1 BS/EN 50014 (1993) (Including Amendment 1 (1994))

180 Electrical installations in hazardous areas



particular protection concept Standard and, in this case, that Standard will,

in its scope where exclusion is total, or in its detailed text where exclusion is

only partial, define those parts of BS/EN 50014 (1993)' which do not apply.

The basic matters dealt with in BS/EN 50014 (1993)' and its predecessor,

BS 5501, part 1 (1977) are as follows:

1. Definitions of terms used in BS/EN 50014 (1993)' and its subsidiary

protection concept Standards.



2. The method by which apparatus is divided into groups and temperature

classes for utilization purposes.

3. Requirements for enclosures.

4. Requirements for fasteners used for enclosure securing, interlocking

devices used to prevent protection being bypassed, bushings used for

connections, and cements which may be used in enclosure construction.



5. Requirements for connection of conductors to the apparatus and for entry

of cables to the apparatus.

6. Requirements which additionally apply to particular types of apparatus

such as rotating machines, luminaires, fuses, etc.

7. Requirements for ex-components, which are components which form

part of an apparatus but may be used in several types of apparatus, and

be examined for compliance with the appropriate parts of the standard

separately to avoid repeat evaluation.



8. Requirements for marking of complying apparatus to ensure that appropriate information is given to the purchaser.

It is worthy of note that BS/EN 50014 (1993)' draws attention to the fact

that apparatus constructors need also to ensure that basic electrical safety

requirements are met, and are assumed to guarantee this by application of

the marking required by the Standard. (BS 5501, part 1 (1977) required a

specific form of declaration for this purpose.)



8.1.1 Definitions



There are some 25 definitions in the Standard, some dealing with terms

such as apparatus and some dealing with the meaning of some of the

symbols used in the Standard and its supplementary protection concept

Standards. In common with earlier chapters they will be detailed at the

end of this book.While in the main, these definitions cover a l of the group

l

of Standards, they are not complete but need to be added to by additional

definitions in the protection concept Standards (showing the problems associated with limiting the meaning of terms by definition).



General requirements for explosion protected apparatus 18 1



8.1.2 Division of apparatus into sub-groups and surface temperature

classes



As earlier indicated, electrical apparatus is divided into groups and classes

according to its performance with regard to ignition capability. Initially,

the apparatus is divided into Group I, which is intended for use in gassy

mines (principally but not exclusively coal mines), and Group I1 which

covers apparatus for use in other industries (which in effect means surface

industry). This book is concerned with Group I1 Apparatus and the requirements for Group I will not be further explored.



Apparatus sub-grouping

As already stated, apparatus is divided into Group I and Group 11. Group I1

apparatus may be further divided into sub-groups to identify particular

factors appropriate to its use. At the moment, only one sub-grouping system

is in common use and that is related to the energy which may be released

in a spark within an explosive atmosphere, or with the ability of flame to

transmit through small gaps and ignite any explosive atmosphere downstream of the gap.

Intrinsic safety and similar protection concepts do not seek to prevent the

release of electrical energy, but only to limit its value to that which cannot

cause ignition. Apparatus and systems which are said to be intrinsically

safe will be sub-grouped as follows:Sub-group IIA: Apparatus and systems which will not ignite the most

easily ignitable mixture of propane/air when tested in accordance with

Clause 10.4 of BS/EN 50020 (1993). This test corresponds approximately to

an equivalent released energy of 160 microjoules from an inductive circuit

where energy release is very efficient. Gases, vapours and mists in mixture

with air are associated with this sub-group where the minimum current

required to cause their ignition (MIC) is more than 0.8 of that needed to

ignite the most easily ignitable mixture of laboratory methane and air (laboratory methane is more than 95 per cent pure) in a spark test apparatus (see

Annex B of BS/EN 50020 (1993)' using the calibration circuit specified in

that Annex).

Sub-group IIB: Apparatus and systems which will not ignite the most

easily ignitable mixture of ethylene/air when tested in accordance with

Clause 10.4 of BS/EN 50020 (1993). This test corresponds approximately to

a released energy of 80 microjoules from an inductive circuit where energy

release is very efficient. Gases, vapours and mists in mixture with air are

associated with this sub-group when their MIC is between 0.45 and 0.8 of

that needed to ignite the most easily ignitable mixture of laboratory methane

and air when tested in a spark test apparatus (see Annex B of BS/EN 50020

(1993)l using the calibration circuit specified in that Annex).

Sub-group IIC: Apparatus or systems which will not ignite the most

easily ignitable mixture of hydrogen/air when tested in accordance with



182



Electrical installations in hazardous areas



Clause 1 . of BS/EN 50020 (1993). This test corresponds approximately to

04

a released energy of 40 microjoules from a n inductive circuit where energy

release is very efficient. Gases, vapours and mists in mixture with air are

associated with this sub-group when their MIC is less t a 0.45 of that

hn

necessary to ignite the most easily ignitable mixture of laboratory methane

and air when tested in a spark test apparatus (see Annex B of BS/EN 50020

(1993) using the calibration circuit specified in that Annex).

In the case of sub-group I C the statement may sound a little odd as

I,

the apparatus or system is tested with hydrogen/air which has an MIC of

around 0.45, and more sensitive gas/air mixtures may be ignited by the

apparatus. The fact is that hydrogen/air is the most sensitive gas known

and more sensitive gas mixtures can only be produced by additional oxygen

in the mixture. Such mixtures are outside the scope of this Standard and

need to be treated specially.

Flameproof enclosure and similar concepts do not limit the release of electrical energy within the apparatus but seek to prevent the progress of the

flame to any explosive atmosphere outside their enclosure. To do this they

have any apertures closely controlled to ensure that any internal conflagration is sufficiently cooled or otherwise treated such that no external ignition

will take place. Flameproof apparatus for a specific group (the groups being

I A W and IIC as before) is tested in specific gas/air mixtures to ens=

I,

that the transmission cannot take place. These test mixtures are arranged

to give a safety factor in use and the situation is more complex than is the

case for intrinsic safety. Gases, vapours and mists are, in this case, associated with the groups on the basis of tests in a speaal test apparatus. This

has 2 5 m m long flanged joints of adjustable aperture, as described in IEC

79 - 1A (1975), titled 'Electrical Apparatus for Explosive Gas Atmospheres,

Part 1: Construction and Test of Flameproof Enclosures of Electrical Apparatus. First Supplement 'Appendix D Method of Test for Ascertainment of

Maximum Experimental Safe Gap' (seeFig. 8.1).The apertures are adjusted

until flame propagation from inside the test apparatus to the surrounding

explosive atmosphere does not take place. This aperture is termed the

maximum experimental safe gap (MESG). The mixtures, both inside the test

apparatus and outside, are mixtures of the same gas and air but that inside

is the stoichiometric mixture (the mixture where all the fuel gas and oxygen

are consumed) and outside is the most easily ignited mixture, which may be

slightly different. The reason for this is that the internal mixture produced

gives the worst conditions for flame transmission and the outer mixture the

worst condition for ignition if transmission occurs.

Sub-group ZZA:Apparatus wl not ignite an external explosive atmosphere

il

when filled with and surrounded with a mixture of 55 per cent hydrogen

i

in a r (equivalent to an MESG of 0.65mm) at atmospheric pressure. Gases,

vapours and mists in mixture with a r wl be associated with sub-group

i il

IIA when their MESG is measured by the method described in IEC 79-1A

(see Fig. 8.1) at above 0.9mm. A safety factor is produced by using a more

sensitive test mixture for the apparatus.



General requirements for explosion protected apparatus 183



sphere is the

most easily ignitable

mixture of the test

gas and air



Fig. 8.1 Method of determination of maximum experimental safe gap. (For full details

see IEC 79-1A (19754, Appendix D)



Sub-group IIB: Apparatus will not ignite a surrounding explosive

atmosphere when filled with, and surrounded by a mixture of 37 per cent

hydrogen in air (equivalent to an MESG of 0.35mm) at atmospheric

pressure. Gases, vapours and mists in mixture with air will be associated

with sub-group IIB when their MESG is measured by the method described

in IEC 79-1A (see Fig. 8.1) ia between 0.5mm and 0.9mm. A safety factor

is produced by using a more sensitive test gas for the apparatus.

Sub-group IIC: Apparatus testing is very different here as achieving a

safety factor on the test gas is more difficult. The safety factor is achieved

by increasing the gaps specified by the manufacturer and then testing with

the most sensitive mixtures of both hydrogen and acetylene with air. (These

are 28 per cent hydrogen in air and 7.5 per cent acetylene in air). Gases,

vapours and mists in mixture with air will be associated with sub-group

IIC when their MESG is measured in accordance with the method described

in IEC 79-1A (see Fig. 8.1) at less than 0.5 mm. In this case, the safety factor

normally achieved by enlargement of gaps is rather more sensitive than test

mixtures.

Once again, as in the case of intrinsic safety the IIC statement is based

upon the fact that hydrogen and acetylene are the two most sensitive gases

known and more sensitive mixtures can only be achieved by adding further

oxygen, which is outside the scope of this Standard and requires special

treatment.

Fortunately there is a relationship between MIC and MESG and to allocate

a gas, vapour or mist to a particular sub-group it is only necessary to carry



184 Electrical installations in hazardous areas



out either an MIC or an MESG test and not both. This ceases to be true,

iis

however, at the upper lmt and where both need to be carried out. In such

cases the following will be true: where MIC is between 0.8-0.9mm, MESG

will determine the sub-group; where MIC is between 0.45-0.5mm, MESG

will determine the sub-group; where MESG is between 0.5-0.55mm, the

MIC will determine the sub-group.

These criteria are necessary to ensure that the most sensitive parameter is

used to determine the sub-group.

To assist in sub-grouping gases, vapours and mists which have not been

tested, it is often possible to identify them as one of a range of materials

of similar structure, in which case it is highly likely that their sub-group

will be the same as other gases, vapours and mists within similar materials

which have a lower molecular weight. In all cases, however, care must be

taken with materials not already allocated to a sub-group to ensure that

no special feature of the material may make allocation unacceptable. Ethyl

nitrate, for instance, will produce an explosion pressure in excess of any

allocated material. There is no guarantee that flameproof apparatus will

withstand an internal explosion of this material and special precautions are

necessary.

The system for sub-grouping now used was preceded by different

systems intended to achieve the same objective in the TJK and other

countries. The approximate relationship between the current system and

these historic systems is given in Table 8.1.



Surface temperature classification

Any unprotected surface to which an explosive atmosphere has access may

cause ignition. This means that while for such protection concepts as flameproof enclosure or pressurization, only the external enclosure temperature

is important. When intrinsic safety and increased safety are considered, the

temperature of internal components becomes important as the explosive

atmosphere has access to them and there is no method of preventing flame

transmission. For all apparatus surfaces where any ignition caused would

produce uncontrolled burning, be they inside or merely on the outer enclosure of the apparatus, it is necessary to identify the attained temperature

in the worst case of operation, which includes supply variation (which in

the case of mains-fed apparatus is normally plus or minus 10 per cent.)

This is done by temperature classifying apparatus into six temperature

classes on the basis of the maximum temperature it reaches in the extreme

of its designed operating conditions (with a safety factor) and associating

gases, vapours and mists with those classes on the basis of their ignition

temperatures, and giving apparatus a temperature classification . Because

of the greater difficulty in causing thermal ignition due to the effects of air

hs

movement, etc., the safety factor in ti case is smaller than that normally

used for grouping. The temperature classes are as follows.



General requirements for explosion protected apparatus 185



Table 8 1 Current european grouping system showing relationship

.

to historic German and UK systems and current USA

system

Test

gases



European

grouping



Historic

UK

groups and

classes

FLP

IS

GRP

CLS



us



German

class



Propane



IIA



I1



2c



1



Ethylene



IIB



I11



2d



2



Hydrogen



IIC



rv



2e



3a



Acetylene



IIC



Iv'



2f



3c



Carbon

disulphide



IIC



IV'



2f



groups and

classes



3b



All gases



Class I2

Group D

Class I*

Group C

Class 1'

Group B

Class I*

Group A

Not

specifically

allocated



3n

~



~



~~



~



~~~~



Notes:

1 Although Group IV was allocated for these gases the Standard appropriate at

the time (BS 229) excluded construchon requirements for these gases and thus

no equipment exists.

2 Class I in the USA National Electrical Code was for gases, vapours and mists only.

Dusts were Class 11, and fibres and flyings Class 111.



T1



For T1 the maximum apparatus temperature must not exceed 440°C. (As

temperature classification is normally done at 40 "C ambient temperature,

this usually means an elevation due to self heating of 400°C.) Gases,

vapours and mists associated with this temperature class will have ignition

temperatures in excess of 450 "C.



T2



For T2 the maximum apparatus temperature must not exceed 290°C. (As

temperature classification is normally done at 40 "C ambient temperature,

this usually means an elevation due to self heating of 250°C.) Gases,

vapours and mists associated with this temperature class will have ignition

temperatures of between 300 "C and 450 "C.



186 Electrical installations in hazardous areas



T3



For T3 the maximum apparatus temperature must not exceed 195°C. (As

temperature classification is normally done at a 40 "C ambient temperature,

this usually means an elevation due to self heating of 155"C.) Gases, vapours

and mists associated with this temperature class will have ignition temperatures of between 200 "C and 300 "C

T4



For T4 the maximum apparatus temperature must not exceed 130°C. (As

temperature classification is normally done at a 40°C ambient temperature, this usually means an elevation due to self heating of 90°C.) Gases,

vapours and mists associated with this temperature class will have ignition

temperatures of between 135°C and 200°C.

T5



For T5 the maximum apparatus temperature must not exceed 95°C. (As

temperature classification is normally done at a 40°C ambient temperature, this usually means an elevation due to self heating of 55°C.) Gases,

vapours and mists associated with this temperature class will have ignition

temperatures of between 100"C and 135"C.

T6



For T6 the maximum apparatus temperature must not exceed 80°C. (As

temperature classification is normally done at a 40°C ambient temperature, this usually means an elevation due to self heating of 40°C.) Gases,

vapours and mists associated with this temperature class will have ignition

temperatures of between 85 "C and 100"C.

As indicated, the self elevation of the apparatus permitted in all cases

depends upon the ambient temperature at which temperature classification

is carried out. If an item of apparatus is temperature classified as T3 at an

ambient temperature of 100"C its permitted self heating will be reduced to

95 "C, as the overall maximum temperature must remain the same.

It is also recognized that small components can exceed the ignition

temperature of a particular gas, vapour or mist without causing ignition,

and this has been demonstrated as the case. In general, this difference

depends upon factors such as the convection performance of the particular

gas, vapour or mist and the configuration of the hot surface. For this reason,

there is no general relaxation (except in the case of intrinsic safety, see

Chapter 13) and each type of small component must be treated individually.

To do this it is necessary to determine the temperature at which the surface

in question actually ignites a gas representative of the most sensitive in the



General requirements for explosion protected apparatus 187



temperature class, and then ensure that the surface does not exceed the

following temperatures in service:

T1 - ignition temperature minus 50 "C

T2 - ignition temperature minus 50 "C

T3 - ignition temperature minus 50 "C

T4 - ignition temperature minus 25 "C

T5 - ignition temperature minus 25 "C

T6 - igrution temperature minus 25 "C

A system of temperature classification similar in concept to this existed in

Germany and the relationship between this and the current system is shown

in Table 8.2. In the United States of America there was initially a system

whereby apparatus classification (their equivalent of grouping) determined

the maximum apparatus temperature (see Table 8.1)but in recent years they

have adopted a variation on the European temperature classification system

(see Table 8.2).

Table 8.2 European temperature classification system and its relationship

with German and US systems

Temperature



European

system



US systems



(Note 1)



Historic

German

system

(Note 2)



450 "C



T1



G1



T1 (842°F)



Groups A, B, C and D



300 "C



T2



G2



T2 (572 "F)

T2A (536°F)

T2B (500 O F )

T2C (446 O F )

T2D (419°F)



Groups A, B, C and D

Groups A, B, C and D

Group C

Group C

Group C



200 "C



T3



G3



T3 (392 O F )

T3A (356°F)

T3B (329 O F )

T3C (320°F)



Group C

Group C



Current

(Note 3)



135 "C



T4



G4



T5



G5



T6



-



T5 (212°F)



85 "C



-



T4 (275 @F)

T4A (248



100 "C



Historic

(Notes 3 and 4)



O F )



T6 (185°F)



Notes:

1 The UK did not have a temperature classification system prior to the introduction of the

European system.

2 The historic German system did not have the equivalent of T6.

3 The USA works on the Fahrenheit scale but the basic T classes are equivalent.

4 The original US temperature classification was associated with its grouping system whereby

the following was the case: Groups A and B were associated with a maximum temperature

of 536 "F Group C was associated with a maximum temperature of 356 "F.



188 Electrical installations in hazardous areas



8.1.3 Requirements for enclosures



In all protection concepts, the enclosure of the apparatus is, to some degree,

important to the continued security afforded to the apparatus. It is clearly

important that the integrity of the enclosure is maintained and it has to

be understood that there are often components within the enclosure which

either retain ignition capable electrical charge, or ignition capable temperature after isolation of the apparatus for some identifiable time. For these

reasons enclosures are subjected to significant impact and drop tests to

ensure that they have sufficient strength for their purpose. They are tested

with the contents inside them and the strength of fixings within are also

tested at the same time. A apparatus is subjected to impact tests at a basic

U

level of 7 joules or, if the apparatus is intended to be installed where impact

risks are small, 4 joules. In general, this means the 7joule approach is used

to give maximum flexibility of use, although in such cases as indoor risks

in pharmaceutical plants, the reduced level can be of assistance. Guards

on parts of the apparatus can also mitigate the impact test requirement

and Table 8.3 shows the possible reductions of impact severity. In addition,

portable apparatus will be subjected to a drop test from a height of 1m to

verify its reliability in service. The tests are normally carried out at normal

laboratory temperature as the effects of changes in ambient temperature

(normally -20 "C to +40 "C) are not considered signrficant. JX, however, this

is not true because of unusual ambient temperature variations or particular

features of enclosure material (plastic enclosures are dealt with later in this

section) it may be necessary to carry out impact and, if appropriate, drop

tests at the limits of ambient temperature envisaged.

Glass is recognized as a weakness, when used for such things as viewports in enclosures, and is subjected to a thermal shock test by spraying

water at a relatively low temperature onto it when it is elevated to its

maximum service temperature.

Several of the protection concepts identify a required enclosure integrity

against the ingress of solid or liquid foreign bodies, or both. This is identified

as an 'P number, such as IPM. The first numeral identifies the protection

I'

against solid foreign bodies, such as tools or dust, and the second numeral

identifies the protection afforded against liquid ingress. I specification is

P

derived by testing enclosures in accordance with BS/EN 60529 19912 and

Table 8.4 identifies the numerals used in IP rating. Typical IP ratings are:

IP20: This is considered as sufficient protection to prevent insertion of

fingers, etc., and is assumed to prevent electric shock.

IP54: This is considered as weather proof for outdoor mounting. All

outdoor-mounted apparatus be it for hazardous-area mounting or not

should meet at least this criterion.

IP65 This is considered to be dust tight. Once again it must be stressed

that the enclosure integrity is often identified by the general situation and

not by the minimum permitted by the protection concept. For example,

outdoor apparatus is almost always IP54 to ensure operational reliability,



General requirements for explosion protected apparatus 189



Table 8.3 Test Requirements for resistance to

impact

Mechanical details



Impact energy

in Joules



Risk of mechanical danger



High



Low



1



Guards, protective covers,

fanhoods, cable entries



7



4



2



Plastics enclosures

Light metal or cast metal



7

7



4



3



7



4



4



5

6



enclosures

Enclosures of other

materials than 3 with

wall thickness of less

than 1mm

Light transmitting parts

without guard

Light transmitting parts

with guard (tested



4



4

2



without guard)



(from BSEN 500241)

Note: Impact test is normally with a 1 kg mass with a hardened

steel 25cm ball at its impact point. The height for dropping

purposes is one tenth of the required impact energy i metres.

n



and apparatus for use in the presence of conducting dusts is likewise almost

always required to be IP65.

As far as residual charge or temperature after isolation is concerned, the

only way to overcome this problem if it cannot be avoided by design is to

identify the time taken for decay, and label the apparatus with a warning

indicating a delay which should be observed before opening. For residual

charge, figures are given as follows:

Group IIA



The residual charge must not exceed 0.2mJ when the enclosure is opened;

Group IIB - The residual charge must not exceed 0.06mJ when the

enclosure is opened;

Group IIC - The residual charge must not exceed 0.02mJ when the enclosure is opened.

-



These figures allow the calculation of the necessary time to be included on

the warning label.

Where the charge cannot be dissipated in a sensible time, or the temperature likewise is retained for a very long time, an alternative approach is to



190 Electrical installations in hazardous areas



Table 8.4 Degrees of protection of enclosures

Number



First numeral

Protection

against solids

afforded



0



No protection of persons

against contact with live or

moving parts. No protection

of equipment against ingress

of solid foreign bodies.



No protection



1



Protection against accidental

or inadvertent contact with

live or moving parts inside

the enclosure by a large

surface of the human body,

for example, a hand, but not

protection against deliberate

access to such parts.

Protection against ingress of

large solid foreign bodies.

Protection against contact

with live or moving parts

inside the enclosure by

fingers. Protection against

ingress of medium size solid

foreign bodies.



Protection against drops of

condensed water. Drops of

condensed water falling on the

enclosure shall have no harmful

effect.



2



Second numeral

Against liquids



Protection against drops of liquid.

Drops of falling liquid shall have

no harmful effect when the

enclosure is tilted at any angle up

to 15" from the vertical.



3



Protection against contact

with live or moving parts

inside the enclosure by tools

wires or such objects of

thickness greater than

2.5 mm. Protection against

ingress of small solid foreign

bodies.



Protection against rain. Water

falling in rain at an angle of up to

60 with respect to the vertical

shall have no harmful effect.



4



Protection against contact

with live or moving parts

inside the enclosure by tools

wires or such objects of

thickness greater than 1mm.

Protection against ingress of

small solid foreign bodies.



Protection against splashing.

Liquid splashing from any

direction shall have no harmful

effect.



5



Complete protection against

contact with live or moving

parts inside the enclosure.

Protection against harmful



Protection against water jets. Water

projected by a nozzle from any

direction under stated conditions

shall have no harmful effect.