2 Protection of Electrical Apparatus for Gas, Vapour and Mist Risks

Design philosophy for electrical apparatus for explosive atmospheres 167



encapsulation of the electrical circuits, and pressurization of the enclosure

containing the electrical circuits with non-flammable gas.

Oil immersion (symbol ‘0’)



In this protection concept (type of protection) the electrical components

are immersed in oil within their enclosure, thus preventing access of the

explosive atmosphere. It is dealt with in detail in Chapter 9. There are limitations on the type of oil which may be used and requirements to ensure

the security of the oil within the enclosure, so that the electrical circuits

remain immersed. The technique is considered as suitable for Zone 2 only,

at present, within the UK but is used in more hazardous areas in other countries. This is likely to change in the not-too-distant future (see Chapter 9)

and will be the protection concept permitted in Zone 1.

Encapsulation (symbol ‘m’)



This protection concept is dealt with in detail in Chapter 9. Again, the explosive atmosphere and electrical circuits are separated from one another, this

time by encapsulating (potting) material. The types of material used are

defined and the encapsulated block is arranged so that surfaces presented

to the outside are hard enough to give protection. The technique is considered as suitable for both Zone 1 and 2.

Pressurization (symbol ‘p’)



This protection concept is dealt with in detail in Chapter 11. In this technique the explosive atmosphere is kept out of the equipment enclosure by

pressure of air or inert gas. Air is normally used as there are asphyxiation

problems with inert gas. Because the protection is by a gas (air) it is almost

always necessary to provide ancillary equipment to ensure that the air pressure is maintained in spite of small leakages, which are almost certain to

occur. Because of this the technique almost always requires a pressurecontrol system, together with a purge system to ensure that on start-up any

internal explosive atmosphere is removed, and this gives added complication which must be set against its added flexibility because of the ease

of depressurizing for repair. The technique is considered as suitable for

both Zone 1 and 2 or only Zone 2, depending upon the pressure-control

systems used.



7.2.2Prevention of sparking (criterion b)

There is only one formal protection concept which is suitable for both

Zone 1 and 2 in this area.



168 Electrical installations in hazardous areas



Increased safety (symbol ‘e’)



This protection concept is dealt with in detail in Chapter 12. The technique

accepts that there is ignition-capable energy within the equipment but relies

upon the quality of construction to ensure that the energy is not released

in a way which could cause ignition. This is achieved by the quality of

construction of the equipment. The technique is considered as suitable for

both Zone 1 and 2.

7.2.3Containment of explosions (criterion c)



There are two protection concepts which use this technique.

Flameproof equipment (symbol Id’)



This is the oldest protection technique. The explosive atmosphere is

permitted to enter the equipment enclosure and to be ignited by the

components within. The equipment is, however, provided with a very

special strong enclosure which will withstand the internal explosion without

sustaining damage and, in addition, will prevent the flames associated with

the internal explosion from exiting the enclosure in a way which would

permit them to ignite any surrounding explosive atmosphere. The technique

is considered as suitable for both Zone 1 and 2.

Powder filling (symbol ‘q’)



Here the equipment enclosure is filled with quartz, sand or some similar

inert small-grained filling. The technique is an extension of an old technique

involving the use of stone chambers to prevent the passage of flame upducting. These chambers served to quench the flame, or so it was thought.

The filling does not prevent the explosive atmosphere from accessing the

equipment and coming into the proximity of a source of ignition. The finegrained filling, however, serves to quench any ignition, thus preventing an

explosion. The technique is considered as suitable for Zone 2 only in the

UK, but is used in more hazardous areas in other countries and is expected

to be acceptable for use in Zone 1 in the UK in the not-too-distant future

(see Chapter 9).

7.2.4 Energy limitation (criterion d)



There is only one protedion concept in t i area but it is the protection

hs

concept offering the highest level of security.

Intrinsic safety (symbol ‘i’)



In this technique the electrical energy fed to the equipment is below that

which will cause ignition if released in a spark or as a hot surface, and



Design philosophy for electrical apparatus for explosive atmospheres 169



all energy storage in the equipment is closely controlled to ensure that

stored energy is limited. As with pressurization, this equipment relies to

a large extent upon other equipment, which feeds it with electricity and

the technique is somewhat complex. It is also limited in its application to

instrumentation because of the energy limitations imposed, but within this

limitation it is very flexible.

There are two grades of intrinsic safety, the higher grade ‘ia’ being suitable for Zone 0, Zone 1 and Zone 2, but the lower grade ’ib’ being limited

to Zone 1 and Zone 2.

7.2.5 Special situations



While the methods of equipment protection given in Sections 7.2.1 to 7.2.4

cover most situations normally encountered, there is an additional type of

protection for use where the types already described are not appropriate

because of advances in technology or special circumstances.

Special protection (symbol ‘s’)



Special protection is not a fixed type of protection, as in the above but is

used to describe equipment which is suitable for use in explosive atmospheres on the basis that its individual type of protection is equivalent to

one of the more classic types, although different. Its typical use is to permit

advances in technology where such advances do not easily fit into standard

types of protection. It has been typically used for encapsulated equipment

’m’ prior to the publication of the construction Standard for that type of

protection, and for sintered flame arresters prior to those being included in

type of protection ‘d‘ (flameproof enclosure). It has also been used where

techniques contrary to existing standards such as bi-pin tubes in type ‘e‘

(increased safety) fluorescent fittings are used on the basis of equivalent

safety. It can also be used where equipment which has two independent

types of protection, each of which is complete in itself (e.g., an increased

safety ’e’ terminal chamber in a flameproof enclosure), is used provided

that the two protection concepts are each independent of the other. This

latter approach must, however, be treated with caution.

Type ‘sf equipment is usable as specified in each particular case, and is

only usable in Zone 0 and 1 where this is specifically stated.



7.3 Situation in respect of Zone 2 apparatus

While the techniques described in Sections 7.2.1 to 7.2.5 are all suitable for,

at least, Zone 1 and 2 (with the exception of the current situation in respect

of protection concepts ’0’

and ’q’) they offer a standard of protection which

is higher than is necessary for Zone 2. For this reason a further type of

protection has been developed.



170 Electrical installations in hazardous areas



Type of protection ‘n’ (historically in the UK, type of protection IN’)



This type of protection includes industrial equipment which does not get

hot or spark, which was historically selected by users, and adds simplified

forms of protection for sparking equipment or that which gets hot, based

upon the Zone 1 types of protection. By producing detailed requirements,

it removes the doubt which used to exist when only selection by somewhat

general criteria was the order of the day, which led to excessive use of

Zone 1 equipment in Zone 2, needlessly increasing operational expense.



7.4 Protection of electrical apparatus for dust risks

Unlike the situation in respect of gases, vapours and mists, the problems

in relation to dusts are the settling properties which dusts have, and their

conductivity. If dust enters an equipment enclosure in significant quantities, it will settle on the electrical components and by insulating them

could cause excessive heating and, if it is conductive, partial short circuits

and similar faults. The objective with dusts, therefore, is always to prevent

significant amounts penetrating the enclosure. This leads to a very much

simpler situation than that which exists with gases, vapours and mists, as

there is effectively only one form of protection, which is to ensure the dust

remains on the outside of the enclosure.

While some equipment constructed for gas/vapour/mist risks is also

suitable for dusts, this does not necessarily follow, and it should not be

assumed that because equipment is suitable for the one type of medium

it is automatically suitable for the other. It is very important to remember

this as dust risks and gas/vapour/mist risks often coincide and, in such

cases, the electrical equipment must be protected for use in both types of

risk, and the elements of this protection may be separate in each case.



7.5 Apparatus construction Standards

The harmonization of Standards is of paramount importance for members

of the EU as these Standards will be acceptable throughout the EU and,

by an agreement between the EU and the European Free Trade Association (EFTA), in EFTA countries also. To this end, those Standards which are

harmonized may be called up in EU Directives as Standards with which

compliance demonstrates fulfilment of the Directive, provided this is verified by a recognized (notified) certification body. Therefore, the Standards

for equipment protection for gas, vapour and mist risks (with the exception

of those methods restricted to Zone2 use only) have been historically

referred to in 76/117/EEC2 and compliance with them allows the use of

the Distinctive Community Mark (see Chapter 1).With the introduction

of the newer Directive 94/9/EC5, the EEC policy was changed to include

essential technical requirements within the Directive. These are included



Design philosophy for electrical apparatus for explosive atmospheres 171



in a relatively general form and it is intended to identify the harmonized

standards (EN’S) as a method of satisfying the essential requirements, by

their approval by publication in the European Journal7. Unfortunately, it

has been realized that the second editions of these current Standards do

not necessarily satisfy the essential requirements and the Standards writing

body, the Centre Europeen de Normalization Electrique (CENELEC), has

been remitted to produce third editions which do in general, in 1997. This

means that the new Directive5 cannot be practically introduced until some

time in 1997, and in the meantime the older Directive2 will remain the only

vehicle by which the Distinctive Community Mark can be awarded. Unfortunately, this Directive still refers to the first editions of the harmonized

Standards and the second editions, now current, will not permit the use of

the mark. Urgent action is being taken to introduce the second editions of

the harmonized Standards into 76/117/EEC2 but until that time certification

in accordance with that Directive will be in line with the first editions.

While this seems to be a problem, it is not really, since the second

editions do not vary widely from the first, and most equipment manufacturers are ensuring that their equipment will comply with both until

the new Directive5 comes into force . When this Directive5 is enacted the

third editions of the harmonized Standards will exist, and any equipment

certified after that will comply to the new provisions.



7.5.1 Zone 0 and/or Zone I compatible apparatus for gases,

vapours and mists



As already stated, there is a long world-wide history of utilizing only thirdparty certified equipment in Zone 0 and 1 and in some countries this is

required by law. At the moment this is not the case in the UK, but the

new Explosive Atmosphere Directive5 from the EU will effectively ensure

this by requiring that all equipment marketed in the EU complies with it

(the Directive itself requiring third-party type certification as a minimum).

While this will cause some restriction in the UK the impact will not be

great, as industry in the UK has historically used only third-party certified

equipment by choice. Therefore, subject to the resolution of the difficulties

already identified in this chapter, the situation will not change greatly.

What this historic situation has led to is a very high degree of detailed

construction Standards which have become more and more international

due to the international level of operation of the industry which needs

the equipment. The current situation is that the Standards for electrical

equipment intended generally for Zone 0 or Zone 1 (or both) are fully

harmonized European Standards, and within the next 10 years will become

fully harmonized International Standards. There are, however, historic

national and European Standards which address this area and equipment is

still available complying with these Standards. There is no suggestion that,

in general, such equipment is not still safely usable in many circumstances,

and the titles of those Standards are referred to in Chapters 8 to 15. Many of



172 Electrical installations in hazardous areas



those standards are, however, obsolete or withdrawn and they should not

be used for construction of new equipment. Where equipment complying

with both current and historic Standards is available, the equipment to the

current standards is always preferable.

The relatively large number of methods by which electrical equipment

may be protected has meant that a large number of Standards exist, and

it has been recognized that many of the requirements of these standards

are common. The standardization structure is therefore to have a general

Standard and a series of additional Standards which refer to the general

Standard for many of their requirements, but contain additional requirements particular to the method used to protect the equipment. These are

all European Standards and are structured as follows.



BS/EN 50014 (1993) - General requirements (see Chapter 8)



This is the general or common Standard and contains any requirements

which apply to more than one of the specific methods of protection. Its

requirements apply to all of the protection concepts (with the current

exception of ’ ’ , unless they are specifically excluded and should be

n)

read in conjunction with all of the following individual protection concept

Standards. It is currently at its second edition stage which should supersede

the first edition, but equipment certified in accordance with this and one of

the subsidiary protection concept Standards cannot, as already described,

bear the Distinctive Community Mark referred to in Chapter 1, unless it

also satisfies the first edition of the Standard. Therefore, it will not have

free access to EU Member States (and EFTA States).

This, as already identified, is because Directive 76/117/EEC2 currently

refers to the first edition and the mark is controlled by the Directive, not

the Standard. Again, as already identified, work is in hand to modify

76/117/EEC2 and it is expected to allow the use of the second editions of

this, and its subsidiary Standards, for equipment relating to the Directive

in late 1996. A further complication which is preventing the activating of

the new Directive5is the fact that there appear to be discrepanciesbetween

the second editions of these Standards and the essential requirements given

in the new Directive. (As already stated, all new approach Directives carry

the essential safety requirements within them, unlike their older counterparts which referred to Standards). This means that third editions of These

standards will need to be produced and the production is expected to be

complete in late 1997.When published, equipment complying with the third

editions of these Standards will be deemed to comply with the Directive

and will consequently be able to carry the ’Distinctive Community Mark’

in accordance with the Directive’s provisions. The need for continuing with

the older Directive, 76/117/EEC2, will then cease to exist for new equipment, although the older equipment will continue to be acceptable until the

beginning of the twenty-first century (see Chapter 1).



Design philosophy for electrical apparatus for explosive atmospheres 173



This Standard, as already indicated, is supplemented by a series of specific

Standards.

BS/EN 50015 (1994) - Oil immersion ‘0’ (see Chapter 9)



This is the second edition of this particular Standard. It suffers from the

same problems in use as BS/EN 50014, second edition, and the resolution

of those problems is being dealt with in the same way.

BS/EN 50016 (1996) - Pressurized apparatus ‘p’ (see Chapter 11)



This is the second edition of this particular Standard. It suffers from the

same problems in use as BS/EN 50014, second edition, and the resolution

of those problems is being dealt with in the same way.

BS/EN 50017 (1 994) - Powder filling ‘q’ (see Chapter 9)



l h s is the second edition of this particular Standard. It suffers from the

same problems in use as BS/EN 50014, second edition, and the resolution

of those problems is being dealt with in the same way.

BS/EN 50018 (1995) - Flameproof enclosures ‘d’ (see Chapter 10)

This is the second edition of this particular Standard. It suffers from the



same problems in use as BS/EN 50014, second edition, and the resolution

of those problems is being dealt with in the same way.

BS/EN 50019 (1994) - Increased safety ‘e’ (see Chapter 12)



This is the second edition of this particular Standard. It suffers from the

same problems in use as BS/EN 50014, second edition, and the resolution

of those problems is being dealt with in the same way.

BWEN 50020 (1995) - Intrinsic safety ‘i’ (see Chapter 13)



This is the second edition of this particular Standard. It suffers from the

same problems in use as BS/EN 50014, second edition, and resolution of

those problems are being dealt with in the same way.

EN 50028 (1987) - Encapsulation ‘m’ (see Chapter 9)



This is the first edition of this particular Standard. It is dual numbered and

is also known as BS 5501, Part 8 (1987). It is not being rewritten at this time

as it is felt to remain current and will not be amended until a third edition

is published to comply with the essential requirements of 94/9/EC5. This



174 Electrical installations in hazardous areas



produces a further complication as it refers to the first edition of EN 50014

and should be used with that edition. This is not, however, a major problem

as those parts of EN 50014 which apply can be derived from either edition

of EN 50014 without problems (see Chapter 9).

EN 50039 (1980) - Intrinsically safe electrical systems ‘i’

(see Chapter 13)



This is the first edition of this particular Standard and is dual numbered as

BS 5501, Part 9 (1982). Fortunately it does not directly relate to EN 50014

but is stated to be a supplement to EN 50020 (1977) (BS 5501 part 7 (1977)),

the first edition of that Standard. There is no practical problem here and this

Standard can equally be used with the second edition of EN 50020 which

is current (BS/EN 50020) (see Chapter 13).

7.5.2 The marketing situation in respect of European Standards



The value of the European Standards referred to in Section 7.5.1 lies in the

fact that equipment which complies with them in their appropriate issue

may bear the Distinctive Community Mark and has free access to both the

EU and EFTA (European Free Trade Association) States. The problem lies

in the difficulty in harmonizing the political (EU) documentation with the

standards which are written by a separate technical body (CENELEC). For

the time being, equipment must comply with the first editions of these

standards to bear the Distinctive Community Mark but will also need to

comply with the second editions of the Standards because of the imminent

modification of the older Directid. It is likely, for the time being, that

equipment will need, to provide maximum flexibility for the manufacturer,

to comply with both the first and the second editions.

In addition, because two of the above Standardsare still current in their first

editions, the situation appears even more complicated. These two Standards,

however, make limited reference to EN 50014 and the change of edition of

that Standard will make little practical difference. Also, the ‘n’ Standard is

relatively contemporary and will include much of current thinking.

In addition to the above Standards, a certification Standard was issued

for type of protection ‘s‘ by one of the UK third-party certification bodies.

This may sound odd as, if an item of equipment is special it infers that it

is non-standard. These was, however, a good reason for t i issue and the

hs

Standard is used by both UK third-party certification bodies for the issue

of national certificates, as opposed to the European certificates issued for

equipment complying with European Standards.

SFA 3009 (1972) - BASEEFA Standard for type of protection ‘s’

(see Chapter 9)



This Standard included many of the techniques developed, but not

included, in formal Standards at the time (e.g., sinters, encapsulation, etc).



Design philosophy for electrical apparatus for explosive atmospheres 175



It has now been largely taken over by the second editions of the European

Standards, but equipment certified to its requirements is still available and

used in the UK.

7.5.3Zone 2 compatible apparatus



forgases, vapours and mists



In the case of Zone 2 apparatus there is no long history of third-party

certification as in the more hazardous zones, and industry has historically

selected its own using a British Standard Guide1 for assistance. Within

the UK, however, industry has always been slightly uneasy at this and

has sought third-party certification for Zone 2 equipment also. Historically,

there was no avenue for this and as no detailed certification requirements

existed it was difficult to arrange one. This unhappy situation led to HM

Factory Inspectorate issuing ‘Letters of no Objection’ for some items of

equipment, which effectively said that the Factory Inspectorate had no

knowledge or evidence to suggest the equipment was unsuitable. This difficult state of affairs led to the production in the UK of a Standard for Type

’N’ equipment6 to permit formal Certification. The reason for the upper case

‘ N was that there was an international symbol for this equipment ‘n‘ and

it was hoped that the use of the upper case letter would avoid confusion

with the international concept, for which there was no Standard. There still

remains only a National Standard for Type ’ N equipment and, unlike the

situation in Section 7.4.1 where certification is valid across Europe, type ’N’

certificates issued within the UK are valid only in the UK. The Standard in

question is:

BS 6941 (1988) - Electrical apparatus for explosive atmospheres with type

of protection ‘N’ (see Chapter 14)

This Standard covers all of the requirements for all of the methods used for

protection of electrical equipment which is intended for Zone 2. It is based

upon an international document published by the International Electrotechnical Commission - EEC 79/15 (1987), but because of the fact that the UK

did not agree with some of the contents of IEC 79/15 there are significant

differences.

IEC 79-1 5 (1987) - Electrical equipment for explosive gas atmospheres

Pari 15: electrical apparatus with type of protection ‘n’ (see Chapter 14)



This international document was the subject of considerable controversy as

several countries objected to some parts of it. The principal objection of the

UK was to the restricted breathing concept being used for sparking contacts

and t i concept does not appear in BS 6941. Because of these objections it

hs

was only possible to issue IEC 79-15 as an IEC Report not a Standard. This

means that it is only a guidance document despite having the appearance

of a Standard.



176 Electrical installations in hazardous areas



7.5.4 Electrical apparatus for dust risks



The British Standard for dusts based upon the International Standard is

not yet published and the current British Standards3are limited to Zone 21

and 22 (Zone Z and Y .There are currently, therefore, no Standards for the

)

construction of electrical equipment for Zone 20.

For Zone21 and 22, equipment construction Standards exist. These

mainly cover the degree of enclosure of the equipment, together with

additional construction requirements covering such things as strength, etc.

In one case, the relevant Standard covers the additional requirements

necessary to make equipment constructed for gas/vapour and mist risks

suitable for use in the explosive atmospheres of combustible dusts. The

relevant Standards follow:

BS 6467 (1985) - Electrical apparatus with protection by enclosure for use

in the presence of combustible dusts, Part 1: specification for apparatus

(see Chapter 15)



It will be noted that the Standard particularly draws attention to the fact that

protection is by enclosure. In addition, the Standard differentiates between

equipment suitable for Zone21 and Zone22, and that suitable only for

Zone 22.

BS 7353 (1992) - Guide to the use of electrical apparatus complying

with BS 5501 or BS 6941 in the presence of combustible dust

(see Chapter 15)



This Standard essentially specifies in what circumstances equipment

complying with European Standards (ENS)is suitable for use in explosive

afmospheres of combustible dust and air and what extra constructional

features are necessary. It refers to these European Standards as BS 5501, as

their first editions were given the numbers that follow:

BS 5501, Part 1 (1977) - EN 50014, first edition

BS 5501, Part 2 (1997) - EN 50015, first edition

B 5501, Part 3 (1977) - EN 50016, first edition

S

BS 5501, Part 4 (1977) - EN 50017, first edition

BS 5501, Part 5 (1977) - EN 50018, first edition

BS 5501, Part 6 (1977) - EN 50019, first edition

BS 5501, Part 7 (1977) - EN 50020, first edition

B 5501, Part 8 (1988) - EN 50028, first edition

S

BS 5501, Part 9 (1982) - EN 50039, first edition



While, as before, these European Standards are now, or soon will be,

replaced by their second editions (which are numbered BS/EN5O and

upwards with the exception of BS 5501, Parts 8 and 9) the differences so

produced are not likely to be sigmiicant and it is likely that in most cases



Design philosophy for electrical apparatus for explosive atmospheres 177



the second editions will, when used with this standard, produce satisfactory

results.

IEC 1241-1 - Electrical apparatus for use in the presence of combustible

dust, Part 1: electrical apparatus protected by enclosure (see Chapter 6)



This is the IEC document which is relevant to dust hazards and is a much

newer document.

The requirements of this Standard are not dissimilar to BS 6467, Part 1

and at some time in the future it is expected that the two Standards will

be harmonized. Until that time, however, it is expected that equipment to

at

either the IEC Standard or BS 6467, P r 1will be acceptable in the UK.

A further Part of IEC 1241 is also envisaged and will address at least

some of the matters addressed by BS 7535. Work is in hand on sections

to cover intrinsic safety 'i' and pressurization 'p' in dust risks. When they

are published, unless the UK votes against them, they are expected to be

acceptable in the UK as alternatives to BS 7535.



References

1 BS 4137 (1967) Guide to the Selection of Electrical Equipment for use in

~~



2 76/117/EEC



3 BS 6467



4 RoSPA/ICI



5 94/9/EC



6 BS 4683

7



Division 2 Areas.

Council Directive of 18 December 1975 (on the approximation of the laws of Member States concerning electrical

equipment for use in Potentially Explosive Atmospheres)

Electrical Apparatus with Protection by Enclosure for Use

in the Presence of Combustible Dusts. Part 2 (1985)Guide

to Selection, Installation and Maintenance.

Engineering Codes and Regulations, Electrical Installations in Flammable Atmospheres (1973). Group C

(Electrical), Volume 1.5.

Directive of the European Parliament and Council of 23

March 1994 (on the approximation of the laws of Member

States concerning equipment and protective systems intended for use in Potentially Explosive Atmospheres)

Electrical Apparatus for Explosive Atmospheres Part 3

(1972). Type of Protection N.

Official Journal of the European Communities.