UNIT 5: Ex d Protection ‘Flameproof’

This unit will provide an understanding of the

principles of Ex d protection, the construction
requirements to EN/IEC 60079-1 and the installation
requirements to EN/IEC 60079-14.
Flameproof Ex d (or EEx d)

Flameproof is one of the original methods of explosion protection

developed for use in the mining industry. It has a wide range of
applications, typically junction boxes, lighting fittings, electric
motors etc.

The letter ‘d’, which symbolizes this type of protection, is from the
German word ‘druckfeste’(kapselung), which roughly translated
means ‘pressure tight’ (enclosure).

Flameproof apparatus, when properly installed in the intended

location, enables components such as switches, contactors and
relays etc. to be safely used in hazardous areas.

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Flameproof Ex d (or EEx d)

Flameproof is the only one of the nine different methods of

explosion protection in which an explosion is permitted.

This explosion, however, must be contained by the robustly

constructed flameproof enclosure.

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Flameproof Ex d (or EEx d)

DEFINITION
A type of protection in which the parts which can ignite an
explosive atmosphere are placed in an enclosure 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.
Zone of Use: 1&2
AMBIENT CONDITIONS
Flameproof enclosures are normally designed for use in ambient
temperatures in the range -20°C to +40°C unless otherwise
marked.

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Flameproof Ex d (or EEx d)

PRINCIPLE OF OPERATION
Flameproof enclosures are not gas tight and a gas or vapour will
enter the enclosure where, for example, joints or cable entries
exist.

These enclosures are designed to contain components which are

an ignition source, ignition of the gas/vapour may occur, and the
resulting explosion pressure can reach a peak value of around 150
psi.

The enclosure must be strong enough to contain this explosion

pressure, and the gaps at the joints and threads of cable entries
must be long/ narrow to cool the flames/hot gases before they
reach, and cause ignition of a flammable atmosphere which may
exist out with the enclosure.

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Flameproof Ex d (or EEx d)

PRINCIPLE OF OPERATION
Typical materials used for the construction of flameproof apparatus
include cast iron, aluminium alloys, and where corrosion resistance
is required, gun metal bronze, phosphor bronze and stainless steel
may be used.

Plastic materials are also used but the free

internal volume must not exceed 10 cm3.

Both standards specify that ‘THERE SHALL BE NO INTENTIONAL

GAP AT THE COVER JOINTS’ and that the average roughness Ra
of the flame path surfaces must not exceed 6.3 m.

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Flameproof Ex d (or EEx d)

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Flameproof Ex d (or EEx d)

GAP DIMENSIONS
Although the standards specify that there shall be no intentional
gap at the joints of flameproof equipment, gaps will normally exist
due to manufacturing methods, tolerances and economics, but
must not be in excess of the dimensions specified in the tables of
the relevant standards for a given hazard.

Factors which influence the dimension of the gap are:

(a) the width of the joint;

(b) the gas group;

(c) the internal volume of the enclosure;

(d) the type of joint.

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Flameproof Ex d (or EEx d)

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Flameproof Ex d (or EEx d)

FLAMEPATH JOINTS
The diagrams below show three types of joints specified in
the British standard BS EN50 018 for use in flameproof
apparatus.

In a flanged joint, the machined surface on the cover makes

face-to-face contact with the corresponding surface on the
base to give a gap dimension normally less than that
specified in the tables when the cover is properly bolted
down.

This type of joint will be used at the covers of, for example,
junction boxes.

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Flameproof Ex d (or EEx d)

FLAMEPATH JOINTS
This standard does not permit the use of flanged joints when
a IIC gas such as acetylene is the hazard.

Flanged joints may be used for other IIC gases or vapors but
the enclosure volume must not exceed 500 cm3.

Spigot joints will be used at junction box covers and motor

end shields.

Threaded joints are used for cover joints, cable gland and
conduit entries. An adequate flame path length is normally
achieved with a thread engagement of five full threads.

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Flameproof Ex d (or EEx d)

(a) Flanged joint

(b) Spigot joint

(c) Screwed joint

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Flameproof Ex d (or EEx d)

Flame Path Joints Types (Rotating Machines)

(d) Cylindrical (shaft gland) joint

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Flameproof Ex d (or EEx d)

Flame Path Joints Types (Rotating Machines)

(e) Labyrinth joint for shafts

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Flameproof Ex d (or EEx d)

FLAME PATH JOINTS (OTHER EXAMPLES)

(f) Push-button spindle

(g) Cable (gland) entry

• Only threaded entries are
permitted for all cable glands
or conduit entering
flameproof enclosures -
clearance entries are not
permitted.

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Flameproof Ex d (or EEx d)

UNUSED CABLE OR CONDUIT

ENTRIES
It is important that unused cable/conduit entries in
flameproof enclosures are closed using
appropriate stoppers as specified in the standards
or supplied by the manufacturer.

These must be ‘component certified’ metal

stoppers - plastic stoppers are unacceptable -
which are fully engaged by 5 full threads.

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Flameproof Ex d (or EEx d)

PRESSURE PILING
If a flammable mixture is compressed prior to ignition, the resulting
explosion will be considerably higher than if the same mixture was
ignited at normal atmospheric pressure.

Pressure piling can materialize as a result of sub-division of the

interior of a flameproof enclosure, which prevents the natural
development of an explosion.

An explosion at one side of an obstacle pre-compresses the

flammable mixture at the other side, resulting in a secondary
explosion which can reach an explosion pressure around three
times that of the first or normal explosion pressure.

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Flameproof Ex d (or EEx d)

Manufacturers, guided by relevant construction standards, must

ensure that, in any cross-section within an enclosure, there is
adequate free space (typically 20 - 25% of the total cross-section)
around any potential obstruction, which may be a large component
or a number of components.

This will ensure that pressure piling is kept under control.

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Flameproof Ex d (or EEx d)

In rotating electrical machines, sections with

appreciable free volume normally exist at each
end within the main frame of the machine.

These sections are linked by the air gap

between the stator and rotor cores.

In the illustration, an explosion in section ‘1’

must be prevented from migrating to, and
causing ignition of the flammable mixture in
section ‘2’ which will have been pressurized by
the initial explosion.

The air gap acts as a flame path.

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Flameproof Ex d (or EEx d)
OBSTRUCTION OF FLAME PATHS
The Code of Practice recommended that
obstruction of flameproof enclosures,
particularly those with flanged joints, should
be avoided.

A solid obstruction such as a wall,

steelwork, conduit, brackets, weather
guards or other electrical apparatus etc., in
close proximity to the opening at the joint
can, in the event of an internal explosion,
reduce the efficiency of the flame path to
the extent that ignition of the external gas
or vapour could occur.

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Flameproof Ex d (or EEx d)
OBSTRUCTION OF FLAME PATHS

The minimum distances between the flame path

opening and an obstruction are:

Group Distance

IIA 10 mm

IIB 30 mm

IIC 40 mm

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Flameproof Ex d (or EEx d)

WEATHERPROOFING
Flameproof equipment must have a level of ingress protection to
suit the environmental conditions in which the equipment is
installed.

Equipment should have, as part of their approved design, seals or

gaskets to prevent the ingress of water and/or dust.

Where environmental conditions are extreme, consideration of

additional measures may be necessary if this is permissible after
consultation with relevant standards, or the manufacturer or other
authority.

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Flameproof Ex d (or EEx d)

WEATHERPROOFING
THE USE OF NON-SETTING GREASE ON THE MACHINED
SURFACES OF FLAME PATHS HAS TWO ADVANTAGES SINCE,
IN ADDITION TO PROVIDING AN ADDITIONAL LEVEL OF
INGRESS PROTECTION; IT ALSO INHIBITS THE FORMATION OF
RUST ON THESE SURFACES.

SILICONE BASED GREASES REQUIRE CAREFUL

CONSIDERATION IN ORDER TO AVOID POSSIBLE DAMAGE TO
THE ELEMENTS OF GAS DETECTORS.

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Flameproof Ex d (or EEx d)

MAINTENANCE
Prevent clearance gaps from becoming excessive due to wear
and from becoming fouled with foreign matter.
Keep all joint surfaces clean.

Slight surface greasing with suitable water-repellent grease is

permissible.

Ensure that all bolts, screws, studs and nuts are present, and
are tight and secured against working loose. Replacement bolts,
screws, studs and nuts must satisfy the requirements of this
specification, or flame-proofness will be impaired.

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The diagrams illustrate the
location of gaskets or rubber
‘O’ rings for ensuring a high
level of ingress protection.

The gaskets etc. must be an

integral part of the original
design, i.e. they cannot be
added at a later date to an
enclosure manufactured
without gaskets.

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DIRECT / INDIRECT ENTRY
The selection of cable glands for flameproof apparatus is
influenced by several factors, one of which is the method of entry
into the apparatus.

There are two entry methods, namely direct and indirect.

Direct entry comprises a single flameproof chamber within which
components such as switches, relays or contactors may be
installed.

Flameproof apparatus with indirect entry has two separate

chambers, one of which contains only terminals for connection of
the conductors of incoming cables or conduit.

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DIRECT / INDIRECT ENTRY
Connection to the arcing components in the second
compartment is made via these flameproof terminals
which pass through the flameproof interface between
the two compartments.

Direct entry Indirect entry

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Direct & Indirect Entry

Indirect Entry

Direct Entry
ELECTRICAL PROTECTION
Flameproof enclosures are tested for their ability to
withstand internal gas explosions only; they are not
capable of withstanding the energy which may be
released as a result of an internal short-circuit. In order
to avoid invalidation of the certification, it is important
that properly rated/calibrated electrical protection, e.g.
fuses and/or circuit breakers, are utilized.

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 Cable Glands

Compound filled gland

(barrier)

Standard gland
(displacement seal)

Compound filled gland

(barrier)

Cable glands for Ex d equipment must be Ex d certified

Gland Selection for Ex d Direct, Indirect Entry
Yes
MICC
cable? Internal Sources of Ignition:
No
Include sparks or Equipment
Use Exd MICC
gland Temperatures occurring in normal
Elastomeric or
No Thermoplastic or Yes operation which can cause ignition.
Thermosetting cable
with extruded bedding? An enclosure containing terminals
only or an Indirect Entry enclosure is
No Circular non- considered not to constitute an
hygroscopic
filled cable? internal Source of Ignition.
Yes
EN / IEC 60079-14

Yes Internal source No

of ignition?

Yes Gas Group No Use a suitable

IIC hazard? flameproof cable
entry device with a
sealing ring
Use either:
Barrier Gland, Yes Zone 1 No
Stopper Box or area?
Sealing Chamber

Yes Volume No
greater than
2 litres?
Gland Selection

What cable gland would we use on the I.S. cable?

Ex d
Ex d [ ia ] Gland
Thread Engagement

All cable/conduit entries into Ex d enclosures should be

threaded – Clearance holes are not permitted.

Volume
≤ 100cm³ > 100cm³
Thread Thread
Axial Length Axial Length
Engagement Engagement
≥ 5 Full Threads ≥ 5mm ≥ 5 Full Threads ≥ 8mm
Ref. EN/IEC 60079-1

Where thread adaptors are required, there shall be no more

than one per entry.
Conduit Filler/Stopper Boxes

Conduits entering Ex d enclosures should be sealed

 Sealing Unused Entries

Stopper/blank plugs must be Ex d certified

Stopper/blank plugs should not be used with thread adaptors

MODIFICATION OF FLAMEPROOF ENCLOSURES
Flameproof enclosures are normally supplied complete with all internal
components fined and certified as a single entity by a recognized test
authority.

The testing procedure will take into consideration the free internal volume
after all the components have been fitted, the temperature rise (determined
by the maximum power dissipation), creepage and clearance distances,
and the rise in pressure as a result of an internal explosion using a gas/air
mixture in its most explosive proportions.

The certification, therefore, “seals” the design of the apparatus so that any
unauthorized modifications will effectively invalidate the
approval/certification.
Modifications will modify the original test results recorded by the
test/certification authority and, consequently, the following points should be
observed

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MODIFICATION OF FLAMEPROOF ENCLOSURES
(a) Replacement components should always be exactly
the same as the original specified components in order
to avoid infringement of the certification. For example,
a component larger or smaller than the original will
affect the internal geometry of the enclosure. Pressure
piling is a possibility if a larger component is fitted, and
increased volume will result if a smaller component is
fitted.

Original Replacement of ‘A’ Replacement of ‘A’

arrangement with a larger item with a smaller item
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MODIFICATION OF FLAMEPROOF ENCLOSURES
(b) Adding components is also forbidden because of
the possibility of increased explosion pressure as
a result of pressure piling.

Addition of component ‘C’

(c) The removal of components should also be avoided

since an increase in the free internal volume will result.
The original test results, prior to certification, would be
compromised as a result of a modification such as this.

Removal of component ‘B’

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MODIFICATION OF FLAMEPROOF ENCLOSURES

(d) Drilling and tapping of cable gland/conduit entries should only

be carried out by the manufacturer of the enclosure, or his
approved agent.

The threads of the entries are required to be compatible with those

of cable glands or conduit in terms of type of thread, thread pitch
and clearance tolerance since flame paths exist at these points.

Correct alignment of the threaded entry is also important since the

flame path length at one side will be reduced if the cable gland or
conduit is not fitted perpendicular to the face of the enclosure.

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 MODIFICATION OF FLAMEPROOF ENCLOSURES

The strength of a flameproof enclosure may be

impaired if the number and size of entries exceeds
that permitted in the original design certified by the
test authority.

Compliance with the original design is paramount with

regard to number, size and location of entries to
ensure the enclosure will contain an internal
explosion.

(e) Gaskets can only be replaced; they must not be

added retrospectively if not included as part of the
original design.
The use of unauthorized sealants should also be
avoided when it is required to maintain or improve the
IP rating.

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