2 Basic Types of Apparatus for Use with Combustible Dusts (Apparatus in Accordance with BS 6467, Part 1)

Protection concepts for apparatus for dust risks 459



is dust tight as no visible dust will have entered during the tests for enclosure

suitability which includes the enclosure being operated in the dust-test atmosphere at reduced pressure to give dust every opportunity to enter. Such

enclosures must be tested for Degree of Protection IP6X in accordance with

BS/EN 605294 (for older enclosures produced prior to the introduction of

BS/EN 60529 the Standard used was BS 5490 and for swichgear,which is now

included in BS/EN 60529 and therefore does not have a specific unique Standard, BS 54206 was historically appropriate). It is notable that even rotating

machines must comply with this requirement rather than the requirements

of EN 60034, Part 57 (produced in the UK as BS 4999, Part 1058)which allow

variations specifically relating to enclosures of rotating machines and are in

normal circumstances considered as sufficient in other areas. For apparatus

for outdoor use is a total enclosure integrity package of IP65 is required and

this enclosure is considered as necessary for all apparatus used in Zone Z

and, in conducting dust situations, apparatus for Zone Y.



Dust-protected enclosures



This is a lower level of enclosure integrity and what is required here is

that the enclosure is at least IP5X to BS/EN 605294 or the more historic

equivalent Standards already quoted. In addition, because of the different

intended use such apparatus is restricted to Zone Y and only then acceptable for non-conducting dusts). In addition, enclosures offering IP5X to

EN 60034, Part 57 (BS 4999, Part 1058)are acceptable in these circumstances

even though they offer a slightly lower level of protection due to the requirement of normal rotating machines. In tlus case, however, there is a specification additional to those in the IP Standards quoted above in that the

amount of dust entering the enclosure during the test must not exceed the

equivalent of 10gm/m3 as derived from the mass of dust entering divided

by the free volume (after deducting the volume of components and assemblies within the enclosure from the total internal enclosure volume) of the

enclosure. This is an approach to defining the criteria for passing the test

and reduce the reliance on the opinion of the expert executing the test,

and again it must be remembered that the dust level quoted only refers to

non-conducting dust. This means an IP54 enclosure for use outdoors.



15.3 Operational requirements

The apparatus may or may not spark within the enclosure as the enclosure constitutes the protection. Thus there is no requirement for energy

limitation in a spark or arc but only of limitation of the maximum external

surface temperature of the enclosure. The enclosure temperature needs to be

determined in the following conditions: first, with the enclosure mounted

in the most unfavourable normal mounting position (that which gives the

maximum temperature on any part of the surface of the enclosure); second,



460 Electrical installations in hazardous areas



the supply voltage must be adjusted to any value within its operating

tolerance to give the maximum temperature in the above position (where

no supply voltage tolerance is quoted it is normal to utilize f10 per cent).

In the above circumstances the maximum surface temperature of the

apparatus must be 10°C lower than the maximum permitted temperature

for any dust cloud or layer in which the apparatus is intended to be used.

These permitted temperatures are as follows.

First, the maximum surface temperature of the apparatus enclosure must

be not more than two thirds less 10°C of the ignition temperature of the

dust cloud in which the apparatus may operate. (This means, for example,

that for a dust with a cloud ignition temperature of 280°C (sulphur) the

maximum surface temperature of the enclosure may not exceed 176°C

{[280 x 2/31 - 10) at the maximum ambient temperature (a surface temperature elevation of 136 "C at an ambient of 40 "C)).

Second, the maximum surface temperature of the apparatus enclosure

may not exceed a temperature 85°C lower than the minimum ignition

temperature of a 5mm dust layer. (This means that for lignite, which has

an layer ignition temperature of 225"C, the maximum enclosure surface

temperature permitted will be 140°C (225 - 75 - 10) or an elevation of

100"Cat an ambient of 40 "C).

The 10"Cmargin is normally included in the maximum operating surface

'

temperature specified for apparatus to BS 6467,Part 1 and thus it is only

necessary to multiply the dust cloud temperature by two thirds and subtract

75 "Cfrom its layer ignition temperature in order to identify the suitability

of the apparatus. Where apparatus complying with these requirements is

also used in gas and vapour risks and is thus temperature classified, the



Table 1 . Equivalent minimum dust ignition temperature for

51

gas/vapour temperature classifications

Minimum acceptable dust

ignition temperature

Protection

Type of protection

concepts

'N('n')



Temperature

classification

' 're,

d



d i t P,

,



do,



tP'

d

q

'



Cloud

T1 (450°C)

T2 (300°C)

T3 (200°C)

T4 (125°C)

T5 (100°C)

T6 (85°C)



Layer



Cloud



Layer



675 "C'

450 "C'

308 "C'

195"C

158°C

135°C



525 "C

375 "C

280 "C

205 "C'

180"C'

165°C'



685°C'

465 "C'

315 "C'

203 "C

165"C

143°C



535 "C

385 "C

285 "C

210°C'

185"C'

170"C'



Notes: 'As a dust cloud cannot practically exist without dust layers forming,

s

n

and dust layers are a result of dust clouds, this figure i predominant i the

apparatus selection procedure and should always be used.



Protection concepts for apparatus for dust risks 461



10 "C margin is not necessarily taken into account during temperature classification. For all types of protection appropriate to gas or vapour risks

with the exception of type of protection ' N a factor of only 5 "C is used for

temperature classes T3-T6 with the 10 "C only being applied to T1 and T2.

(For type of protection 'N' no factor at all is used.) Where apparatus which

is temperature classified to BS/EN 500149 is used in dust situations without

change because, in other than surface temperature considerations, it can be

shown to satisfy BS 6467, part 1,*the permissible usage of the apparatus is

shown in Table 15.1.



15.4 Basic constructional requirements

As may be expected, the requirements for the construction of this type of

apparatus revolve principally around the needs of the enclosure integrity

which have already been dealt with in Section 15.2. There are, however,

requirements for physical strength and electrical connection but these are

very general and not dissimilar to those applied to apparatus for gas and

vapour risks.

15.4.1 Enclosure materials and mechanical strength



Enclosure materials



The basic enclosure material requirement is that it will withstand the environmental and other operating conditions which it may be called upon

to deal with in operation. These are mainly a matter of ensuring that the

manufacturer's material specification matches chosen use and thus selection criteria. There is, however, a specific requirement to ensure that all

enclosure materials are not operated outside their manufacturer's ratings

when subjected to a temperature 10"C higher than the maximum temperature which they will achieve at maximum ambient and in the most onerous

service conditions for which they are designed. This is clearly identifiable

at the construction stage and is necessary to ensure that enclosure strength

and IP-rating can be maintained. This requirement is, of course, no more

onerous than those applied to apparatus for gas and vapour risks.

As with enclosures for apparatus for gas and vapour risks, there is a

restriction on the magnesium content of any of the externally exposed light

metal enclosures or parts thereof to minimize the risk of thermite sparking

if the enclosure is struck by an oxidized (rusty) steel item.

A final requirement is that materials must not be combustible or

propagate flame if ignited. This is not principally an enclosure requirement

as if the enclosure is ignited from outside the source of dust ignition

already exists. It is, however, important for both enclosures and all other

parts inside the enclosure, as ignition may occur due to electrical fault

before the electrical protection operates, and it must be remembered that

there is no internal constructional requirement other than normal industrial



462 Electrical installations in hazardous areas



requirements to prevent such ignition. If it did occur there is a real danger

that the elevated temperature may damage the enclosure integrity leading to

ignition of any surrounding dust cloud or layer and thus the requirement

assumes a greater importance in this context. Methods of demonstrating

non-combustibility and prevention of flame transmission are not specifically

defined in relation to this particular risk and typical industrial methods of

demonstration are acceptable (e.g., BS 2782, BS 6458 and BS 6334 are typical

Standards which may be used for this purpose but they are not exclusive

and other recognized methods may be used).



Mechanical strength



The mechanical strength of the enclosure is required to be such that it will

not be damaged other than superficially by impact and drop tests. These

tests are identical to those used for apparatus for gas and vapour risks (see

Chapter 8) except that, because the enclosure is all important, the lower

level of impact test for enclosures intended for installation only in areas

where the risk of mechanical impact is low is not acceptable, the higher

figure always being required.



15.4.2Joints intended to be opened in normal setvice



Non-gasketted joints



Metal/metal and similar joints in enclosures which do not use compressible

gaskets may be used provided that the enclosure may be shown to achieve

the desired IP-rating. Such joints have minimum requirements applied to

them to ensure that they are repeatable and these are that the joint be close

hc

fitting and have a minimum width over w i h this is achieved, defined as

the dust ingress path. No figure is given for close fitting but it is recommended that a maximum separation limit of 0.5mm be applied with the

proviso that the enclosure has to satisfy the necessary E'-rating. This would

allow the use of all flameproof enclosures which satisfied the necessary IP

be applied for

test. A minimum joint flatness requirement of at least 6.3

the same reason. Typical joints are shown in Fig. 15.1.

The minimum dust ingress paths (widths) for such joints need to be as

follows:

5mm

Flat flanged joints

Spigot joints

3mm

Cylindrical joints

3mm

Conical joints

3mm

Threaded joints

5mm

These joints may not use additional measures such as non-setting sealing

compounds to assist in the achievement of the necessary IP-rating but must



Protection concepts for apparatus for dust risks 463



r"lrjl



r1



(Note)



(Note)



A25rnrn

(X+Y)25mm

Note :

Joint faces to be as close as possible (no deliberate gap)



Flanged joints



B23rnm



B23rnm



Spigot joint



Cylindrical joint



B23mm

Conical joint



Note :

Joint faces to be as close fitting as possible (interterencefit)



Threaded joint

Note :

Three full threads must be engaged.



Fig. 15.1 Minimum dimensions for ungasketted joints



achieve the necessary IP-rating 'dry' on the basis of the fit of the two parts

of the joint alone.

Threaded joints, in addition to satisfying the length requirements, must

have at least three threads fully engaged, whether parallel or taper threads.

It is necessary to identify these joints if they are intended to be opened in

normal service in view of their importance to the protection offered by the

apparatus. Additionally and particularly it is necessary to identify the fact

that no compound is used in their completion. The purchaser of the apparatus must be aware of these matters if the apparatus is to be used safely.



Joints having gaskets and other types of sealing device



Plane joints may use flat gaskets to prevent the ingress of dust into

enclosures as shown in Fig. 15.2, provided that the gasket width is at least



464



Electrical installations in hazardous areas



=7F

Gasket



A 2 3mm



Gasketted joints



(X+Y) 2 3 mm



\



A

,



I

4



'0' ring

(no defined minimum dimension)



'0' ring sealed joints



Fig. 15.2 Use of gaskets and '0'rings. Note: Gaskets must be secured to one face

of joint and ' 'rings in slots (by adhesive or other means which does not

0

adversely affect sealing of joint)



3mm. These joints include plain flanged joints and spigot joints where the

gasket is in the plane part and thus the spigot ceases to be important. In

this case the gaskets must be effectively continuous in that any joints within

them need to be bonded together and not just rely on pressure. The gaskets

should be fixed to one of the joint parts, possibly with adhesive, to prevent

their displacement but their material of construction should be such that

they do not stick inadvertently to mating faces. They must also be fabricated

of a material which is not damaged by the extremities of their service

conditions either by ageing or operation outside their material specification.

0

Other types of sealing unit, such as ' ' rings, may also be used and in

this case securing of the ring to one side of the joint will not be necessary

as such rings need to be fitted in recesses which effectively perform the

necessary action. Lip seals are also possible where the seal is by a shaped

ring at the edge of the joint. Again, these seals are profiled and positively

located by their form.

15.4.3 Semipermanent joints



Joints which form part of an apparatus which is not addressed in normal

service and which are seldom opened, need in principal to comply with the

foregoing requirements. A relaxation is, however, permitted in the case of



Protection concepts for apparatus for dust risks 465



flanged joints and threaded joints, whereby the dust path through the joint is

reduced to 3mm provided the joint surfaces are coated with a non-setting

sealing compound of adequate specification before assembly. Such nonsetting compounds may, unlike the situation in the case of regularly opened

joints, be used to assist exclusion of dust in all cases where the joint is

seldom opened in service and is flanged, cylindrical conical or a spigot

joint and is static.

The basic requirements for such non-setting compounds is that they be

chemically and thermally stable, and inert between a temperature 5 "C

below the minimum ambient temperature specified for the apparatus and

20 "C above the maximum temperature which they can reach, with the apparatus operated at the maximum ambient temperature in the most adverse

supply and mounting conditions. They also need to be impervious to deterioration as a result of the environment (e.g., moisture) or be protected from

contact with it.

15.4.4 Spindles and shafts



Spindles and shafts which move or rotate should be sealed against the unacceptable entry of dust by suitable bushings, the fitting of a dust exclusion

seal, a labyrinth joint or a combination of these types of device. Simple

cylindrical joints are not considered as suitable for this use. In addition,

the dust exclusion properties of the joints must be maintained when any

grease or oil applied to the joint for operational reasons is absent and the

joint relies only on its solid parts for its performance.

No minimum lengths of dust path are specified for such joints and their

dimensions are solely affected by their ability to achieve the necessary

IP-rating.



15.4.5 Light-transmitting parts



Light-transmitting parts need to be fixed to the enclosure by one of the joint

techniques already mentioned or be fixed by cementing or similar methods.

Any transparent or translucent material may be used provided it complies

with the materials requirements already stated, and securing to the enclosure by cement alone is acceptable provided that the cement adheres to both

the light-transmitting part and the enclosure. The assembly including the

light-transmitting part needs to achieve the necessary IP-rating and must

not be damaged by the impact tests already described. The cements used

for cemented light-transmitting part assemblies needs to satisfy the same

requirements as those for non-setting sealing compounds already specified

(except of course that the cement is required to set). If the light-transmitting

part is glass then a thermal shock test of spraying a l m m jet of water at

10-15°C onto the glass when it is at its maximum operating temperature

is required.



466 Electrical installations in hazardous areas

15.4.6 Fasteners



The fasteners used to secure parts of the enclosure play an important roll in

the protection concept. Unless there is a specific additional device to prevent

the ingress of dust via their mounting or fixing holes, these should not enter

the enclosure and the thickness of metal or other enclosure material between

their sides and bottom and the inside of the enclosure needs to be not less

than one third of the hole diameter to ensure the enclosure is strong enough

to prevent cracking in service.

Hinged lids are acceptable provided that the hinge does not apply the

pressure necessary for sealing. A flexible hinge which allows lid movement during tightening of fixings is therefore necessary in such cases. Also,

because such movement may allow the displacement of the lid during tightening, locating pins may be necessary.



15.5 Specific additional requirements for particular types of

electrical apparatus and connections

As is the case with other protection concepts there are specific requirements

concerning connections, cable entry and for specific types of apparatus.

These are generally similar to those required for types of apparatus used

in gas and vapour risks and may generally be described as those applied

to industrial apparatus but with a few additional specifications.



15.5.1 Connection facilities

The requirements for connections to apparatus are very similar to those

required for apparatus for gas and vapour risks. This requires the

provision of connection facilities (except for apparatus manufactured with

a permanently connected cable) which are: accessible and allow effective

clamping of incoming cable, connection of conductors without relying on

any plastic part in the case of terminals and other connectors which rely

on pressure to hold the conductor; have sufficient separation so as to

minimize the risk of inadvertent short circuits to the normal industrial

degree; and have enclosures which ensure the required IP-rating after fitting

and connection of the conductors.

Earth connections are required within the terminal enclosure close to the

other circuit connections and, where the enclosure or any accessible part

of it is conducting, outside the enclosure also. The internal and external

earth connection facilities need to be able to accommodate a conductor of

at least the size required by Table 15.2, with a minimum size for :he external

conductor of 4mm which is not dissimilar to the specific requirement for

apparatus for gas and vapour risks. No external earth connection facility is

necessary for an enclosure which is wholly insulating as the objective of this

is to bond the enclosure locally to remove any risk of it being at a different



Protection concepts for apparatus for dust risks 467



Table 15.2 Required protective conductor crosssectional area relative to phase

conductors

Phase conductor crosssectional area



(mm2)

4



Minimum cross-section

required for

protective conductor

(mm2)



6



As the phase

conductor



>16 but 535

> 35



16



Half of the phase

conductor area



potential to the local metalwork. The internal earth connection facility may

also be omitted where it has no purpose (e.g., in double insulated apparatus)

or where it is specifically not permitted for a particular type of apparatus.

15.5.2 Cable and conduit entries



Where cables or conduits enter the enclosure, the method of entry must

be such as to ensure that the enclosure integrity is maintained. This may

involve the fitting of washers at the entry point and sealing of the interstices

of the cable, particularly where tape bedded cable is used.

Conduits will almost certainly require to have a stopper box fitted close

to the point of entry as otherwise the conduit will need to be considered as

part of the enclosure.

Cable glands need to clamp the cable sheath or armour to secure the

cable and prevent stress on the electrical connections. The requirements

here are identical to those for cable entries for apparatus for gas and vapour

risks (see Chapter 8), requiring a test on the clamped cable constituting

application of a specific tensile stress of specific values for specific times to

the clamped cable without any significant movement of the cable. Details

for these tests are given in BS/EN 500149.

15.5.3 Fuses and switchgear



Enclosures of fuses and remotely operated switchgear, including apparatus

with switching contacts, should be interlocked to prevent the enclosure

being opened while the apparatus is energized and, in both cases, the interlock needs to be on-load rated. If interlocking is not provided, the enclosure

needs to be fitted with a label warning against opening in a hazardous area

while the fuses or switchgear enclosed are energized. The term 'remotely



468 Electrical installations in hazardous areas



operated’ is used in relation to switchgear as locally operated switchgear

is always assumed to have mechanical interlocking to prevent enclosure

opening unless the switchgear is in the isolated position. In the unlikely

event that this is not so, then the interlocking or warning requirements

apply to locally operated switchgear also.

Disconnectors (isolators) are a particular problem as they are ’off load’

devices and are not intended to be operated on-load. These therefore require

that an interlock preventing their on-load operation is fitted, or a specific

warning against such action is provided.

The interlocking devices used in any circumstance must be such that

they cannot be readily defeated by use of such tools as screwdrivers or

pliers which are generally carried by personnel. The objective of this is to

minimize the risk of mistakes and is a similar requirement to that used in

the case of interlocks for apparatus for gas and vapour risks.



15.5.4 Plugs and sockets

As with apparatus for gas and vapour risks, plugs and sockets need to

be interlocked to prevent energized separation or, if not interlocked, to be

identified by warning as items which must not be so separated. The fixed

part of the plug and socket (that forming part of the apparatus) is also

required to maintain the IP-rating of the enclosure when the moving part

is not present. Again, the interlocking devices, if fitted, must be on-load

operable and not defeatable by normally carried tools.



15.5.5 Luminaires and similar apparatus

Apparatus, such as luminaires, where enclosures are more readily opened

than is the case for other apparatus, because of the needs of relamping, etc.,

should also carry a label warning against opening while energized unless

they are similarly interlocked. This is necessary as it is normal in standard

industrial circumstances to carry out such practices as relamping live and

this must not be the case in hazardous areas.



15.6 Apparatus conforming to the protection concepts

appropriate to gas, vapour and mist risks

There are large quantities of apparatus designed and often certified/approved for operation in explosive atmospheres of gas, vapour or

mist, and air. These are subject to detailed construction requirements

which are not dissimilar or inferior to those required for apparatus for

use in explosive atmospheres of dust and air. Third-party certification of

construction of apparatus for gas/vapour/mist air risks is provided which

gives a high degree of confidence in its construction. In addition many

hazardous areas are created by both dust/air and gas/vapour/mist/air



Protection concepts for apparatus for dust risks 469



risks requiring apparatus to be suitable for operational situations as, even

though apparatus may be incapable of igniting a dust/air mixture, it may

ignite a gas/air mixture which itself may form the source of ignition of the

dust/air mixture. For the above reasons (and also economy of design and

construction) it is advantageous to utilize one type of apparatus for both

risks whether they occur together or separately. For this reason consideration has been given to the suitability of apparatus constructed to the BS/EN

500 range of Standards in relation to BS 6941" for use in dust risks with

the following results.

15.6.1 Spark ignition



Apparatus used in explosive atmospheres of gas, vapour or mist, and air

and in which sparking in either normal or recognized fault conditions

occurs, is required to provide protection for sparking by the use of

such techniques as intrinsic safety (Chapter 13), flameproof enclosure

(Chapter 10) or use of non-incendive techniques (Chapter 14). This is

because it is not normally possible to exclude mixtures of gas/vapour/mist

and air from enclosures. In the case of dust/air mixtures it is, however,

possible to do this and so, provided the enclosure of apparatus is adequate,

sparking becomes less of a problem for dusts in general.

15.6.2Hot surface ignition



Surface temperature classification as carried out for gas/vapour/mist and

air risks is not immediately appropriate to dust situations, as it often

includes temperatures within the apparatus which are accessible to the

gas/vapour/mist and air mixture but not to dusts where the enclosure

criteria for dust exclusion are applied. In addition, the factors used in

surface temperature classification are different to those applied for the dust

risks already explained in this chapter.

The simplest method of selection of gas or vapour risk apparatus for

use in dust risks on the basis of Surface Temperature Classification is to use

Table 15.1,but it must be recognized that this will often give a very onerous

solution because of its consideration of internal temperatures. Alternatively,

the apparatus could be tested for maximum external surface temperature

and the figure resulting used for selection purposes. In this latter case,

however, the 10°C safety factor explained earlier in this chapter should

be used.

15.6.3 Basis of selection of apparatus with protection concepts

appropriate to gas/vapour/mist and air risks

As already shown, the basic requirements for apparatus for use in dust

risks are almost the same in many cases to those required for apparatus for