Explosion Proof May Not be the Answer
Authors: Justin Bishop, Ph.D., P.E., CFEI; Mark Fecke, P.E., CFEI; Russ Ogle, Ph.D., P.E., CSP
Introduction
Dust explosions are a known hazard in bulk solid manufacturing and process facilities. In fact,
281 confirmed dust incidents occurred from 1980 through 2005, taking 119 lives and causing
718 injuries. i One technique that is used routinely to mitigate the risk of a dust explosion is the
removal of potential ignition sources. Potential ignition sources include, but are not limited to,
mechanical ignition sources such as mechanical sparks and frictional heating; chemical ignition
sources such as self-heating, spontaneous combustion, or other exothermic reactions; and
electrical ignition sources such as static electricity and electrical equipment.
Of the three groups of ignition sources described above, electrical ignition sources are regularly
identified as potential ignition sources during dust explosion investigations. During these
investigations, it is common to find electrical equipment that is not appropriate for the classified
(hazardous) location that it is located within even though facilities personnel thought the
equipment was appropriate. Understanding how dust explosions occur and what electrical
equipment is appropriate for a given classified location is critically important when trying to
mitigate the risk of a dust explosion.
Dust Explosion Basics
A dust explosion occurs when a fuel, ignition source, oxygen, confinement, and dispersion come
together. This requirement is traditionally depicted using an explosion pentagon as shown in
Figure 1. If any of the five items identified in the pentagon are removed from the pentagon, an
explosion can be prevented. Dust explosion protection becomes stronger as additional items of
the pentagon are removed, but some items are more easily managed than others. Electrical
ignition sources are one of those more easily managed items, but only if you know what to look
for.
Hazardous (Classified) Location Basics
The National Electrical Code (NEC) identifies three specific areas where explosion hazards may
exist, as it relates to electrical equipment. These areas are defined as Class I, II, and III. Class
I areas are those where an explosion or fire hazard exists due to the presence of flammable
gases, flammable liquid-produced vapors, or combustible liquid-produced vapors. Class II
areas are those where an explosion or fire hazard exists due to the presence of combustible
dusts, and Class III areas are those where an explosion or fire hazard exists due to the
presence of ignitable fibers/flyings. ii The NEC refines each Class into either Division 1 or
Division 2. Additionally, materials such as flammable gases, combustible dusts, and others that
might be considered when evaluating whether an area should be classified as hazardous per
the NEC are separated in to Groups based on their material and physical properties.
Representative lists of materials in the material Groups may be found in reference material such
as National Fire Protection Association (NFPA) 497 iii and NFPA 499. iv
Bulk solid manufacturing and process facilities likely fall into the Class II (i.e., combustible dusts)
area; however, depending on the process, other materials that require Class I or Class III
classifications may be possible and should be considered. To remove potential electrical
ignition sources from an area that has been determined to be hazardous per the NEC, the
electrical equipment that will be used and installed in the area needs to be evaluated and
confirmed to meet the NEC requirements for electrical equipment within the specific hazardous
area.
The NEC describes requirements for electrical equipment that is intended for use in each of the
Classes described above and also identifies how to tell if a piece of electrical equipment is
appropriate for a given Class, Division, and Group. Remember that electrical equipment within
hazardous (classified) locations must be appropriate for not only the Class and Division but also
the Group. To get the full benefit of using classified equipment, the equipment must be installed
appropriately for the area. Classified equipment may not work properly if it is installed
improperly.
Figure 1. Representation of an explosion pentagon.
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Electrical Motors in Class II Locations
Electrical motors are found throughout industry and are commonly found in classified locations.
However, based on experience, there is often confusion about what types of motors are
appropriate for Class II locations.
Two questions that are frequently asked by facility personnel regarding the choice or
appropriateness of a given motor are:
1) Is an explosion-proof motor appropriate for a Class II, Division 1 area?
2) Is a totally enclosed, fan-cooled (TEFC) motor appropriate for a Class II,
Division 2 area?
Question 1: Is an explosion-proof motor appropriate for a Class II, Division 1 area?
The short answer is maybe, but additional information such as the Group designation for the
material in the atmosphere around the equipment is needed. Also, the expression “explosionproof” is associated with Class I equipment, whereas the analogous expression for Class II
equipment is “dust-ignition-proof.” The NEC addresses this question, in part, as shown in
Figure 2. v
Figure 2. Excerpt from the 2014 edition of the NEC
As shown in Figure 2, there are two parts to the NEC requirement. The first is that explosionproof equipment is not required when trying to meet the equipment requirements for Class II,
Division 1 areas. The second part is if explosion-proof equipment is chosen, then the
equipment must also be identified as being appropriate for a Class II area and must also be
identified as being appropriate for the Division and Group where the equipment will be used.
How could a piece of explosion-proof equipment that is appropriate for a Class I, Division 1
area, not be appropriate for a Class II, Division 1 area? To answer this question, one must
understand that the hazard or ignition scenario that the equipment is trying to prevent is different
for different Classes.
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Recall that the NEC’s definition of explosion-proof equipment is:
“[E]quipment enclosed in a case that is capable of withstanding an explosion of a
specified gas or vapor that may occur within it and of preventing the ignition of a
specified gas or vapor surrounding the enclosure by sparks, flashes, or explosion of
the gas or vapor within and that operates at such an external temperature that a
surrounding flammable atmosphere will not be ignited thereby.” vi
However, the definition of dust-ignition-proof equipment is:
“[E]quipment enclosed in a manner that excludes dusts and does not permit arcs,
sparks, or heat otherwise generated or liberated inside of the enclosure to cause
ignition of exterior accumulations or atmospheric suspensions of a specified dust on or
in the vicinity of the enclosure.” vii
As is apparent from the definition, explosion-proof equipment is designed to prevent ignition of a
specified gas or vapor and not combustible dusts. While explosion proof equipment is designed to
prevent the ignition of specified gases or vapors surrounding the equipment when an explosion
occurs inside the equipment, explosion-proof equipment is not necessarily designed to keep dust
from entering the equipment. Additionally, explosion-proof equipment is designed so that its
maximum operating temperature will not ignite the surrounding flammable atmosphere. This is
similar to the requirement for dust-ignition-proof equipment; however, recall that the ignition
temperature of flammable gases and vapors is generally higher than the ignition temperature of
combustible dust layers. viii
Figure 3A shows a motor nameplate that indicates the motor is explosion proof (i.e., appropriate
for a Class I, Division 1 area), but it does not indicate that it is appropriate for Class II areas and
therefore would not be appropriate. Additionally, this motor is appropriate for Group D materials
but not for other material Groups. However, some equipment does have a dual rating; Figure 3B
shows the name plate from such an explosion-proof motor. This motor is appropriate for some
Class II, Division 1 locations. Specifically, this explosion-proof motor would be appropriate for
Class II, Division 1 areas where the material (i.e., combustible dust) in the environment around the
motor is in either the Group F or G category. This means that this explosion-proof motor is not
appropriate for environments that contain Group E dusts, which is the other Group designation in
Class II, Division 1 environments.
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Figure 3A. Representative photograph of name plate from an explosion-proof motor
Figure 4B. Representative photograph of name plate from an explosion-proof motor
Question 2: Is a totally enclosed fan-cooled (TEFC) motor appropriate for a Class II,
Division 2 area?
Similar to question 1, the short answer to question 2 is maybe, but additional information such
as the Group designation for the material in the atmosphere around the equipment is needed.
The 2014 edition of the NEC states:
“In Class II, Division 2 locations, motors, generators, and other rotating electrical
equipment shall be totally enclosed nonventilated, totally enclosed, pipe-ventilated,
totally enclosed water-air-cooled, totally enclosed fan-cooled, dust-ignition proof for
which maximum full-load external temperature shall be in accordance with 500.8(D)(2)
for normal operation when operating in free air (not dust blanketed) and shall have no
external openings.” ix
The first part of the requirement above confirms that the TEFC motor style is appropriate;
however, the TEFC motor must also have a maximum full-load external temperature, under
normal operating conditions, in accordance with Table 1 below, and it must also have no
external openings. Note that the maximum full-load external temperature should be in
accordance with Table 1, but this does not mean that it can be above the auto ignition
temperature of the specific combustible dust to be encountered by the equipment, nor can it
exceed 165°C (329°F) for organic dusts that may dehydrate or carbonize.
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Table 1. Maximum Full-Load External Temperature a
Equipment (such as motors or power
transformers) that may be Overloaded
Class II
Group
E
F
G
a
Equipment Not Subject
to Overloading
o
o
C
F
200
200
165
Normal Operation
o
C
392
392
329
200
150
120
Abnormal Operation
o
F
392
302
248
o
C
200
200
165
o
F
392
392
329
Recreation of Table 500.8(D)(2) from the 2014 edition of the NEC
To determine whether a given motor complies with the maximum full-load external temperature
requirements and is below the auto-ignition temperature of the specific combustible dust being
considered, the dust Group classification is needed, and the motor’s temperature code (T-code)
needs to be verified. Table 2 provides a list of T-codes and their associated maximum
temperatures.
Table 2. T-codes and Their Associated Maximum Temperatures b
Maximum Temperature
o
o
C
F
450
842
300
572
280
536
260
500
230
446
215
419
200
392
b
Temperature
Class (T-code)
T1
T2
T2A
T2B
T2C
T2D
T3
Maximum Temperature
o
o
C
F
180
356
165
329
160
320
135
275
120
248
100
212
85
185
Temperature
Class (T-code)
T3A
T3B
T3C
T4
T4A
T5
T6
Recreation of Table 500.8(C) from the 2014 edition of the NEC
Unfortunately, T-codes are not necessarily required on a motor’s name plate unless the motor is
identified as being approved for a specific Class II, Division 1 hazardous environment. x In some
cases, the only way to determine the motor’s T-code is to contact the motor manufacturer and
ask them to evaluate the motor. Sometimes the motor manufacture is able to calculate the
motor’s T-code and other times the manufacture may not be able to determine the T-code.
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What Do I Do Now?
Now that you know what to look for, conduct a classified electric motor survey. Any area of your
facility where combustible dusts are handled may be a classified (hazardous) area per the NEC.
Do your motors measure up? The only way to find out is to get out on the shop floor and look at
motor nameplates. It is not unusual for a facility to have tens, if not hundreds, of electrical
motors, so it is best to get organized. First, determine the proper hazardous area classification
for your facility. Remember, some areas will be classified, and some will not.
Create a checklist of all the motors in a given area to be surveyed and grab a few essential
inspection items:
•
•
•
•
•
A general arrangement drawing
Notebook or clipboard for taking notes
Flashlight for illuminating dark nameplates
Machine shop towel for cleaning dirty nameplates
Small digital camera for taking photographs of each non-complying motor.
Electric motors that are in compliance can be left alone, but if you find a non-complying motor,
you will need to decide if you are going to replace it to reduce the risk of a dust explosion. You
can indicate the location of the motor on the drawing, so you will know where to find the motor
later. Document the name plate so you know the motor specification when purchasing a new
one. Once this action is complete and non-complying motors are replaced, you will have taken
a significant step toward preventing dust explosions at your facility.
Conclusion
Understanding how dust explosions occur and what electrical equipment is appropriate for a
given classified location is critically important when trying to mitigate the risk of a dust explosion.
Removal of potential electrical ignition sources is a common technique used to mitigate the risk
of dust explosions; however, there continues to be some confusion regarding the type of
electrical equipment that is appropriate for Class II locations. For example, an explosion-proof
motor may not be appropriate for a Class II, Division 1 location, and a TEFC motor may not be
appropriate for a Class II, Division 2 location. Additional, critical information is needed to verify
whether these types of motors are appropriate for the hazardous (classified) location, and
therefore, would address the goal of reducing the risk of dust explosions by removing potential
electrical ignition sources.
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Contribution Authors
Justin A. Bishop, Ph.D., P.E., CFEI, Managing Engineer
630.658.7522 • jbishop@exponent.com • Bio
Dr. Bishop applies his knowledge of electrical/electronic system failure
modes and electrical codes to the analysis and investigation of marine,
residential, commercial, and industrial incidents and fires; alleged product
and equipment failures; electrical work practices; and hazard studies.
Mark Fecke, P.E., CFEI, Principal Engineer
630.658.7512 • mfecke@exponent.com • Bio
Mr. Fecke’s mechanical engineering expertise focuses on steam, hot water,
and heat generating systems, especially boilers, burners, and fluidized bed
systems for electric power utilities, industrial plant utilities, agrochemical and
specialty chemical processing plants, hospitals, multi-tenant buildings, and
marine vessels.
Russell A. Ogle, Ph.D., P.E., CSP, Principal Engineer
630.658.7502 • rogle@exponent.com • Bio
Dr. Ogle applies his expertise as a chemical engineer to the scientific
investigation and prevention of accidents, with particular emphasis on fires,
explosions, and chemical releases. He specializes in the investigation of
complex industrial accidents, catastrophic fires and explosions, and
incendiary fires (arson).
i
US Chemical Safety and Hazard Investigation Board (CSB), “Combustible Dust Hazard StudyInvestigation Report,” 2006-H-1, Washington DC, November 2006, page 1.
ii
National Electrical Code, 2014 edition, article 500.
iii
NFPA 497: Recommended practice for the classification of flammable liquids, gases, or vapors and of
hazardous (classified) locations for electrical installations in chemical process areas, 2012 edition,
Chapter 4. National Fire Protection Association.
iv
NFPA 499: Recommended practice for the classification of combustible dusts and of hazardous
(classified) locations for electrical installations in chemical process areas, 2013 edition, Chapter 5.
National Fire Protection Association.
v
National Electrical Code, 2014 edition, article 502.
vi
National Electrical Code, 2014 edition, article 100.
vii
National Electrical Code, 2014 edition, article 500.
viii
Electrical Installations in Hazardous Locations, P. J. Schram, R. P. Benedetti, M. W. Earley, 3rd Edition,
Chapter 4, 2009.
ix
National Electrical Code, 2014 edition, article 502.
x
National Electrical Code, 2014 edition, article 430 and article 502.
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