6 Apparatus Conforming to the Protection Concepts Appropriate to Gas, Vapour and Mist Risks

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



470



Electrical installations in hazardous areas



gas and vapour risks. This means that, with little or no modification, such

apparatus may be shown to be suitable for dust risks subject to temperature

classification selection as already described.



Oil-immersed apparatus ‘0’

(Chapter 9)



This type of apparatus will be suitable in the following circumstances.

First, its enclosure must be IP5X or IP6X, depending on the intended Zone

of use (IP5X for Zone Y or Zone 22, and IP 6X for Zone Z or Zone 21). While

the variation of enclosure integrity for conducting dusts is not appropriate

here, there is a risk that some dusts may degrade the oil and in such cases

IP6X is always required.

Second, in the case of this protection concept, the surface temperature

classification is for the surface of the oil, which is practically equivalent

to the enclosure temperature and no advantage can be gained by added

surface temperature testing. The maximum surface temperature reached

should therefore be the maximum permitted for the surface temperature

class given.



Pressurized apparatus ‘p’ (Chapter 11)



This type of apparatus will be acceptable in the following circumstances.

First, the general requirement for the enclosure is that it satisfies IP6X

(for Zone Z or 21) or IP5X (for Zone Y or 22) in circumstances where the

pressure is not present. In this case the conducting dust requirement of

IP6X in all cases is relevant as consideration is given in the absence of

pressurization. Additionally it is necessary to consider the effects of outlets

for exhaust of gas in the hazardous area. These need to be considered in

the IP-rating, unlike the gas inlets which do not as they will not be exposed

to the dust atmosphere.

Second, surface temperature classification will have been carried out

on the outer surface of the enclosure and there is no advantage in

attempting a determination of external surface temperature. Thus the

surface temperature will be assumed to be the maximum permitted for

the surface temperature class awarded.

Powder-filled apparatus ‘q’ (Chapter 9)



This type of apparatus is acceptable in the following circumstances.

First, this type of apparatus should be selected on the basis of the IPselection criteria (IP6X for Zone Z and 21 and IP5X for Zone Y and 22).

Once again, however, it is necessary to have IP6X in all cases where the

dust is conducting as, if it enters the enclosure and mixes with the powder,

it could adversely affect the operation of the apparatus. There is little



Protection concepts for apparatus for dust risks 471



likelihood of the dust reacting with the powder as the latter is normally

quartz or glass.

Second, surface temperature classification will have been carried out on

the surface of the powder, which is effectively the enclosure temperature,

and thus no advantage will accrue by measuring enclosure temperature.

The surface temperature will, be the maximum temperature permitted by

the surface temperature classification awarded.



Flameproof enclosure ‘d’ (Chapter 10)



This type of apparatus is acceptable in the following circumstances.

First, the choice of flameproof apparatus for use in dust risks is on the

basis of enclosure integrity. IP6X is required for Zone Z and 21 and IP5X

for Zone Y and 22. For conducting dusts IP6X is always required as entry

of dust into the enclosure could give rise to an internal dust explosion, and

the performance of the enclosure in such circumstances is not known.

Second, temperature classification of flameproof enclosures is carried

out on the enclosure outer surface and no advantage will result from

measuring enclosure temperature. The surface temperature is assumed to be

the maximum permitted by the surface temperature classification awarded.



Increased safety apparatus ‘e’(Chapter 12)



This type of apparatus is acceptable in the following circumstances.

First, increased safety is a technique which relies in part on enclosure

integrity and requires an IP54 enclosure, making it normally usable in

Zone Y and 22. Increasing its enclosure integrity to a minimum of IP64

will make it suitable for Zone Z an 21. One point to be stressed is that

increased safety relies to a significant extent on the protection equipment

in its supply, particularly in the case of rotating machines. This is no less

true in the case of dust risks and te time particularly must be adhered to.

One significant point is that temperature classification is based upon the

hottest part of the apparatus, be it within or outside the enclosure. If its

temperature classification is found to be too restrictive using Table 15.1

then temperature measurement of the outside of the enclosure may produce

significant gains. This is even true of rotating machines when the machine

is rotor sensitive (Le., the rotor temperature decides the surface temperature

classification).

Intrinsic safety 7’ (Chapter 13)



Apparatus of this type will be acceptable in the following circumstances but

as intrinsic safety does not rely on its enclosure for security then a more

flexible approach is possible.



472 Electrical installations in hazardous areas



If the intrinsically safe apparatus has an enclosure of IP6X it will be

suitable for use in Zone Z or 21 and if it has an enclosure of IP5X for

Zone Y or 22 without further consideration. As before, taking this approach

the enclosure will need to be IP6X even for Zones Y or 22 if the dust is

conducting.

The above approach takes no account of the added security of intrinsic

safety in that faults are required before an installation can become ignition

capable. This is true whether the installation is 'ia' or 'ib'. In this case,

however, there is much to be gained by considering the internal circuits. If

the enclosure is not of sufficient integrity then the dust can enter. When,

for instance the dust is non-conduding then it will only have the effect of

providing a layer over the components in the enclosure. This is unlikely

to sigruficantly alter the temperature classification of the apparatus and as,

'without fault', 'with one fault' in the case of 'ib' and with two faults in the

case of 'ia', any internal sparking is non-incendive the apparatus should be

suitable for Zone Z.

If the dust is conducting then any conducting parts within the apparatus may be assumed to be interconnected unless they are either coated

(varnished), encapsulated or insulated. In these circumstances the apparatus should be assessed on this basis and if sparking and hot surfaces

can still be shown not to be incendive in conditions of two faults for 'ia'

with these additional internal connections, then the equipment remains

suitable for ZoneZ or 21. Likewise, if the same evaluation is done for

'ib' apparatus it will be suitable for Zone 22 or Zone Y. In conditions of

humidity or other situations in which the dust can be assumed to be moist

then it should be treated as conducting dust, even if when dry it is nonconducting.

Extending this approach, an item of 'ia' apparatus which satisfies the

above requirements for either nonconducting or conducting (or moist)

dusts, and which is also enclosed in an IP6X enclosure, has a much

higher level of protection than any other of the foregoing equipment.

This apparatus should be safely usable in the new Zone 20 for the

relevant type of dust for which no other protected equipment is currently

available.

Encapsulation 'm' (Chapter 9)



Encapsulated apparatus, by definition, excludes dust to at least IP6X and

such apparatus is suitable for Zone Z and 21 (and thus Zone Y and 22)

without further consideration. The surface temperature class is based upon

the external temperature of the encapsulant and the maximum temperature should normally be taken as the maximum permitted by the surface

temperature classification.

It should, however, be noted that the connection facilities for such

apparatus will normally utilize alternative protection concepts and their

requirements should also be considered.



Protection concepts for apparatus for dust risks 473



Apparatus with type of protection ‘N’ (n) (Chapter 14)



This type of apparatus is normally only acceptable in Zone Y or 22 and only

then if it has an enclosure integrity of IP5X for non-conducting dusts and

IP6X for conducting dusts. As its surface temperature may be produced on

the basis of internal components, measurement of the enclosure temperature

may be helpful when the maximum temperature permitted by the surface

temperature classification is not acceptable.



References

1 BS 6467



Electrical Apparatus with Protection by

Enclosure for Use in the Presence of

Combustible Dust. Part 1, (1985). Specification for Apparatus. Part 2 (1988). Guide to

Selection, Installation and Maintenance.

2 BS 7535 (1992)

Guide to the Selection of Apparatus

Complying with BS 5501 or BS 6941 in the

Presence of Combustible Dust.

Engineering Codes and Regulations, Elec3 RoSPA/ICI

trical Equipment in Flammable Atmospheres

(1973).Group C (Electrical),Volume 1.5.

4 BS/EN 60529 (1991)

Specification for Degrees of Protection

Provided by Enclosure.

5 BS 5490 (1977) (withdrawn) Specification for Degrees of Protection

Provided by Enclosure. This Standard has

been replaced by BS/EN 60529 (1992).

6 BS 5420 (1977) (withdrawn) Specification for Degrees of Enclosure of

Switchgear and Control Gear for Voltages up

to and including 1OOOV ac and 1200V dc.

This Standard has been replaced by BS/EN

60947-1 (1992).

General Requirements for Rotating Electrical

7 EN 60034

Machines. Part 5 (1986). Classification of

Degrees of Protection Provided by Enclosures for Rotating Machinery.

General Requirements for Rotating Electrical

8 BS 4999

machines. Part 105 (1988). Classification of

Degrees of Protection Provided by Enclosures

for Rotating Machinery.

Electrical Apparatus for Potentially Explo9 BS/EN 50014 (1993)

sive Atmospheres. General Requirements.

Electrical Apparatus for Explosive Atmos10 BS 6941 (1988)

pheres with Type of Protection N.



Other methods of protection and

future apparatus requirements

To fully understand the future approach to apparatus for use in

explosive atmospheres it is necessary to consider the problems with

the present system which is based upon Directive 76/117/EEC1 and

its supplementary Directives. These define the detailed constructional

standards with which apparatus must comply to be suitable for formal

conformity certificationby a ‘notifiedbody‘, as described in earlier chapters.

This method of definition effectively restricted the types of equipment

which could be used in Zone 0 and 1 as, although the Directive only

addressed conformity certification as a method of European market

admission, it effectively defined what could be used in those Zones because

user industry was unwilling, in the main, to utilize any other type of

equipment. This did not apply to Zone 2 as the approach was much more

relaxed due to the much lesser risk envisaged.

In the case of Zone 0 and 1, however, the approach led to considerable restriction and inability to deal with technological advance and special

circumstances. Accordingly, within a national arena, the UK exploited the

protection concept known as special protection ‘s‘. The objective of this

concept was to provide a vehicle which would allow formal approval of

apparatus which, while not complying (or not complying fully) with any of

the standardized protection concepts (’d’, ’e’, ‘i’, ’m’, ‘o’, ’p’, ‘4’) achieved

the minimum required level of security in respect of ignition capability. The

objective of this was not to certify ’near misses’ but to allow for advances

in technology. The proof of the value of this approach is clear when it is

recognized that it has been used for such protection concepts as encapsulation ’m’ and devices without measurable flamepaths, such as sinters,

before these were included in the relevant protection concept Standard

and specifically recognized by the Directive. More recently, the approach

has been used to permit bi-pin tubes used in the cold cathode mode in

luminaires on the basis of multiple (at least four) connections allowing

redundancy, an approach not recognized in increased safety ‘e’, and to

recognize gas detectors measuring oxygen, and similar apparatus where

the oxygen concentration of the explosive atmosphere was enhanced and

not covered by the standard protection concepts, on the basis of special

testing (usually in oxygen enriched atmospheres).

Within the UK, national certification to an agreed set of requirements

has been historically available on the same basis as it has been for

the more classic approaches to construction. Directive 76/117/EEC’



Future apparatus requirements 475



attempted to address this problem by introducing the Inspection Certificate

which allowed certification of special protection methods. This certificate,

however, was noticeably different to the Certificate of Conformity and

required, before issue, the agreement of all of the European notified

bodies who had the right to perform their own tests at the expense of

the manufacturer before granting such approval. The result of these two

situations made the Inspection Certificate approach very unattractive and

resulted in very few certificates being issued and, resultantly, slow advance

in technology in the European scenario. In addition, 76/117/EEC addressed

only the gas, vapour and mist scenario, leaving dusts outside European

legislation.

In the production of the new Directive 94/9/EC2 an attempt has been

made to overcome both of these problems. The new Directive does not

refer to European Standards for definition of construction requirements

but includes them as ’essential requirements’ within its own text. This

allows a much simpler method of addressing European Standards by merely

referring to them in the EU Journal3 as Standards defining constructional

requirements which satisfy essential requirements and removes the previous

cumbersome Inspection Certificate approach by using Certificates of

Conformity in all cases and providing a more rapid acceptance route for

technological advance. In addition, apparatus for dust risks is now included

together with protective systems, such as those used with pressurized

apparatus ’p’ which were hitherto not adequately addressed.

The one negative aspect of the new Directive, however, is its requirement for compliance. Under 76/117/EEC1 use of conforming apparatus

was optional, the Directive merely stating that member states could not

prevent its import. Directive 94/9/EC2 is quite different in that it prohibits

marketing and putting into use of apparatus not complying with it thereby

reducing widely used national flexibility. While it permits the 76/117/EEC

approach until the end of June 2003, and hence the use of apparatus subject

to a Certificate of Conformity or an Inspection Certificate until that date,

the marketing in member states of apparatus which does not conform to

76/117/EEC or 94/9/EC is already in force, having been introduced with

f

the implementation o the new Directive.



16.1 Acceptance of technical requirements

As already stated, the essential requirements for construction of apparatus

in order to comply with 94/9/EC are contained within the Directive itself.

These are, however, considered as insufficiently detailed to allow construction of apparatus and CENELEC4 has the mandate to produce Standards

which satisfy the ’essential requirements’ and contain the required level of

i,

detail. This activity is expected to be complete for ’d’/ ’e’, ’’ ‘m’/ ’o’, ’p’ and

‘q’ in mid-1997 and for some time later. The situation in regard to ‘n‘ is not

quite so difficult as the Directive maintained the lower level of certification

formality previously used in Zone 2 equipment.



476 Electrical installations in hazardous areas



When produced, these European Standards will be identified in the EU

Journal as satisfymg 'essential requirements' and any future additions will

also follow this route. Thus technological progress is placed clearly in the

hands of the technical-standardsking body and a much more rapid route

to recognition is made available. How this will ultimately operate is not yet

clear but if the approach is positive then a much more flexible situation

should emerge.



16.2 Essential requirements

The 'essential requirements', not withstanding their technical impact were,

in fact, written by employees of the European Commission taking such

advice as they felt necessary. They tend to specify the objectives to be

achieved in a general way and leave the technical rules necessary for the

achievement of these objectives to others. While this approach is generally

reasonable it does throw up some curiouS anomalies as will be seen later

in this chapter.

The requirements apply, unlike those of 76/117/EEC,' to both electrical

risks and mechanical risks, although implementation in the case of mechanical risks is delayed as there are currently no Standards which can be used

to identify detailed requirements. They are divided into three parts:

16.2.I General requirements



These identify five basic requirements which apparatus must meet:

Principles



The equipment is required to prevent, if possible, any explosive atmosphere

being formed by any gas, vapour, mist or dust which it may itself release

and prevent, as far as possible, any ignition of an explosive atmosphere

formed by is own release or otherwise. Where it cannot do this it is required

to contain the explosion as far as possible to prevent damage or injury.

Environmental requirements



The apparatus or protective system (protective systems, such as shut-down

systems, are included) is required to be capable of operation without

damage or deterioration in the environmental conditions in which it

is intended to be used. This is not uniquely an explosive atmosphere

requirement but is very important in the explosive atmosphere context; if

the apparatus deteriorates excessively then so may its explosion protection

elements. Thus any identified problems with its environment must not

adversely affect its explosion protection.



Future apparatus requirements 477

Basic construction and use



The apparatus or protective system must be designed and manufactured

so that within its specified tolerances, in normal operation and with

any expected faults having occurred, and taking account of any required

checking and maintenance envisaged by the designer, it does not create

a dangerous condition. Dangerous condition in this context means an

unacceptable risk of ignition.



Information



Sufficient information must be supplied with the equipment to allow its

safe use. Manufacturers may well be required to release sufficient information to permit repair; a level of information which some manufacturers are

reluctant to release because of the possibility of copying of designs.



Marking



All apparatus complying with the Directive is required to carry the

Community Mark (Fig. 16.1), the Explosion Protection Mark (Fig. 16.2) and

a mark to indicate the hazard which it is intended to address (which is ’G’

in respect of explosive atmospheres of gas, vapour or mist, and ’ D if it is

intended for use in respect of dust risks). This, importantly, represents an

extension of the use of the ’Ex’ mark (Fig. 16.2) to protective systems as

well as protected apparatus. The use of the ’G’ and ’ D are also new and

may cause some confusion unless clearly identified with the ‘Ex’ mark.



Fig. 16.1 The community mark



478 Electrical installations in hazardous areas



Fig. 16.2 The explosion protection mark



Apparatus and protective systems are also required to be identified by

type, serial number and the year of construction, the latter being a requirement not previously applied.



76.2.2Materials of construction



The 'essential requirements' include basic requirements for the material

used in construction in that:

They must not themselves trigger off an explosion in any stress applied

in use. This requirement identifies the problems which could occur if

light metals were used without restriction on outer parts of an enclosure

because of the frictional (thermite) sparking risk which certain of these

materials exhibit when struck by rusty steel.

The material of construction must not be such as to react with any

flammable gases, vapours, mists or combustible dusts in a way which

would impair explosion protection. The principal problem is likely to be

in respect of plastic and elastomeric parts which could be degraded by

contact with chemicals and damage enclosure integrity or reduce insulation quality. Any such reaction between any materials of construction

and the explosive atmosphere needs to be considered.

Material of construction must be stable within the operating tolerances

o the apparatus. Instabilities (e.g., the possible corrosion or change of

f

state of materials due to temperature) must be identified and shown not

to affect explosion protection or the operation of protective systems.



Future apparatus requirements



479



16.2.3General design and constructional requirements

The basic requirements for the components used in protected apparatus and

protective systems are that they should only be used within their manufacturer’s ratings and, in the light of the state of technical knowledge at

the time of their construction, they will be capable of performing their

duty throughout their projected lifetime. (Apparatus in general use would

normally be expected be capable of operating, with a minimum of maintenance, for around ten years).

The Directive goes on to identify requirements for control of leakage from

apparatus, allowance for dust layer formation without danger, overloading

of equipment, identification of hazards introduced by necessary opening

of apparatus, and the control of internal ignitions. In addition it identifies

sparks, arcs, surface temperatures, static electricity, and pressure changes

as possible ignition sources which need to be guarded against.

The general requirements go on in much the same vane but in all cases

identify problems and require that they be guarded against without identifying in detail how this is to be done. Thus the production of Standards and

their reference in the EU journal3 is of great importance as without them it

is not possible to have confidence in compliance with the Directive.



16.2.4Specific requirements for particular types of apparatus and

protective systems

In addition to the general requirements which apply to all explosion

protected apparatus and protective systems there are additional

requirements which apply to specific types of explosion protected

apparatus. The delineation is basically by the level of safeguarding applied

relating to the intended Zone of use.



Category 1 apparatus and protective systems



Category 1 apparatus and protective systems are, by their requirements,

intended for operation in all hazardous areas including Zone 0 if for gas,

vapour or mist risks and Zone 20 if for dust risks. No delineation between

the two is present in the basic requirement which is: that the apparatus

must be protected by at least two independent means of protection such

that, when one fails the other remains operative; or that the apparatus must

remain non-incendive in both normal operation and with up to two faults

applied.

The two-fault criteria is readily recognizable as that used in intrinsic

safety (see Chapter 13) but the dual protection approach is new and not

has been fully addressed internationally in any technical forum. It thus

represents an unknown quantity and one which will require considerable

technical consideration before it can be fully implemented. The question as

to what is an ‘independent method of protection’ is a vexed one. Flameproof