Preface
In nine out of ten cases, a hazardous location is classified much too conservatively.
The reasons for this conservative approach are a lack of knowledge and a
misunderstanding of the actual concept of safety and danger. In the majority of cases,
hazardous areas are classified Division 1 when the location could have been classified
Division 2, and areas which are classified Division 2 could have been classified
nonhazardous. In other cases, the location is classified nonhazardous when it should
have been classified Division 1 or Division 2. It must be kept in mind that a location
classified Division 1 requires explosion-proof equipment, which ranges in price from
two to four times the cost of general-purpose electrical equipment, some of which are
allowed in Division 2 locations. Therefore, it is important to strive to achieve a
classification of a lower yet acceptable level of safety, which is commensurate with
an acceptable risk and reduces the cost of electrical installations.
To establish such a point, it is necessary to evaluate the characteristics of the
flammable products, along with the conditions under which the product must
operate. By listing this information on appropriate forms, the evaluation of the
degree of hazard and its boundaries can be correctly performed, and, as a result, the
proper electrical equipment can be selected under the provisions of the NEC.
A total of 126 tables and illustrations have been developed to assist the
engineer in establishing the degree of danger and its boundaries for locations with
flammable products.
This publication is divided into three parts with an appendix. Part 1 discusses
the flammable and combustible principles of hazardous products and other pertinent
information associated with an area classification. Part 2 discusses the
environmental conditions in hazardous locations. A number of specific illustrations
are included in this section. Part 3 discusses the application procedure for
classifying NEC Class I locations. Examples are also included in this section.
Following these sections is an appendix listing properties of flammable liquids,
gases, and vapors.
The application of the information explained herein is mainly for flammable
liquids, vapors, and gases that are processed, handled, stored, and/or transported. A
small portion of this publication explains the classification of coal handling
facilities.
Where there may still be cases that are difficult to solve, sound engineering
judgment should be applied in compliance with the content of this publication.
Contents
List of Figures, Tables, and Reports ………………………………………………….. xv
Figures ……………………………………………………………………………. xv
Tables ……………………………………………………………………………. xx
Reports …………………………………………………………………………. xxiii
Part 1
Fundamentals
1 Flammable and Combustible Principles of Hazardous Products …….. 3
A. GENERAL ……………………………………………………………………………. 3
B. FLAMMABLE AND COMBUSTIBLE LIQUIDS ………………………. 3
B.1. Classification of Flammable and Combustible Liquids …………. 3
B.2. Flash Point ……………………………………………………………………. 4
B.3. Ignition Temperature ……………………………………………………… 4
B.4. Vapor Density ……………………………………………………………….. 6
B.5. Explosion Range of Flammable Gas or Vapor …………………….. 6
B.6. Explosion Hazard as a Function of Temperature
and System Pressure ………………………………………………………. 8
B.7. Extent of Hazard as a Function of Molecular Weight …………. 10
B.8. Vapor Traveling Distance ……………………………………………… 11
B.9. Vapor Volume of Flammable Mixtures ……………………………. 12
viii Contents
B.10. Liquefied Petroleum Gases ……………………………………………. 13
B.11. Grouping of Flammable and Combustible Products ……………. 13
C. COMBUSTIBLE COKE AND COAL DUST ……………………………. 14
D. FIRE AND EXPLOSION HAZARDS IN NEC CLASS I
LOCATIONS ………………………………………………………………………. 14
D.1. General ………………………………………………………………………. 14
D.2. Principles of Fires and Explosions …………………………………… 15
D.3. Early and Remote Permanent Non-Electric Ignition Sources .. 17
D.4. Ignition Sources …………………………………………………………… 19
D.5. Causes of Fires and Explosions ………………………………………. 19
2 Classifying Sources of Hazard……………………………………………………. 20
A. SOURCES OF HAZARD………………………………………………………. 20
A.1. Static and Dynamic Sources of Hazard…………………………….. 20
A.2. Open and Closed Sources of Hazard………………………………… 21
A.3. Mini, Small, and Large Sources of Hazard ……………………….. 21
A.4. Determining Small and Large Dynamic-Type
Sources of Hazard ………………………………………………………… 23
B. WHEN A LOCATION IS HAZARDOUS ………………………………… 24
C. SAFETY VERSUS HAZARD IN NEC CLASS I LOCATIONS …… 28
D. REQUIREMENT FOR NEC CLASS I, DIVISION 1,
AND DIVISION 2 LOCATIONS ……………………………………………. 29
D.1. General Requirements …………………………………………………… 29
a. “Dangerous” Locations ……………………………………………. 29
b. “Remotely Dangerous” Locations ……………………………… 30
D.2. Specific Requirements for Classifying a Location
as Division 1 ……………………………………………………………….. 30
a. Open Sources of Hazard ………………………………………….. 30
b. Closed Sources of Hazard Frequently Leaking
or Opened ……………………………………………………………… 31
c. Closed Sources of Hazard Not Sufficiently Ventilated ….. 31
d. Increasing Wear ……………………………………………………… 31
e. Simultaneous Failures ……………………………………………… 32
f. Early Ignition ………………………………………………………… 32
D.3. Special Requirements for Classifying a Location
as Division 2 ……………………………………………………………….. 33
a. Sufficiently Ventilated Closed Sources of Hazard ………… 33
b. Failure of Process and Electrical Equipment ……………….. 33
c. Adjacent Locations …………………………………………………. 34
D.4. Summary of Conditions for which NEC Class I Locations
must be Classified ………………………………………………………… 36
D.5. Classification by European Standard IEC …………………………. 42
a. General …………………………………………………………………. 42
b. Definition of IEC Zones …………………………………………… 43
Contents ix
c. Types of Electrical Equipment ………………………………….. 44
d. Grouping According to IEC ……………………………………… 45
3 The Extent of Explosion Danger for NEC Class I Locations …………. 46
A. GENERAL ………………………………………………………………………….. 46
B. THE DIMENSIONAL OUTLINE OF A DIVISION 1 AND
DIVISION 2 ZONE………………………………………………………………. 47
C. QUANTITY OF FLAMMABLE SUBSTANCES VERSUS
EXTENT OF EXPLOSION DANGER …………………………………….. 49
D. FACTORS INFLUENCING QUANTITIES OF FLAMMABLE
GASES AND VAPORS ………………………………………………………… 51
E. EARLY AND REMOTE PERMANENT IGNITION SOURCES….. 77
F. THE EXTENT OF EXPLOSION DANGER FOR CLASS II
FLAMMABLE PRODUCTS………………………………………………….. 78
G. TRANSITION ZONES FOR NEC CLASS I LOCATIONS …………. 81
H. ADDITIONAL DANGER ZONES ………………………………………….. 85
H.1. General Requirements …………………………………………………… 85
H.2. Additional Danger Zones for Heavier-Than-Air Products ……. 86
H.3. Additional Danger Zones for Lighter-Than-Air Products …….. 91
H.4. Safe Distances for Lighter-Than-Air Products …………………… 91
H.5. Safe Distances for Heavier-Than-Air Products ………………….. 91
I. DANGER ZONES ABOVE GROUND ……………………………………. 92
J. CLASSIFICATION OF SOURCES OF HAZARD IN PUMP
STATIONS OCCUPYING 50, 75, OR 100% FLOOR SPACE …….. 92
K. FUME HOODS ………………………………………………………………….. 100
K.1. General …………………………………………………………………….. 100
K.2. Process Areas in Compliance with Figure 1-10A ……………… 100
K.3. Laboratory-type Fume Hood Enclosures in Compliance
with Figures 1-10B and 1-10C ……………………………………… 102
L. STORAGE AND DISPENSING OF FLAMMABLE LIQUIDS ….. 107
L.1. Storage and Dispensing Areas ………………………………………. 107
L.2. Suitable and Non-Suitable Containers ……………………………. 108
L.3. Storage Rooms Inside a Building—Classification and
Ventilation Requirements …………………………………………….. 109
L.4. Dispensing in Storage Areas Inside a Building in Rooms
Without External Walls ……………………………………………….. 110
L.5. Dispensing in Cut-Off Rooms Inside a Building ………………. 111
L.6. Liquid Warehouses …………………………………………………….. 111
L.7. Drum Filling Stations ………………………………………………….. 112
L.8. Storage Cabinets ………………………………………………………… 113
M. SEGREGATION ………………………………………………………………… 113
4 Spatial Considerations …………………………………………………………….. 115
A. INDOOR AND OUTDOOR LOCATIONS …………………………….. 115
B. ROOFED SPACES IN HAZARDOUS AREAS……………………….. 118
x Contents
C. NONHAZARDOUS SPACES ABOVE OR BELOW
HAZARDOUS AREAS ……………………………………………………….. 123
D. SPACES GIVING ACCESS TO HAZARDOUS AREAS ………….. 125
5 The Degree of Explosion Danger for NEC Class II Locations ……… 130
A. GENERAL ………………………………………………………………………… 130
A.1. Division 1 Locations …………………………………………………… 130
A.2. Division 2 Locations …………………………………………………… 131
B. DETAILED REQUIREMENTS FOR NEC CLASS II,
DIVISION 1 LOCATIONS ………………………………………………….. 131
C. DETAILED REQUIREMENTS FOR NEC CLASS II,
DIVISION 2 LOCATIONS ………………………………………………….. 132
D. THE DEGREE OF EXPLOSION DANGER IN FOSSIL
POWER PLANTS ………………………………………………………………. 133
D.1. Coal Fuel Unloading Systems ………………………………………. 133
D.2. Coal Pulverizers …………………………………………………………. 135
D.3. Crusher Houses ………………………………………………………….. 139
6 Ventilation Requirements………………………………………………………… 141
A. GENERAL ………………………………………………………………………… 141
B. NATURAL VENTILATION ………………………………………………… 144
C. MECHANICAL VENTILATION………………………………………….. 147
C.1. Pressure Fans …………………………………………………………….. 147
C.2. Suction Fans ……………………………………………………………… 148
D. APPROXIMATE LOCATION OF MECHANICAL
VENTILATION …………………………………………………………………. 158
E. CANOPY FUME HOODS …………………………………………………… 164
F. DEMARCATION LINE ………………………………………………………. 165
G. LOW AND HIGH INTEGRITY SEAL CONNECTORS …………… 167
H. SAFEGUARDS………………………………………………………………….. 169
I. WIRING DIAGRAMS FOR SAFEGUARDS ………………………….. 170
a. Manual Starting ……………………………………………………. 171
b. Manually Stopping ……………………………………………….. 173
Standby Mode ……………………………………………………………………. 173
a. Mechanical Failure of Fan F1 …………………………………. 173
b. Power Failure in Circuit #1 …………………………………….. 173
7 Electrical Equipment for NEC Class I Locations ……………………….. 177
A. GENERAL ………………………………………………………………………… 177
B. ELECTRICAL EQUIPMENT REQUIRED FOR DIVISION 1
LOCATION ………………………………………………………………………. 178
C. ELECTRICAL EQUIPMENT REQUIRED FOR DIVISION 2
LOCATION ………………………………………………………………………. 179
D. INTRINSICALLY SAFE ELECTRICAL EQUIPMENT …………… 180
Contents xi
E. MARKING OF ELECTRICAL EQUIPMENT ………………………… 181
F. CONSTRUCTION OF EXPLOSION-PROOF ENCLOSURES ….. 181
G. GROUPING OF ELECTRICAL EQUIPMENT ……………………….. 182
8 Electrical Equipment for NEC Class II, Group F Locations ……….. 185
A. GENERAL ………………………………………………………………………… 185
B. CLASS II, DIVISION 1 LOCATIONS …………………………………… 186
C. CLASS II, DIVISION 2 LOCATIONS …………………………………… 186
9 Intrinsically Safe Equipment and Wiring ………………………………….. 188
10 Installation of Electrical Instruments in Hazardous Locations ……. 191
A. TYPE Z PURGING …………………………………………………………….. 192
B. TYPE Y PURGING ……………………………………………………………. 192
C. TYPE X PURGING ……………………………………………………………. 193
11 Hydrogen Gas ………………………………………………………………………… 194
12 Cathodic Protection ………………………………………………………………… 196
13 Static Electricity …………………………………………………………………….. 198
14 Groundings of Tanks, Pipelines, and Tank Cars ………………………… 201
15 Grounding Requirements for Electrical Equipment …………………… 206
A. GENERAL ………………………………………………………………………… 206
B. INTERNAL AND EXTERNAL GROUNDING
CONDUCTORS…………………………………………………………………. 208
C. SUPPLEMENTARY GROUNDING SYSTEMS ……………………… 211
16 Application of Seals in NEC Class I Locations …………………………… 215
A. GENERAL ………………………………………………………………………… 215
B. CLASS I, DIVISION 1 LOCATIONS ……………………………………. 228
C. CLASS I, DIVISION 2 LOCATIONS ……………………………………. 229
17 Application of Seals in NEC Class II Locations ………………………….. 231
xii Contents
Part 2
Application of Fundamentals
18 Environmental Conditions in NEC Class I Hazardous Locations … 235
19 General Requirements for Group A …………………………………………. 249
A. GENERAL REQUIREMENTS FOR GROUP A ……………………… 249
Classified Division 2 …………………………………………………………… 259
Classified Nonhazardous ……………………………………………………… 260
Storage in the Basement ………………………………………………………. 260
20 General Requirements for Group B …………………………………………. 264
21 General Requirements for Group C …………………………………………. 278
22 General Requirements for Group D …………………………………………. 304
23 General Requirements for Group E …………………………………………. 310
24 General Requirements for Group F………………………………………….. 314
25 General Requirements for Group G …………………………………………. 322
26 General Requirements for Group H …………………………………………. 329
27 General Requirements for Group I…………………………………………… 341
28 General Requirements for Group J ………………………………………….. 349
29 General Requirements for Group K …………………………………………. 369
Contents xiii
Part 3
Examples
30 Application Procedure for Classifying NEC Class I Locations …….. 379
A. GENERAL ………………………………………………………………………… 379
B. STEPS TO BE FOLLOWED FOR CLASSIFYING A
HAZARDOUS LOCATION…………………………………………………. 380
C. EXAMPLES ……………………………………………………………………… 381
Example 1. Pump Station …………………………………………………….. 388
Example 2. Holding Basin ……………………………………………………. 390
Example 3. Crude Oil-Fired Power Plant ………………………………… 392
Classification of Area 1……………………………………………….. 395
Classification of Area 2……………………………………………….. 395
Classification of Area 3……………………………………………….. 401
Example 4. Platform Reactor ………………………………………………… 405
Appendix: Properties of Flammable Liquids, Gases, and Vapors ……….. 409
Definitions …………………………………………………………………………………….. 417
Bibliography …………………………………………………………………………………. 426
Index …………………………………………………………………………………………… 428
