, gas group, T-class, EPL)
All cover bolts fitted and spanner-tight
All unused entries sealed with certified Ex d stopping plugs
Cable glands tight and correct type
Flamepath faces free of paint, sealant, corrosion, and damage
Flamepath gaps within certified limits (Detailed inspection — feeler gauges)
Minimum clearance maintained between flamepath opening and obstructions (IIA: 10mm, IIB: 30mm, IIC: 40mm)
Quick Check — 5 Questions
Test key concepts from this unit before moving on.
Q1Summary
A rubber bung is found sealing an unused cable entry in an Ex d junction box. What category is this deficiency?
A rubber bung completely defeats the flamepath at that entry. This is a Category X deficiency. The enclosure must be removed from service and the bung replaced with a certified Ex d stopping plug before the area can be re-energised.Ref: IEC 60079-17 / IEC 60079-1
Q2Summary
What is the minimum number of full thread turns required for a metric threaded cable entry into an Ex d enclosure?
IEC 60079-1 requires at least 5 full thread turns of engagement for metric threaded cable entries. At least 6 are normally provided in the enclosure to allow for a margin of error. The thread itself forms the flamepath at this entry point.Ref: IEC 60079-1 — Threaded Entries
Q3Summary
Jointing compound is found applied to the flamepath face of an Ex d enclosure cover. What action is required?
Jointing compound (sealant) is strictly prohibited on Ex d flamepath faces. It alters gap dimensions, may act as fuel, and prevents correct seating of the cover. The compound must be removed, the flamepath inspected for damage and dimensional compliance using feeler gauges, and the equipment verified before return to service. This is a Category X deficiency requiring immediate withdrawal.Ref: IEC 60079-1 / IEC 60079-17
Q4Summary
Why cannot IIB-rated Ex d equipment be used in a hydrogen atmosphere?
Hydrogen has a MESG of approximately 0.29mm. IIB flamepaths allow gaps up to 0.9mm — nearly three times the MESG of hydrogen. Internal explosion gases can propagate through an IIB flamepath and ignite the external hydrogen atmosphere. Only IIC equipment (flamepath ≤0.5mm) provides adequate protection.Ref: IEC 60079-1 — Gas Group and MESG
Q5Summary
A flanged joint Ex d enclosure is to be installed in a Zone 1, IIC area near steelwork. What minimum clearance must be maintained between the flamepath opening and the steelwork?
IEC 60079-14 specifies minimum clearances between flamepath openings and obstructions: IIA = 10mm, IIB = 30mm, IIC = 40mm. An obstruction too close to the flamepath opening can prevent hot gases from dispersing, increasing the risk of external ignition. The 40mm requirement for IIC is the most stringent.Ref: IEC 60079-14 — Flamepath Obstruction Clearances
EX Academy — independent CompEx-style preparation. Not official CompEx course materials.
Unit 3 · Ex01 & Ex02 · Full Manual
Flameproof Enclosures — Ex d
Flameproof protection is the most widely used Ex concept on offshore platforms and in heavy process industries. Unlike all other Ex protection methods, an internal explosion is permitted — the enclosure must survive and contain it. This unit covers the flamepath principle, construction requirements, cable entry rules, and inspection requirements in full detail.
Learning Objectives
State the IEC 60079-1 definition of a flameproof enclosure and explain the flamepath principle
Describe the four main flamepath joint types: flanged, spigot, cylindrical, and threaded
State the minimum thread engagement for metric threaded cable entries (5 full threads)
Explain flamepath gap dimensions, why they differ by gas group, and what pressure piling is
State minimum separation distances between flamepath openings and obstructions for IIA, IIB and IIC
Distinguish between direct and indirect (Ex de) cable entry and explain when a barrier gland is required
List unauthorised modifications that invalidate Ex d certification
State the inspection items and deficiency categories for Ex d equipment to IEC 60079-17
3.2 Explosion Protection Concepts
Several methods may be used to make electrical equipment safe in potentially explosive atmospheres. The choice of which method to use is based on the probability of a flammable atmosphere being present and the degree of ignition risk from the equipment. Protection concepts covered in IEC 60079 include: Ex d (flameproof), Ex e (increased safety), Ex n (Type N), Ex p (pressurised), Ex i (intrinsic safety), Ex m (encapsulation), Ex o (oil immersion), Ex q (powder filling).
3.3 Flameproof Ex d — Protection Principle
Flameproof is the only Ex protection concept in which an explosion inside the equipment is permitted to occur. The enclosure is designed to contain the explosion and prevent it from reaching the external atmosphere.
IEC 60079-1 Definition
An enclosure in which the parts which can ignite an explosive atmosphere are placed, and which can withstand the pressure developed during an internal explosion of an explosive mixture, and which prevents the transmission of the explosion to the explosive atmosphere surrounding the enclosure.
The letter 'd' comes from the German druckfeste Kapselung — roughly 'pressure-tight enclosure'. Flameproof is one of the original methods of explosion protection, developed initially for use in mining.
Equipment Protection Level: Gb (may also be used where Gc is required). ATEX Category 2. Zones of use: 1 and 2 (unless risk assessment indicates otherwise). Ambient temperature: -20°C to +40°C unless otherwise marked.
Typical applications: junction boxes, lighting fittings, electric motors, control stations, solenoid valves, and any equipment containing arcing or sparking components.
3.3.2 Principle of Design and Operation
Flameproof enclosures are not gas-tight. Gas or vapour will enter through joints or cable entries. When the arcing or sparking components inside ignite this gas, the explosion pressure can reach values in excess of 10 bar (150 psi) — for IIC gases such as hydrogen, up to 13 bar.
The enclosure must therefore be strong enough to contain this pressure. Typical construction materials: cast iron, aluminium alloys. For corrosive environments: gunmetal bronze, phosphor bronze, stainless steel. Plastic materials may be used but the free internal volume must not exceed 10cm³ and the material must pass a flammability test.
The gaps at joints and thread entries must be long and narrow enough to cool flames and hot gases below the ignition temperature of the external atmosphere before they escape. These gaps are called flamepaths.
3.3.3 Flamepaths and Joint Types
Flamepath
Any small joint or gap in a flameproof enclosure through which internal explosion gases must travel before reaching the external atmosphere. The geometry of the flamepath (length and maximum gap width) determines whether escaping gases are cooled sufficiently. Gaps are measured with calibrated feeler gauges — they are too small to assess visually.
The latest standards specify that there shall be no intentional gap at joints. However, manufacturing tolerances mean gaps will always exist and must be controlled within the limits specified in IEC 60079-1 Tables 1 and 2.
Factors that influence the permitted gap dimensions:
The width (axial length) of the joint
The gas group (IIA, IIB, IIC)
The internal volume of the enclosure
The type of joint
Four Flamepath Joint Types
Joint Type
Description
Typical Application
a) Flanged Joint
Machined flat surface on cover bears against corresponding surface on base. No intentional gap when correctly bolted. Used with acetylene only under restrictions.
Junction box covers, panel covers
b) Spigot Joint
A cylindrical spigot on the cover engages a corresponding recess in the base. Combines cylindrical and flat flamepath sections.
Junction box covers, motor endshields
c) Threaded Joint
Screw thread provides flamepath. Must engage at least 5 full threads. The thread geometry provides the cooling path for escaping gases.
Used for rotating shafts on motors. Labyrinth joint with narrow cylindrical annular gap provides flamepath around the rotating shaft.
Motor shaft glands
Threaded Joint Requirements (IEC 60079-1)
For threaded cable entries and conduit entries into Ex d enclosures:
Minimum 5 full thread turns of engagement
Minimum axial thread length: 5mm for volumes ≤100cm³; 8mm for volumes >100cm³
At least 6 threads are usually provided in the enclosure — allowing a margin for thread undercut
A locknut is NOT required when 5+ threads are engaged — thread retention is sufficient
Flamepath Gap Dimensions — Summary
Maximum allowable flamepath gaps vary by gas group and enclosure volume. IIC gases require the tightest gaps:
Gas Group
Flanged joint L=6mm, Vol ≤100cm³
Flanged joint L=25mm, Vol ≤100cm³
Group IIA
0.30mm
0.50mm
Group IIB
0.20mm
0.40mm
Group IIC
0.10mm
0.10mm
3.4 Obstruction of Flamepaths
There must be adequate clearance between any flameproof flanged joint and surrounding obstructions — walls, steelwork, pipework, conduit, other equipment. If an obstruction is too close to the flamepath opening, it can prevent hot escaping gases from dispersing freely, potentially allowing ignition of the external atmosphere.
Gas Group
Minimum Separation Distance (Flanged Joints)
IIA
10mm
IIB
30mm
IIC
40mm
These are minimum distances from the flamepath opening (where the gap is exposed) to any solid obstruction. If the equipment has been specifically tested at smaller distances, that tested distance may be used instead.
3.6 Pressure Piling
Pressure piling occurs when the inside of an Ex d enclosure is partially divided into two sections (by a partition, large component, or unauthorised modification). An explosion in the larger section compresses the unburnt mixture in the smaller section before it ignites. When this pre-compressed mixture ignites, the resulting explosion pressure can be approximately three times that of a normal (unobstructed) explosion — potentially causing structural failure of the enclosure.
To prevent pressure piling, manufacturers ensure adequate free space around any obstruction in each cross-section — typically 20–25% of the total cross-section must remain unobstructed. This is another reason why unauthorised addition, removal, or replacement of internal components with different-sized equivalents is prohibited.
3.7–3.9 Ingress Protection, Weatherproofing and Corrosion Protection
Gaskets may be incorporated into Ex d enclosure designs for weatherproofing and ingress protection. They must be:
Metallic or made from non-flammable compressed material in a metallic sheath
An integral part of the original certified design — cannot be added retrospectively
Replaced with manufacturer-approved types only — home-made gaskets invalidate the certificate
⚠ What May and May Not Be Applied to Flamepath Faces
Permitted: Non-setting grease (petroleum jelly / Vaseline, PBC high-temperature grease, silicone grease) — anti-corrosion only. These do not seal the gap. Prohibited: Jointing compound, silicone sealant, PTFE tape, paint on flamepath faces, hardening agents, any material that fills or seals the gap.
Paint on complete assembly (not flamepath): Permitted — but aluminium paint must not be used. Paint particles in the gap must be removed before reassembly. Non-hardening grease reduces paint penetration.
Denso tape (non-hardening grease-bearing textile tape): May be applied outside straight flanged joints only — one layer, short overlap. Permitted for IIA equipment and IIB (if gap <0.1mm). NOT for IIC or IIB+H₂ equipment.
3.10 Stopping Plugs and Thread Adaptors
Every unused cable entry in an Ex d enclosure must be sealed. The requirements are:
Certified Ex d stopping plug (blanking element) — must be component certified
Correct thread size and type for the entry
Minimum 5 full threads of engagement
Metal stoppers only — plastic stoppers are unacceptable
Fitted with certification details visible for inspection
One thread adaptor maximum per entry — a blanking element cannot be combined with a thread adaptor
A flameproof thread adaptor (reducer) may be used when the entry thread does not match the cable gland thread. The adaptor itself must be component certified and must engage at least 5 full threads in both the enclosure entry and the cable gland.
3.11 Cable Gland Selection for Ex d
Cable gland selection for Ex d depends on the type of entry — direct or indirect — and on whether the enclosure contains IS associated apparatus.
Direct Entry — Ex d Barrier Gland Required
A single Ex d chamber containing arcing components (switches, contactors, relays). The cable gland must be certified Ex d, Ex de, or a barrier gland. The barrier gland contains epoxy resin compound which consolidates the cable cores and provides a pressure-tight seal at the cable entry. In the event of an internal explosion, this seal prevents combustion gases from being forced through the cable cores into the safe area or into other Ex equipment.
Indirect Entry (Ex de) — Ex e Gland Acceptable
The Ex d enclosure has a separate Ex e terminal chamber for cable entry. Arcing components are only in the Ex d inner chamber. The gland in the Ex e terminal chamber may be any certified Ex e or dual-certified Ex d/Ex e type gland — a barrier gland is not required for the Ex e chamber entry.
IS Associated Apparatus Inside Ex d Enclosure
If an Ex d enclosure contains IS associated apparatus, ALL glands entering the enclosure (including those on IS output cables) must be Ex d types. The IS cable is in the hazardous area and the gland must maintain Ex d flamepath integrity regardless of the low-energy IS circuit inside.
Other Non-Gland Entry Methods
Conduit — must maintain IP rating of enclosure; minimum 5 full threads into Ex d entry; sealing devices required where conduit enters/leaves hazardous area
Plugs and sockets certified as parts of equipment
Entry devices included in the original certification
3.13 Unauthorised Modifications to Ex d Enclosures
The certification of an Ex d enclosure accounts for the complete design: internal volume, component layout, explosion pressure calculations, temperature rise, creepage and clearance distances. Any change from the certified design invalidates the approval.
Increases free volume — test results no longer valid
Adding components
Pressure piling risk; thermal effects not tested
Removing components
Increases free volume beyond tested values
Drilling additional entries
Must match certified thread type; may weaken enclosure; only by manufacturer or approved agent
Wrong stopping plug size or type
May not provide required flamepath
Combining adaptor and blanking plug
Prohibited — one adaptor maximum per entry, no blanking plug with adaptor
Attaching brackets to cover bolts
Cover bolts must only secure the cover — mechanical loading changes bolt stress and flamepath clamping force
Paint or sealant on flamepath
Alters gap dimensions; potential fuel source
3.14–3.15 Inspection of Ex d Equipment
IEC 60079-17 provides inspection schedules for Ex d equipment. Key inspection checks include:
Inspection Item
Typical Deficiency if Wrong
Grade
Nameplate data matches area drawing (zone, group, T-class, EPL)
Wrong gas group or EPL — Category X
Visual
All cover bolts fitted and spanner-tight
Missing bolt — Category X (flamepath not clamped)
Visual/Close
All unused entries sealed with certified Ex d stopping plugs
Rubber bung or plastic cap — Category X
Visual
Cable glands correct type and tight
Non-barrier gland on direct entry — Category A/X
Close
Flamepath faces free of sealant, paint, corrosion, damage
Jointing compound — Category X
Close
Flamepath gaps within certified limits
Out-of-tolerance gap — Category X
Detailed (feeler gauges)
Obstruction clearances maintained
Insufficient clearance — Category A/B
Visual/Close
No unauthorised modifications
Additional holes, wrong plug types — Category A/X
Visual/Close/Detailed
Cable insulation undamaged
Damaged sheath — Category A
Visual
Conductors correctly terminated in terminals
Loose connection — Category A
Detailed
Unit 3 Knowledge Check — 10 Questions
CompEx-style questions covering the full unit content.
Q1Unit {num}
What is the minimum axial length of thread required for a metric threaded cable entry into an Ex d enclosure with volume >100cm³?
For volumes greater than 100cm³, IEC 60079-1 requires a minimum axial thread length of 8mm AND a minimum of 5 full thread turns. For volumes of 100cm³ or less, 5mm axial length applies with 5 full thread turns. The thread itself forms the flamepath at cable entry points.Ref: IEC 60079-1 — Threaded Joint Requirements
Q2Unit {num}
An Ex d enclosure rated IIB T4 is found in a Zone 1 area processing hydrogen sulphide (H₂S, Group IIB). Is the equipment suitable?
Hydrogen sulphide (H₂S) is classified as gas Group IIB (MESG 0.65mm). The equipment is marked IIB — correct for IIB group. T4 allows 135°C max surface temperature; H₂S ignition temperature is 270°C, so T4 is acceptable. The equipment is suitable for this application provided zone and EPL are also confirmed.Ref: IEC 60079-0 — Gas Group Compatibility
Q3Unit {num}
What is pressure piling in an Ex d enclosure?
Pressure piling occurs when a partition or obstruction divides the interior of an Ex d enclosure into two sections. An explosion in the larger section compresses the unburnt mixture in the smaller section. When this compressed mixture ignites, the resultant pressure can be approximately three times the normal explosion pressure — potentially causing structural failure. This is why internal component changes and additions are prohibited.Ref: IEC 60079-1 — Pressure Piling
Q4Unit {num}
Which gasket material is permitted as an original part of an Ex d enclosure design?
Gaskets in Ex d enclosures must be metallic or made from non-flammable compressed material contained in a metallic sheath. They must be an integral part of the original certified design — gaskets cannot be added to enclosures not designed to include them. Replacement gaskets must be the manufacturer's approved type. Home-made gaskets invalidate the certificate.Ref: IEC 60079-1 — Gaskets and Weatherproofing
Q5Unit {num}
A certified Ex d IIB reducing adaptor (M32 to M25) is available. Can it be combined with a certified Ex d IIB stopping plug in the same M32 entry?
IEC 60079-1 explicitly states that a blanking element (stopping plug) shall not be used in combination with a thread adaptor. Each unused entry must be sealed with a stopping plug directly — one adaptor is the maximum per entry, and it cannot be combined with a blanking element. If the entry needs to be sealed, a stopping plug of the correct thread size must be fitted directly.Ref: IEC 60079-1 — Adaptors and Stopping Plugs
Q6Unit {num}
During a Detailed inspection of an Ex d enclosure, the flamepath gap at a flanged joint is measured at 0.45mm. The enclosure is Group IIB, Volume 800cm³. Is this acceptable?
For IIB, flanged joints with axial length 25mm and volume in the range 500–2000cm³, the maximum gap is 0.40mm. A measured gap of 0.45mm exceeds this limit. The specific values must be verified against the equipment certificate (which may reference more specific dimensions), but 0.45mm is outside the general IEC 60079-1 Table 1 limits for this combination.Ref: IEC 60079-1 Table 1 — Flamepath Gap Dimensions
Q7Unit {num}
What type of cable gland is required when SWA armoured cable enters an Ex d enclosure with direct entry (arcing components inside)?
Direct entry Ex d enclosures (single chamber containing arcing components) require a certified Ex d barrier gland. The epoxy resin compound in the barrier gland consolidates the cable cores and provides a pressure-tight seal. Without this seal, internal explosion pressure could force hot gases through the spaces between cable cores — potentially igniting the external atmosphere at the other end of the cable run.Ref: IEC 60079-14 — Cable Gland Selection for Ex d
Q8Unit {num}
Non-hardening grease (petroleum jelly) is applied to the flamepath faces of an Ex d enclosure before reassembly. Is this correct?
Petroleum jelly (Vaseline) and approved non-setting greases are the only permitted materials on Ex d flamepath faces. They provide anti-corrosion protection without filling or sealing the gap. Silicone-based greases are also suitable but should be used with caution around gas detectors. Jointing compound, sealant, PTFE tape, and hardening materials are all prohibited.Ref: IEC 60079-1 / IEC 60079-17
Q9Unit {num}
An aluminium alloy Ex d enclosure has been painted by a maintenance contractor. The paint has penetrated slightly into the flamepath gap. What action is required?
All traces of paint must be removed from flamepath surfaces before assembly. Paint particles left in the gap result in a larger final gap than specified — an ineffective barrier to flame. Cleaning method: solvent or fine steel wool. Do NOT use a scraper or file (these can damage the machined surface). Also: aluminium paint must not be used on any Ex d equipment.Ref: IEC 60079-1 / IEC 60079-17
Q10Unit {num}
Denso tape is applied over the flanged joint of an Ex d IIC enclosure to provide extra weather protection. Is this acceptable?
Non-hardening grease-bearing textile tape (Denso tape) may only be applied outside straight flanged joints for IIA equipment (one layer, short overlap) and for IIB equipment where the gap is less than 0.1mm (one layer, short overlap). Tape must never be used on IIC equipment or IIB+H₂ equipment. The IIC case here means Denso tape is prohibited.Ref: IEC 60079-1 — Use of Tape on Flamepaths