mcmillan, a. (1998). electrical installations in hazardous areas

I
l


Elec trica
I
Installations in Hazard0
us
Areas

Electrical Installations
in
Hazardous Areas
Eur
Ing
Alan
McMillan
C
Eng
FlEE
FlnstMC
c
EINEMANN
Butterworth-Heinemann
Linacre House, Jordan Hill, Oxford
OX2
8DP
225 Wildwood Avenue, Woburn,
MA
01801-2041
A division of Reed Educational

and
Professional Publishing Ltd
e
A member of the Reed Elsevier
plc
group
OXFORD
BOSTON
JOHANNESBURG
MELBOURNE
NEWDELHI
SINGAPORE
First published 1998
0
Reed Educational and Professional Publishing Ltd 1998
All rights reserved.
No
part of
this
publication may be reproduced
in
any material form (including photocopying or storing in any medium by
electronic means and whether or not transiently or incidentally to some
other use
of
this
publication) without the written permission
of
the
copyright holder except in accordance with the provisions of the Copyright,

Designs and Patents Act
1988
or under the terms of a licence issued by the
Copyright Licensing Agency Ltd,
90
Tottenham Court Road, London,
England
WlP
9HE.
Applications for the copyright holder’s written
permission to reproduce any part
of
this
publication should be addressed
to the publishers
British
Library Cataloguing in Publication Data
A catalogue record for
this
book is available from the British Library
Library
of
Congress Cataloguing in Publication Data
A catalogue record for this book is available from the Library of Congress
ISBN
0
7506 3768
4
Typeset by Laser Words, Madras, India
Printed and bound in Great Britain by

Biddles Ltd, Guildford and King’s Lynn
Contents
Preface
xvii
1
Introduction
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
Examples
of
historic incidents
Technological approach
History of development
UK
legislation
European legislation
Certification
Certificate and labelling information
The future of certification
2
Area classification
Philosophy objectives and procedures
Basic properties of flammable and combustible
materials

2.1.1
Flammable gases
2.1.2
Flammable vapours
2.1.3
Flammable mists
2.1.4
Flammable liquids
2.1.5
Combustible dusts
2.2
Basis
of
area classification
2.3
General approach to area classification
2.4
Classification of sources
of
release
2.5
Hazardous zonal classification
2.5.1
Gases, vapours and mists
2.5.2
Dusts
2.5.3
2.1
Relationship between sources of release
and Zones

Information on fuels (gases, vapours
and mists)
2.6
Collection
of
information
2.6.1
2.6.2
Information on fuels (dusts)
2.6.3
Information on process conditions
2.7
Procedures
2.8
Personnel involved
2.9
Results
of
area classification and frequency
of
repeats
1
1
2
4
6
8
10
15
18

22
22
22
22
23
23
24
24
24
25
26
27
27
28
30
30
30
32
34
34
39
41
vi
Contents
3
Area classification practice for gases, vapours and mists in
freely ventilated situations
3.1
3.2
3.3

3.4
Introduction
Containment of flammable materials
3.1.1
Effects of storage conditions
3.1.2
3.1.3
Oxygen enrichment
3.1.4
General consideration of release
Generalized method of area classification
3.2.1
Generalized zonal classification specification
3.2.2
Generalized extents of zones
The source of hazard method of area classification
3.3.1
Types of release
3.3.2
Releases from pipe joints
3.3.3
3.3.4
Special pipe joint circumstances
3.3.5
Releases from moving seals
Other practical well-ventilated situations
Effect of sunlight on storage vessels
Typical extents of Zone
2
from pipe joint

releases
3.4.1
3.4.2
3.4.3
3.4.4
3.4.5
3.4.6
3.4.7
3.4.8
3.4.9
3.4.10
3.4.11
The fixed roof vented stock tank
The floating roof tank
Tanks containing
gas,
vapour or liquefied
vapours
Road and rail tanks for flammable liquids
Oil/water separators
Other open vessels
Open drains
Trenches
Sampling points
Walls and apertures
Vents
4
Calculation of release rates and the extents of
hazardous areas
4.1

4.2
Releases
of
gas and vapour
4.1.1
Examples of gas and vapour release
Release of liquid below its atmospheric boiling point
4.2.1
Example of liquid release below its
atmospheric boiling point
Release of liquid above
its
atmospheric boiling point
4.3.1
Example of liquid release above its
atmospheric boiling point
Summary of use of equations
4.4.1
Gas and vapour releases
4.3
4.4
43
43
44
44
45
45
46
46
46

47
48
50
52
54
65
65
67
67
69
71
72
76
78
78
80
80
85
85
87
88
105
107
113
116
118
119
119
Contents
vii

4.4.2
4.4.3
Releases in areas which are not well ventilated
4.5.1
4.6
Conclusion
Liquid releases below boiling point
Liquid releases above boiling point
Example of gas release using BS/EN
10079-10
formulae
4.5
5
Area classification practice for gases, vapours and mists in
areas which are not freely ventilated
5.1
Typical areas of restricted ventilation
5.1.1
Open areas surrounded by walls
5.1.2
Covered areas (dutch-barn type)
5.1.3
Above-ground rooms
5.1.4
Below-ground rooms
Effect of walls on hazardous areas
Roofs without walls
or
associated with one,
two

or
three walls
5.3.1
Roofs without walls
5.3.2
5.3.2
5.3.4
5.4.1
5.4.2
5.4.3
High integrity ventilation
5.5
Rooms below ground
5.6
Rooms
without any internal release but which abut
external hazardous areas
5.7
Particular circumstances
5.7.1
The paint spray booth
5.7.2
The paint drying oven
5.2
5.3
Roofs associated with one wall
Roofs associated with
two
walls
Roofs associated

with
three
walls
The application of additional general
ventilation
The application of additional local ventilation
5.4
Rooms above ground
6
Area classification practice for dusts
6.1
Properties of dusts
6.1.1
6.1.2
6.1.3
6.1.4
Other important dust properties
Area classification for dust releases
6.2.1
Sources of dust release
6.2.2
Definition of Zones
6.2.3
Extents of hazardous areas
The ignition of dust clouds
The ignition of dust layers
Production
of
flammable gases and vapours
by dusts

6.2
120
120
121
123
123
125
126
126
126
126
127
127
130
130
132
132
133
134
135
140
143
144
145
146
146
146
149
150
151

154
154
155
155
155
156
158
viii
Contents
6.3
Practical situations
6.3.1
Cyclones
and
bag filters
6.3.2
Loading hoppers within buildings
6.3.3
Loading hoppers outside buildings
7
Design philosophy for electrical apparatus for explosive
atmospheres
General approach and applicable standards
7.1
History
7.1.1
7.1.2
Dust
risks
Protection

of
electrical apparatus for gas, vapour and
mist risks
7.2.1
Exclusion of the explosive atmosphere
(criterion a)
7.2.2
Prevention of sparking (criterion b)
7.2.3
Containment of explosions (criterion c)
7.2.4
Energy limitation (criterion d)
7.2.5
Special situations
Situation in respect of Zone
2
apparatus
Protection of electrical apparatus for dust risks
7.5.1
7.5.2
7.5.3
7.5.4
Gas, vapour
and
mist risks
7.2
7.3
7.4
7.5
Apparatus construction Standards

Zone
0
and/or Zone
1
compatible
apparatus for gases, vapours and
mists
The marketing situation in respect of
European Standards
Zone
2
compatible apparatus for gases,
vapours and mists
Electrical apparatus for dust
risks
8
General requirements for explosion protected apparatus
(gas, vapour and mist
risks)
8.1
Apparatus to European Standards
BS/EN
50014 (1993)
(including amendment
1
(1994))
8.1.1
Definitions
8.1.2
Division

of
apparatus into
8.1.3
Requirements for enclosures
8.1.4
8.1.5
8.1.6
8.1.7
Ex
components
8.1.8
Marking requirements
sub-groups
and
surface temperature classes
Fasteners, interlocking devices, bushings
and
cements
Connection facilities
and
cable entries
Additional requirements for particular types of
apparatus
161
161
162
163
164
164
164

164
165
166
166
167
168
168
169
169
170
170
171
174
1
75
176
178
178
1
79
180
181
188
195
199
201
204
204
Contents
ix

9
Apparatus using protection concepts encapsulation 'm', oil
immersion
'0'
and powder filling
'q'
9.1
9.2
9.3
Encapsulation
-
'm'
9.1.1
9.1.2
Specification of the encapsulation
9.1.3
9.1.4
Encapsulated circuits and components
9.1.5
The encapsulation process
9.1.6
Particular component problems
9.1.7
Type testing
Oil immersion
-
'0'
9.2.1
Construction of the apparatus
9.2.2

Containment of the oil
9.2.3
9.2.4
External connections
9.2.5
Type testing
Powder filling
-
'q'
9.3.1
The filling material
9.3.2
The enclosure
9.3.3
Electrical components and circuits
9.3.4
External connections
9.3.5
Type testing
Exclusions from BS
5501,
Part
1
(1977)
Types
of
apparatus for encapsulation
Requirements for the protective fluid (oil)
10
Apparatus using protection concept flameproof enclosure

'd'
10.1
Standards for flameproof apparatus
10.2
Construction and testing requirements for flamepaths
10.2.1
Joints between the interior of flameproof
enclosures and the external atmosphere
10.2.2
Joints for rotary or longitudinal shafts and
operating rods
10.2.3
Joints for shafts and rotating machines
10.2.4
Bearing greasing arrangements
10.2.5
Tests for flameproof joints
10.3
Construction of flameproof enclosures, entry
facilities and component parts
10.3.1
Enclosure construction
10.3.2
Bushings
10.3.3
Arrangements for entry facilities
10.3.4
Fasteners
10.3.5
Component parts

10.3.6
Freestanding flameproof components
209
209
209
210
21
1
212
213
213
215
215
215
216
218
218
218
219
219
219
220
223
223
226
228
230
230
250
252

253
254
256
256
261
261
262
263
269
x
Contents
11
Apparatus using protection concept pressurization 'p'
11.1
Standards for pressurization
'p'
11.2
Methods of pressurization
11.2.1
Static pressurization
11.2.2
Pressurization with leakage compensation
11.2.3
Pressurization with continuous dilution
11.3
Purge and dilution gases
11.4
Flammable materials imported into enclosures
and their containment
11.4.1

Flammable materials imported into enclosures
11.4.2
Containment of flammable materials within
enclosures
11.5
Enclosures, ducting and internal components
11.5.1
Enclosures
11.5.2
Ducting
11.5.3
Internal electrical components, etc
11.6
Safety provisions and devices
12
Apparatus using protection concept increased safety 'e'
12.1
12.2
12.3
The situation in regard to standardization
Basic construction requirements
12.2.1
Construction of enclosures
12.2.2
Terminals and connection facilities
12.2.3
Separation
of
conducting parts
12.2.4

Insulation
12.2.5
Windings
Additional requirements for specific types of
apparatus
12.3.1
Terminal enclosures
12.3.2
Mains powered luminaires
12.3.3
Caplamps
12.3.4
Measuring instruments and transformers
12.3.5
Secondary batteries
12.3.6
Heating devices
12.3.7
Rotating machines
12.3.8
Other types of apparatus
13 Apparatus and systems using protection concept intrinsic
safety
'i'
13.1
The situation in respect of standardization
13.2
Basic application
of
the concept

13.2.1
Intrinsically safe apparatus
13.2.2
Associated apparatus
275
275
278
279
280
283
284
284
284
285
287
287
290
292
293
296
297
297
297
298
300
305
309
311
311
313

317
317
319
322
324
329
331
332
333
333
334
Contents
xi
13.3
13.4
13.5
13.6
13.7
13.8
13.9
13.2.3
Interconnections
13.2.4
Simple apparatus
13.2.5
The intrinsically safe circuit
13.2.6
The intrinsically safe system
Levels of intrinsic safety
13.3.1

Intrinsic safety category ’ia’
13.3.2
Intrinsic safety category ‘ib’
Countable and non-countable faults
13.4.1
Non-countable faults
13.4.2
Countable faults
13.4.3
Effects of other faults
13.4.4
Infallible component, assembly or
Confirmation of intrinsic safety in respect of arc and
spark ignition
13.5.1
Achievement of safety factors
13.5.2
Assessment of circuits
Confirmation of intrinsic safety in respect of hot
surface ignition
Basic component and construction requirements
13.7.1
Safety factors on component rating
13.7.2
Specific requirements for particular
components
Component and circuit failure modes
13.8.1
Wire and printed circuit tracks
13.8.2

Connections
13.8.3
Capacitors and inductors
13.8.4
Resistors
13.8.5
Semiconductors
Apparatus construction requirements
13.9.1
Enclosures
13.9.2
Internal layout (wiring, printed circuit
boards, etc)
13.9.3
Earth conductors and connections (including
terminals)
13.9.4
Encapsulation
interconnection
13.10
Infallible components, assemblies and construction
elements
13.10.1
Infallible transformers
13.10.2
Damping windings
13.10.3
Current limiting resistors
13.10.4
Blocking capacitors

13.10.5
Shunt safety assemblies
13.10.6
Internal wiring and connections
13.10.7
Galvanic separation
335
335
335
336
336
336
337
337
338
338
338
339
339
343
344
353
357
357
359
370
371
371
371
372

372
374
374
375
382
384
385
386
394
394
395
396
398
400
xii
Contents
13.11 Diode safety barriers
13.12 Input and output specifications
13.13 Examples of fault counting in apparatus
13.14 Intrinsically safe systems
13.14.1 Interconnecting cable systems
13.14.2 Cable parameter measurement
14
Apparatus using protection concept
’N
(‘n’)
14.1 The situation in respect of standardization
14.2 Basic requirements of the protection concept
14.3 General constructional requirements
14.3.1 Environmental protection

14.3.2 Mechanical strength
14.3.3 Wiring and internal connections
14.3.4 External connection facilities
14.3.5 Conductor insulation and separation
14.4 Additional requirements
for
certain types of
non-sparking apparatus
14.4.1 Rotating electrical machines
14.4.2 Fuse links and fuseholders
14.4.3 Fixed luminaires
14.4.4 Portable luminaires and other light sources
14.4.5 Electronic and low power apparatus
14.5 Apparatus producing arcs, sparks and/or
ignition-capable hot surfaces
14.5.1 Enclosed break devices and non-incendive
components
14.5.2 Hermetically sealed devices
14.5.3 Sealed devices
14.5.4 Energy limited apparatus and circuits
14.5.5 Restricted breathing enclosures
15
Protection concepts
for
apparatus
for
dust
risks
15.1 Situation in respect of standardization
15.2 Basic types of apparatus for use with combustible dusts

15.2.1 Degrees of enclosure
15.3 Operational requirements
15.4 Basic constructional requirements
15.4.1 Enclosure materials and mechanical strength
15.4.2 Joints intended to be opened in normal service
15.4.3 Semi-permanent joints
15.4.4 Spindles and shafts
15.4.5 Light-transmitting parts
15.4.6 Fasteners
402
406
411
415
419
424
427
427
429
430
430
431
432
433
433
437
437
438
438
441
442

443
443
445
447
447
450
456
456
458
458
459
461
461
462
464
465
465
466
Contents
xiii
15.5
15.6
Specific additional requirements for
particular types of electrical apparatus and connections
15.5.1 Connection facilities
15.5.2 Cable and conduit entries
15.5.3 Fuses and switchgear
15.5.4 Plugs and sockets
15.5.5 Luminaires and similar apparatus
Apparatus conforming to the protection concepts

appropriate to gas, vapour and mist risks
15.6.1 Spark ignition
15.6.2 Hot surface ignition
15.6.3 Basis of selection of apparatus with protection
concepts appropriate to gas/vapour/mist and
air risks
16 Other methods
of
protection and future apparatus
requirements
16.1 Acceptance
of
technical requirements
16.2 Essential requirements
16.2.1 General requirements
16.2.2 Materials of construction
16.2.3 General design and constructional
16.2.4 Specific requirements for particular
requirements
types
of
apparatus and protective systems
16.3 Use of apparatus
17 Selection
of
power supply, apparatus and interconnecting
cabling system for both gas/vapour/mist
risks
and dust risks
17.1 Electrical supply systems

17.2 Electrical protection
17.3 Selection of apparatus
17.3.1 Selection
in
respect of gases, vapours
and mists
17.3.2 Selection of apparatus for dust risks
17.3.3 Selection on the basis
of
zone of risk
17.4 Selection
of
conduit
or
cable systems
17.4.1 Zone
0
and Zone
20
17.4.2 Zone
1
and Zone 21
17.4.3 Zone
2
and Zone
22
18 Installations in explosive atmospheres
of
gas, vapour, mist
and dust

18.1 Standards and Codes
466
466
467
467
468
468
468
469
469
469
474
475
476
476
478
479
479
481
482
482
485
486
487
491
493
497
498
500
501

503
504
xiv
Contents
18.2 Legislation and enforcement
18.3 Basic installation requirements
18.3.1 Pre-installation checks
18.3.2 General installation recommendations
18.3.3 Post-installation checks
18.3.4 Earthing
and
bonding
18.3.5 Application of earthing
18.3.6 Application of bonding
18.3.7 Electrical isolation
18.3.8 Conduit and cable installation
18.4 Additional requirements or relaxations for particular
protection concepts
18.4.1 Hameproof electrical apparatus
‘d’
18.4.2 Increased safety apparatus ’e’
18.4.3 Type
’N
apparatus
18.4.4 Other types of protection
18.4.5 Particular problems
18.5 Heating tapes
19
Installation
of

pressurized apparatus
and
other uses
of
the
pressurization technique
19.1 Standards for pressurization
19.2 Certification/approval
19.3 Basic installation approach
19.4 Pressurization arrangements
19.4.1 Pressurization with leakage compensation
using air
19.4.2 Pressurization with leakage compensation
using inert gas
19.4.3 Pressurization with continuous dilution
using air
19.4.4 Pressurization with continuous dilution using
inert gas
19.5 Safety devices and procedures
19.5.1 Manufacturer provision of control devices
and
systems
19.5.2 Manufacturer provision of control devices
only
19.5.3 Manufacturer provision of control information
only
19.6 Purge gas and operation of control system (gas,
vapour and mist
risks)
19.6.1 Gases used

19.6.2 Basic operation of pressurization system
19.7 Multiple enclosures
19.8 Typical pressurization control system
504
505
506
506
507
507
509
510
514
516
520
520
528
533
533
533
534
538
539
539
540
541
544
546
548
550
551

551
552
553
554
554
554
556
557
Contents
xv
19.9 Pressurized enclosures in dust
risks
19.10 Analyser houses
19.10.1 Pressurization considerations
19.10.2 Analyser house construction and protection
19.11 Pressurized rooms
20
Installation of intrinsically safe apparatudassociated
apparatus and intrinsically safe systems 'i'
20.1 Standards and Codes
20.2 Basic installation requirements
20.3 Cables, conduits and conductors
20.3.1 Conductor temperature
20.3.2 Inductance and capacitance
20.3.3 Cable installation
20.3.4 Conduit installation
20.3.5 Marking of cables, cable bundles, cable trays,
or ducts
and
conduits

20.3.6 Additional requirements for Zone
0
(and
Zone 20 where appropriate)
20.4 Conductor terminations
20.4.1 Terminal construction
20.4.2 Assemblies of terminals
20.5.1
20.5 Earthing and bonding
Typical Zone
1
and Zone 21 intrinsically safe
circuits with bonding in the non-hazardous
area
20.5.2 Typical Zone
1
and
Zone 21 intrinsically safe
circuits with bonding in the hazardous area
20.5.3 Typical Zone
1
and
Zone 21 circuits with
bonding connection at more than one point
20.5.4 Bonding and insulation
of
screens
for
intrinsically safe circuits
20.5.5 Typical Zone

0
and Zone 20 intrinsically safe
circuits
20.5.6 Special circuits
21
Documentation, inspection, test and maintenance
of
explosion protected apparatus, systems and installations
21.1 Documentation
21.2 Detailed inspection requirements
21.2.1 Initial inspection
21.2.2 Inspection after apparatus repair
21.2.3 Inspection after change in area classification,
sub-group or surface temperature classification
21.2.4 Routine inspection
557
558
559
563
563
566
567
568
569
569
570
571
572
572
573

573
574
577
581
584
587
587
589
59
1
595
598
598
599
599
604
604
604
xvi
Contents
21.2.5
Visual inspection
21.2.6
Inspection procedures
21.3
Testing
21.3.1
Necessary testing
21.4
Maintenance of explosion protected apparatus

21.4.1
General maintenance
22
Radio frequency radiation and static electricity
22.1
Basic situation
22.2
Static electricity including lightning
22.2.1
Dealing with static charge
22.2.2
Dealing with lightning
Ignition by radio frequency radiation
22.3.1
Basic safety assessment
22.3.2
Dealing with the hazard
22.3
Glossary
Index
607
608
609
609
612
613
616
616
617
618

619
620
620
626
628
637
The technology of application of electrical equipment in explosive atmo-
spheres is very old, dating almost from the original application of electricity
to apparatus other than lighting. From its origins it has been developed in
most industrialized countries with the United Kingdom, Germany and the
United Sates of America being in the vanguard. As the world moves closer
together this technology has, like all others, been coordinated
so
that its
detail will be the same in all countries, principally to allow free marketing
around the world.
This
has led to more detailed standard requirements
particularly in the case
of
apparatus construction.
In
the UK the considerable standardization of technology is defined in
more than
30
published standards (some national, some European and some
international). While this is basically good, in that it details what is neces-
sary and thus makes the achievement of safety easier in principle, it has
drawbacks in that there is considerable complication which can cause confu-
sion. Despite the longevity

of
the technology
I
can find no serious attempt
in the
UK
to produce a freely published volume, such as
this,
which brings
together the entire technology under one roof, as it were. This fact, together
with the pivotal role played by the
UK
in development through the British
Standards Institution, which brought together all the necessary expertise
to produce the necessary technical standards, the Safety in Mines Research
Establishment (now the Health and Safety Laboratory of the Health and
Safety Executive) and the Electrical Research Association (now
ERA
Tech-
nology), organizations that carried out much of the research work necessary
to permit the current standards to exist and the large contribution made
by
UK industry, led me to write this volume.
1.
The determination of the likelihood and the areas contaminated or likely
to be contaminated by explosive atmospheres produced by fuels such
as gas, vapour, mist, dust or a combination of these.
This
is still the
least researched

of
the areas of this technology, principally because there
are
so
many variations, in particular circumstances occurring in practical
locations.
2.
The construction
of
electrical equipment
so
that it is unlikely to become
an ignition source.
This
has been heavily researched in many countries
because, unlike area classification, it is relatively specific and lends itself
more readily to specification.
3.
The installation, operation, maintenance and inspection of electrical
equipment. This again is heavily influenced by the circumstances
The field can be divided into three facets:
xviii
Preface
occurring at particular situations and is thus not as easily specified as
equipment construction. It is, however, more specifiable than
1
above.
In
writing
this

book I have tied to address all three facets of the tech-
nology and, rather than reproducing all the content of standards and codes, I
have been selective in discussing most of the principal requirements therein,
while at the same time trying to explain the reasoning which led to their
inclusion. Therefore, when applying the technology it will be necessary to
address the appropriate standards and codes in all cases but
this
book
will,
by provision of the background reasoning, make those documents more
understandable.
In
addition, by developing practical examples
of
their use,
it will assist in their application.
This
field
is
not one for inexperienced engineers and technologists and
thus must be approached with care.
In
addition there are many local condi-
tions which can vary the advice given here and those involved need to be
aware of
this
and have sufficient expertise to determine conditions under
which additional requirements
are
necessary and those, much less common,

where relaxations are possible. The onus is, of course, always on the occu-
pier of a location to be able to jus* what is done on safety grounds and
it is hoped that
this
book will assist in
this
activity.
The contents here relate to the situation
in
the
UK
but differences in
Europe and other counties
are
not great and its content should be useful
elsewhere.
Finally, unlike the situation historically existing, where
this
technology
was often applied in isolation, it is now important to recognize that it can
only be applied as a part of an overall safety strategy. That is not
to
say that
its requirements can be ignored if they adversely affect other safety features
but rather that,
if
such
is
the case, an alternative approach to achievement
of its requirements should be sought. It should always be remembered that

electrical installations
in
explosive atmospheres should only exist where
necessary (i.e., where they can be fully justified).
Alan McMillan
7
ln
traduction
Where combustible or flammable materials are stored or processed there
is,
in most circumstances, a possibility of their leaking or otherwise having the
ability to produce what may be described as an explosive atmosphere in
conjunction with the oxygen present in air.
This
is true for gases, vapours,
mists and dusts and, as electricity is widely used in industries and other
places where such explosive atmospheres can occur, the propensity of elec-
trical energy to create sparking or hot surfaces presents a possibility that
the explosive atmospheres may be ignited with resultant
fire
or explosion.
This
hazard
has
been recognized for many decades
-
almost since the use
of electricity was introduced into
mining
and other industries

-
and the
precautions taken to overcome
this
problem date back, in their basic incep-
tion, to the turn of the twentieth century
and
before.
There is no way
in
which explosions can be totally prevented in indus-
tries where explosive atmospheres can occur as all human endeavour is
fallible but it is necessary to develop our operations to a degree where
such explosions are
so
rare that their risk is far outweighed by the benefits
of the processes in which they may occur. Such balance is evident in the
coalmining industry where the overall risks associated with working under-
ground, where explosions are one constituent, have been seen as justifiable
on the basis of society’s need for fuel. It is true that the
risks
are minimized
as far as possible but only to a level consistent with the need to win coal
and accidents still occur. It remains true, however, that the risk of these
accidents has been reduced to a level acceptable to our society and partic-
ularly those working in the industry. That is not to say that when a risk is
identified by an incident nothing is done.
We
always learn
from

these and
invariably they result in changes to our operating systems and equipment
in order
to
minimize the risk of a repeat. Notwithstanding all of our efforts,
however, accidents of significant proportion still occur with a degree of
regularity which causes
us
all concern.
1
.I
Examples
of
historic incidents
The following are examples of the more significant incidents occurring in
the
UK
and, although they were not necessarily caused by electricity, there
is in at least one of the cases a suspicion of electrical initiation and electricity,
as
has already been indicated, is seen
as
an obvious igniting agent.
2
Electrical installations in hazardous areas
Senghennyd colliery
-
191
3
An

underground firedamp (methane) explosion caused a roof fall which cut
off over
400
miners
from
a
shaft in
a
burning section of the colliery. Most
of them died from the resulting suffocation.
Flixborough
-
1974
A
modification to a process plant, said by the accident report not to have
been properly considered, was identified as causing a major release of
flammable gas resulting in an immense aerial explosion.
Loss
of life on the
plant was mercifully low (probably because it was Saturday) but damage to
the plant and surrounding residential and other properties was significant.
Piper Alpha
-
1988
This
oil platform was effectively destroyed by a gas explosion which
resulted from a major release of gas suggested to be due to erroneous
process operation. The initial and subsequent explosions and fire effectively
prevented controlled evacuation of the platform and heavy loss of life was
caused.

Texaco Pembroke refinery
-
1994
A
major vapour explosion occurred leading to a major fire which was
extremely difficult to extinguish. The refinery burned for a considerable
time with consequent adverse effects on the local environment. Casualties
were light but the refinery suffered considerable damage.
The above examples clearly demonstrate the dangers present, particu-
larly in locations where escape of personnel is difficult
and
it is essential,
therefore, that all involved have an understanding of the technology used
to minimize the risk
of
explosion.
1.2
Technological approach
The objective
of
the technology associated with the use
of
electrical
equipment in potentially explosive atmospheres is to reduce the risk of
an explosion to an acceptable level.
To
have an explosion three elements
are necessary
-
namely fuel, oxygen and a source of ignition (see Fig.

1.1).
Oxygen is present
in
air to a sufficient extent to support combustion and
cannot normally be excluded, which leaves only the fuel and ignition
sources as elements to which influence can be applied.
This
has formed
Introduction
3
Fuel (Flammable
gas, vapour,
mist or dust)
Oxygen (Air)
Ignition source (Operation or
maloperation
of
electrical equipment)
Fig.
1.1
The explosion triangle
the basis for technology since the turn of the twentieth century when the
problem was first identified in the
mining
industry. While in other areas
of
risk the approach is often based much more heavily on statistical analysis
than is the case here, the approach in respect of explosive atmospheres is
well established and accepted, having been
in

use since the early
1900s.
The
presence
of
many subjective areas which make statistical analysis difficult
have also limited the statistical approach although there have been many
attempts to apply such an approach. Thus current and foreseeable future
technology is based upon that currently used, and there is no indication
of
a radical change to readdress the technology on a statistical basis as is
done, for example, in the nuclear industry.
A
typical attempt to analyse the statistical level of security achieved in
relation to gas, vapour and mist releases is that
in
a paper by
W.A.
Hicks
and
K.J.
Brown at the
1971
Institution of Electrical Engineers Conference'
which identified the risk
of
ignition as between and Many
others however have produced different figures as the assumptions made
in respect of the subjective elements of the technology vary.
The technology is currently based upon the identification of the risk of an

explosive atmosphere being present in a particular place coupled with the
identification of the likelihood of electrical equipment within the explosive
atmosphere malfunctioning in a way which would cause it to become
a
source of ignition coincident with the presence of that explosive atmosphere.
The objectives are not just to identify these coincidences but to utilize the
information
so
obtained to influence the design of particular process plants
and similar operational situations in a way
so
as to minimize the risk of
creation
of
an explosive atmosphere, and hence the risk
of
an explosion
due to electrical installations.
To
this end, the generality of the approach is
to seek out situations where an explosive atmosphere is normally present
of
necessity due to the process involved, situations where the likelihood of
its presence is high and situations where the likelihood is of its presence is
4
Electrical installations
in
hazardous areas
low but identifiable.
In

this scenario catastrophe does not play a part and
although it is necessary to plan for catastrophe such plans are by
and
large
outside the scope of this technology.
In
addition this technology should not
be used in isolation but as part of an overall safety strategy
for
a location
where the problem occurs.
Having identified the possible presence of an Explosive Atmosphere it
is
then the part of technology to identify those electrical installations which
really need to be present rather than those which convenience would make
desirable, and ensure that these are protected in a way which makes the
overall risk of an explosion sufficiently low.
1.3
History
of
development
The use of electrical equipment in explosive atmospheres was originally the
province of the mining industry and, although the technology was used
in surface industry, significant developments in this latter area are more
contemporary being to a large extent post war. While the present approach
is to minimize the chance of a release of flammable material, or where a
release occurs to minimize the build up of the material in the atmosphere, it
is probably somewhat surprising that in early coal production the method
used to deal with releases of methane (firedamp) was to deliberately burn
off

the explosive atmosphere.
This
was done by a specifically designated
miner called the 'firelighter'. The method used took advantage of the fact
that methane is lighter than air (relative density is around
0.55)
and thus
methane/air mixtures collected preferentially near the roof of the workings.
Warning was given by changes in colour of the flames of the lamps used by
miners and the workings were then cleared. A torch was inserted into the
methane air cloud, igniting it
and
burning
off
the methane. The technique
fell into disrepute for obvious reasons and was replaced by the introduction
of the use of ventilation to restrict the possibility of explosive atmospheres
forming and the employment of a safety lamp (Fig.
1.2)
to minimize the
risk of ignition.
The introduction of electricity in the latter part of the nineteenth century
and the early part of the twentieth century led to significant other risks
being identified. Initially electricity was utilized for lighting and motive
force. The lighting was typically provided by incandescent filament lamps,
none of the more sophisticated lamps having been developed at the time,
and
the motive force usually by either dc or wound rotor ac machines which
were initially typical of the motors available. Both lighting
and

machines
required control equipment (often as simple as a switch) but
this
equipment
also introduced risks associated with hot surfaces and sparks, together with
the possibility of the presence of both methane and coal dust.
The solutions
to
these problems in relation to gas, vapour and mist
releases were developed
in
both the
UK
and Germany along very similar
lines and in very similar time scales.
In
Germany the organization prin-
cipally involved was what
is
now known as the Berggewerkschaftlichen