3 Construction of Flameproof Enclosures, Entry Facilities, Fasteners and Component Parts

Protection concept flameproof

enclosure ‘d’ 257

Non-metallic enclosures may be constructed by moulding or fabrication,

with similar methods of securing one part of an enclosure to another but, as

already stated, they have a volume limit of 3000 cm3 unless the non-metallic

parts form only part of an otherwise metallic enclosure. When this occurs an

individual 500cm’ surface area limit is imposed on the non-metallic parts

(excepting light transmitting parts of luminaires where the limit is increased

to 8000 cm’). After being tested to confirm their capabilities to withstand

pressure in common with all flameproof enclosures, non-metallic enclosures

having non-metallic or partially non-metallic joint surfaces are subjected to

flame erosion tests. In these their flamepaths are subjected to the passage of

burning gas by the carrying out of 50 tests, with plane gaps opened to their

maximum values, using the explosive test mixture for explosion pressure

testing, and to confirm that no significant erosion has taken place. Where

the non-metallic parts are of plastics material, flammability tests are also

carried out on the material.

In addition, non-metallic enclosures which have insulating properties can,

unlike metal enclosures, be used to directly support conductors of differing

voltages. In utilizing this advantage, however, care needs to be taken to

ensure the insulation quality of the material, as unlike material chosen for

insulation alone, it is chosen principally for its dimensional ability and

strength. There are requirements for all apparatus in explosive atmospheres

(see Chapter 8) to minimize the risk of static build up. One way of satisfying

these is to load the plastic with a conducting material to limit its surface

resistivity. Clearly, this will bring its insulating properties into question and

it is therefore necessary to ensure that non-metallic enclosures used for such

support satisfy the requirements of Table 10.14. The comparative tracking

index (CTI) specified in that table is determined in accordance with BS 5901:

198015.

All enclosures, both metallic and non-metallic, are subjected to a

hydraulic pressure test at 1.5 times the reference pressure. It is recognized

that they may be subject to elastic movement during the explosion (they

may temporarily enlarge due to their internal pressure) but no permanent

distortion is acceptable and measurements on the enclosure after pressure

testing are necessary to confirm this. Permanent distortion is not acceptable

either in the enclosure parts themselves (e.g., bowed lids) or in the

flamepaths. Elastic expansion of the flamepaths is, of course, taken account

of during the flame transmission tests. The reference pressure is obtained

by igniting a specific test mixture inside an enclosure with no gaps specially

opened, and monitoring the rise and fall of pressure inside the enclosure

during the explosion. Several tests are carried out and the worst case

pressure taken as the reference pressure. Fig. 10.16 shows a typical pressure

rise and decay curve obtained in such tests. There is a problem in cases

where the interior of the enclosure is divided into separate parts (see

Fig. 10.17) or a component within the enclosure has its own enclosure. An

ignition within one enclosure can lead to pre-pressurization of the gas in

the other as the shock fronts preceding the flame travel faster than the

flame travelling at the speed of sound in the gas. The explosion pressure



258 Electrical installations in hazardous areas



Table 1 . 4 Creepage distance between bare live

01

parts of flameproof apparatus supported by non-metallic enclosure

Maximum rms

working voltage

(U i volts)

n



O
15 < U f 30

30 < U f 60

60 < U f 110

110 < U f 175

175 < U 5 275

275 < U f 420

420 < U 5 550

550 < U 5 750

750 < U 5 1100

1100 < u f 2200

2200 < U f 3300

3300 < U 5 4200

4200 < U f 500

5500 < U f 6600

6600 < U 5 8300

8300 < U 5 11000



Minimum creepage distance (mm)

Material

Group I



Material

Group I1



Material

Group IIIa



1.6

1.8

2.1

2.5

3.2

5.0

8.0

10.0

12.0

20.0

32.0

40.0

50.0

63.0

80.0

100.0

125.0



1.6

1.8

2.6

3.2

4.0

6.3

10.0

12.5

16.0

25.0

36.0

45.0

56.0

71.0

90.0

110.0

140.0



1.6

1.8

3.4

4.0

5.0

8.0

12.5

16.0

20.0

32.0

40.0

50.0

63.0

80.0

100.0

125.0

160.0



(from BS/EN 50019)

Material Group I = Comparative tracking index (CTI)

over 600

Material Group I1 = Comparative tracking index at least 400

but less than 600

Material Group IIIa = Comparative tracking index at least 175

but below 400

Note: Comparative tracking index in accordance with BS 5901

(1980).



is a function of the initial pressure at the time of ignition and if this initial

pressure is raised, the explosion pressure in the second part of the enclosure

is significantly increased and, additionally, the pressure rise time in the

second enclosure reduces. This phenomenon is called pressure piling. The

evidence of such a phenomenon is the reduction of pressure rise time to less

than 5ms, or erratic maximum pressure values which can be identified by

variation of more than a factor of 1.5 in the measured explosion pressures

in separate tests. Enclosure and apparatus construction should, as far as

possible, be arranged so as to prevent this. The use of enclosures where

pressure piling does occur is not, however, precluded.

In small enclosures it is often very difficult to measure explosion pressure

and to determine the pressure to be used in the pressure test. In these cases

the values of pressure in Table 10.15 are used.



.



Protection concept flameproof enclosure 'd' 259



Differential

pressure

(N/m*)



-



0



Maximum rate

, of rise

/

/



/

/

//



' L t -



Time

(seconds)



P = Maximum explosion pressure

t = Pressure rise time



Fig. 10.16 Profile of pressure inside a flameproof enclosure resultingform an internal

explosion

Pressure = p initially



Maximum explosion

pressure = P



Small hole to allow

passage of wires

between enclosures



\



,



Maximum explosion pressure

~picalty

approaches (P + 'p) P



Fig. 10.17 Effects of pressure piling



A flameproof enclosure may be what is called direct entry or indirect

entry (see Fig. 10.18 and 10.19).A direct entry enclosure has the connection

facilities (e.g., terminals) for external connection in the same enclosure,

as the enclosed apparatus and an indirect entry enclosure has a separate

terminal box. This may be connected to the main enclosure by, for example,

bushings and has the advantage that entry to the connection facilities does

not access the main enclosure and, as the main enclosure may contain



260



Electrical installations in hazardous areas



Table 10.15 Test pressure for small enclosures where

reference pressure cannot be measured

Enclosure



Test pressure

(N/m2)



volume



km3)



510



Sub-group



Sub-group



IA

I

>10



Sub-group



IIB



IIC



1 x 106

1.5 x lo6



1 x 106

1.5 x lo6



1 x 106

2 x 106



(from BS/EN 50018)



Fig. 10.18 Indirect entry flameproof enclosure. Note: The main enclosure and

terminal box are entirely separate enclosures



normally sparking parts, it was felt to be an advantage. In BS 229 enclosures

were not permitted to be direct entry and direct entry only became common

with the advent of BS 4683, Part 2. As will be seen when installation is

discussed, there are differences in permissible installation practices for the

two types of enclosure.



Protection concept flameproof enclosure ' d 261



Enclosure



Terminals



I-



'



/



Gland



Cable



Fig. 10.19 Direct entry flameproof enclosure



70.3.2Bushings

A bushing is a method of carrying a conductor through the wall of an

enclosure or between two enclosures with a common wall. It is normally

of insulating material containing one or more conductors. The conductors

passing through the bushing are normally not insulated and adhesion of

the bushing insulating material to the conductors gives the required seal

against flame transmission. The normal construction used is either conductors cemented together and into a metal ring which forms a flamepath with

the enclosure wall, or plastic moulded onto the conductors in whch case

the plastic itself forms the flamepath with the enclosure wall. The bushings

are normally subjected to flame erosion tests and their material, if plastic,

is subjected to flammability testing.

All bushings not unique to one flameproof enclosure are also subjected

to pressure testing at 30 x lo6 N/m2 to ensure that no leaks occur along the

conductors.



70.3.3

Arrangements for entry facilities

The usual method by which cables enter flameproof enclosures is via a cable

gland or conduit. The most common method in the UK is a cable gland but

the situation is quite the reverse in the USA - a fact which is quite important

as flameproof enclosures in the USA are tested with a length of conduit



262



Electrical installations in hazardous areas



attached, whereas in the UK this is not done. Therefore, the requirements

for fitting stopper boxes (described later) are different in the UK to those

used in the USA.

Glands enter the enclosure through its wall and the entry is normally a

threaded hole which must satisfy the requirements for a threaded joint with

the cable gland or conduit in place. This places significant requirements on

the production of such holes and for this reason they are normally machined

into the enclosure at the time of manufacture. This causes problems as

entries may be required in different places for different applications. To

overcome this the manufacturer machines in several holes, some of which

are unlikely to be used in particular installations. To overcome this problem

a series of closing devices have been develop for unused threaded openings.

These devices must be secure and form, with the opening, a threaded

flamepath. The acceptable methods of mounting are as follows: first, the

closing device must be removable only from the inside of the enclosure. It

must be fitted from the inside or secured from the inside after fitting (see

Fig. 10.20); second, the closing device may be fitted from the outside but

shall only have a narrow shoulder to minimize the possibility of its removal

with a wrench, and its installation or removal must be with a hexagon

head or socket of the same specification as those used for bolts and screws

holding enclosures together (see Fig. 10.21);or third, the closing device must

be fitted with a shearing head so that it is permanently installed when the

head has been sheared off (see Fig. 10.22).



10.3.4 Fasteners



The fasteners used for flameproof enclosures are almost invariably screws,

bolts, or nuts and bolts. These devices have to hold the enclosure together

during an explosion but are not usually an integral part of it. These bolts or

screws need to comply with the following : first, their threadform, tolerances

and heads should comply with those for special fasteners to BS/EN 50014

which are discussed in Chapter 8; second, any screws bolts or nuts used



Engaged threads

and length for



Closing device

(no specific head precautions)



nclosure

wall



Fig. 10.20 Internally secured closing device



Protection concept flameproof enclosure ' ' 263

d



Hexagon socket as per



Inside



Fig. 10.21 Closing device with hexagon key type control

This part shears



outside

Residual head narrow



Inside



Fig. 10.22 Closing device with shearing head



as fasteners should have a lower limit yield stress of at last 240N/mm2 in

accordance with IS0 6892 and, unless there is a real necessity to use fixings

of a higher yield stress, it is recommended that these are always used.

The use of any higher yield stress fixings is considered as special and will

require the 'X' mark described in Chapter 8; and third, if any stud screw

or bolt passes through the wall of the enclosure it must form a flameproof

joint, as described in Section 10.2.1, and be non-detachable, being held to the

enclosure by welding or a method which is equally effective (e.g., rivetting).



10.3.5Component parts

In general there are no specific requirements for components within flameproof enclosures except those associated with such things as arcing parts.

Arcing or sparking parts



It is known that arcing parts placed near and in the plane of flange gaps can

seriously affect the flameproof nature of an enclosure and, when this occurs,



264



Electrical installations in hazardous areas



the information in this chapter may not be sufficient to ensure that an enclosure is flameproof. It is difficult to see how such parts could be placed in

similar proximity to spigot and cylindrical parts, but in exceptional circumstances where such possible siting is identified the same problems arise.



Obstacles near flamepaths



Obstacles placed near flamepaths of flameproof enclosures are likewise

known to affect the flamefront and allow transmission of flame where none

would occur in their absence. This is important when constructing a flameproof enclosure where parts of the enclosure may have other items mounted

on them (e.g., a flameproof telephone). Much work has been done on this

problem, mainly to overcome installation problems, but the solutions identified are just as applicable for constructors. To avoid the possibility of an

enclosure with outside fixtures, which otherwise satisfies the requirements

for flameproof enclosures, failing flame transmission tests no obstructions

should be placed in the exit area of a flamepath within 1cm of the flamepath

edge for sub-group IIA, within 3cm of the flamepath edge for sub-group

IIB, or within 4cm of the flamepath edge for sub-group IIC.



Liquids



Flameproof enclosures may contain apparatus which either requires a liquid

to operate (e.g., electrohydraulic devices) or operates on a liquid (e.g., a

chromatograph). These situations are not prohibited but care needs to be

taken to ensure that any liquid entering cannot decompose due to any

situation occurring within the enclosure, even in abnormal conditions, and

create a vapour belonging to a more onerous sub-group than that for which

the enclosure is designed. In these circumstances the enclosure must be

designed for the more onerous sub-group as the vapour produced may exit

the enclosure at the flanges and form the external explosive atmosphere in

addition to being present within the enclosure.

Any liquid or gas within the enclosure which may be pressurized must

not pressurize the enclosure if released therein, or an effect similar to pressure piling may occur. If liquid or gas under pressure is permitted within

an enclosure there must be a method of breathing or draining to ensure that

the enclosure is not pressurized in the worst case of internal release. The

method of breathing or draining needs to maintain the flameproof nature

of the enclosure and is subject to specific construction requirements.

Breathing and draining devices



Breathing and draining devices are for draining liquids and gases from

an enclosure without any significant pressure increase. They may not be



Protection concept flameproof enclosure ‘d’ 265



used where a liquid core gas released within an enclosure increases the

internal pressure by more than 1x lo4N/m2 and they may not be used to

reduce internal pressure caused by an explosion, even though they themselves may withstand such an explosion. Therefore, an enclosure where the

internal pressure is increased by liquid or gas release within it must withstand the internal pressure produced in the absence (blockage) of the device,

and must not transmit the explosion to the outside atmosphere. This will

require special construction requirements not included in those for flameproof enclosures. The internal pressure generated by an internal explosion

must also be considered as that occurring with the device blocked.

Breathing and draining devices, due to their construction (which will be

described later), are often manufactured in copper or brass. While this is

generally acceptable it must be borne in mind that copper in the presence

of acetylene may form copper acetylides, which are in effect solid explosives. For this reason devices containing copper or of alloys containing

more than 60 per cent copper are not permitted in breathing or draining

devices for enclosures for applications where acetylene may be present.

This is necessary because, although users will identify enclosures containing

copper as unsuitable and not use them, it may not be clear to that a coppercontaining breathing or draining device is present in an otherwise copperfree enclosure.

There are two types of breathing or draining device, namely those with

measurable flamepaths through their interstices, and those where these

paths cannot be measured.

The requirements for breathing and draining devices, whose dimensions

are all measurable and controllable to ensure that each measurement is

repeatable within a specific tolerance or against a specific maximum value,

are fairly straightforward. The measured gaps and interstices need not

comply with the gap dimensions specified earlier in this chapter for flange,

spigot and cylindrical gaps, provided the device does not transmit an

internal explosion to the outside atmosphere when tested in the same

manner as these gaps and, in addition, the device withstands an internal

explosion within enclosure without damage when a pressure test is applied

to the enclosure with the device blocked.

As with all divisions there are exceptions and in this case the crimpedribbon breathing and draining device is one. These devices may have

gaps within them which can be specified as to their maximum and

minimum sizes and in this case they are breathing and draining devices

with measurable gaps. In such cases they are tested for their ability to

transmit an internal explosion to the outside atmosphere with the maximum

gaps permitted. Their material of construction is, however, restricted in

that no material used may contain magnesium, aluminium or titanium

because these materials are active in that they can burn and, therefore,

adversely affect the performance of the device in an unquantifiable manner.

A preference for cupronickel or stainless steel is expressed but this is not

mandatory, provided that the metal used is not subject to any reaction with



266 Electrical installations in hazardous areas



the gases involved as this would adversely affect the performance of the

device insofar as its flameproohess is concerned.

There are several types of breathing and draining device whose gaps and

interstices cannot be measured and an alternative strategy is necessary to

define these with regard to their ability to transmit an internal explosion to

the outside atmosphere. The approach used is to define the overall dimensions of the devices. Their density in accordance with BS 5600 Part 316 is

then determined, followed by their bubble pore size in accordance with the

same British Standard. Although this Standard is concerned with sintered

metal elements, the determination methods can be readily applied to other

types of breathing and draining device. Determination and specification of

these two parameters will adequately define the performance of the devices

for the purposes of confidence in their flameproof properties, provided that

they satisfy the flameproof requirements on explosion transmission type

tests.

BS/EN 50018 places requirements on the methods used for determination of porosity or fluid permeability which determine the effectiveness of

the device as a breathing or draining device, requiring them to be determined in accordance with B 5600, Part 3. The reason for this is that the

S

performance of the device as a breathing or draining device is pertinent

to the pressure which can be developed within the enclosure prior to the

initiation of an internal explosion. If the device does not perform effectively

then the entry of fluids into the enclosure is not acceptable as it could lead

to pre-pressurization.

Elements with non-measurable gaps are subject to four basic

constructional requirements. First, crimped-ribbon elements shall, as for

those with measurable elements, of metal and magnesium, aluminium,

titanium, or alloys containing these materials are not acceptable. A

preference for cupronickel or stainless steel is expressed but use of these

metals is not mandatory, provided the metal or alloy chosen does not

react with the fluids present in a way which would adversely affect

the flameproof properties of the device. In particular, where acetylene is

involved the upper limit of 60 per cent copper in any alloy used must

be observed to avoid copper acetylides, which tend to behave as solid

explosives.

Second, pressed wire elements are required to be metal and to satisfy the

same requirements as crimped-ribbon elements, as far as their materials of

construction are concerned. Their initial construction must also begin by

the compression of the wire or wire braid into a die to form a homogeneous matrix which can then be treated as a solid material as far as further

machining is concerned.

Third, sintered metal devices may be constructed of any metal but the

same restrictions as for pressed wire elements apply to these. The preferred

metals in this case are stainless steel or, where acetylene is not present,

90/10 copper - tin bronze.

Fourth, the construction of metal-foam elements is much more specific

because of the method of production of as metal foam. The elements need



Protection concept flameproof enclosure ‘d 267



to be produced by coating a reticulated polyurethane foam with nickel

and then removing the polyurethane by thermal decomposition. The nickel

must then be converted into nickel/chrome alloy by a process such as

gaseous diffusion and subsequent compression of the metal foam produced.

After compression the devices should contain at least 15 per cent chromium

by weight.

Cable glands



Cable glands used in flameproof enclosure may be of two types; a normal

compression gland with a sealing ring (see Fig. 10.23), or a gland using

a sealing compound (see Fig. 1 . 4 . The type chosen depends upon the

02)

type of cable used for a particular installation, in that a compression gland

is only suitable where effective sealing of the cable interstices is possible

by compression. In both cases the thread on the part of the gland which

is screwed into the enclosure is required to satisfy the requirements for

threaded joints and, in addition, required to be at least 8mm long and

comprise at least six f l threads to ensure the five thread minimum speciul

fied for a threaded joint. If the gland has an undercut to ensure that its base

is flush with the enclosure side, washer must be provided and be sufficiently

non-compressible to ensure that the undercut does not enter the female

thread and reduce the engaged thread length to less than five threads.

8rnrn rnin



sheath

(or outer sheath

if cable not

Exd seal (and weather seal

arrnoured)

for unarrnoured cable)



Fig. 10.23 Flameproof compression gland