CONTROLVALVE Review
A mixture of hazardous gases and air
may be ignited by coming into contact
with a hot surface. The conditions under
which a hot surface will ignite a
gas depend on surface area, temperature,
and the concentration of the gas.
The approval agencies test and establish
maximum temperature ratings for
the different equipment submitted for
approval. Equipment that has been
tested receives a temperature code
that indicates the maximum surface
temperature attained by the equipment.
The following is a list of the different
temperature codes:
Class 1 Division 1 Groups ABCD T4
Hazard Type Area Classification Gas or Dust Group Temperature Code
Chapter 9. Standards and Approvals
182
North American Temperature
Codes
TEMPERATURE
MAXIMUM SURFACE
ATURE TEMPERATURE
CODE C F
T1 450 842
T2 300 572
T2A 280 536
T2B 260 500
T2C 230 446
T2D 215 419
T3 200 392
T3A 180 356
T3B 165 329
T3C 160 320
T4 135 275
T4A 120 248
T5 100 212
T6 85 185
The NEC states that any equipment
that does not exceed a maximum surface
temperature of 100 C (212 F)
[based on 40 C (104 F) ambient
temperature] is not required to be
marked with the temperature code.
Therefore, when a temperature code
is not specified on the approved apparatus,
it is assumed to be T5.
NEMA Enclosure Rating
Enclosures may be tested to determine
their ability to prevent the ingress
of liquids and dusts. In the
United States, equipment is tested to
NEMA 250. Some of the more common
enclosure ratings defined in
NEMA 250 are as follows.
General Locations
Type 3 (Dust-tight, Rain-tight,
or Ice-resistance, Outdoor enclosure):
Intended for outdoor use primarily
to provide a degree of protection
against rain, sleet, windblown
dust, and damage from external ice
formation.
Type 3R (Rain-proof, Ice-resistance,
Outdoor enclosure): Intended
for outdoor use primarily to
provide a degree of protection against
rain, sleet, and damage from external
ice formation.
Type 3S (Dust-tight, Raintight,
Ice-proof, Outdoor enclosure):
Intended for outdoor use primarily
to provide a degree of
protection against rain, sleet, windblown
dust, and to provide for operation
of external mechanisms when ice
ladened.
Type 4 (Water-tight, Dusttight,
Ice-resistant, Indoor or outdoor
enclosure): Intended for indoor
or outdoor use primarily to provide a
degree of protection against windblown
dust and rain, splashing water,
hose-directed water, and damage
from external ice formation.
Type 4X (Water-tight, Dusttight,
Corrosion resistant, Indoor or
outdoor enclosure): Intended for indoor
or outdoor use primarily to provide
a degree of protection against
corrosion, windblown dust and rain,
splashing water, and hose-directed
water, and damage from external ice
formation.
Hazardous (Classified) Locations
Two of the four enclosure ratings for
hazardous (classified) locations are
described as follows in NEMA 250:
Type 7 (Class I, Division 1,
Group A, B, C or D, Indoor hazardous
location, Enclosure): For indoor
use in locations classified as
Class I, Division 1, Groups A, B, C or
D as defined in the NEC and shall be
marked to show class, division, and
group. Type 7 enclosures shall be capable
of withstanding the pressures
resulting from an internal explosion of
specified gases, and contain such an
explosion sufficient that an explosive
gas-air mixture existing in the atmosphere
surrounding the enclosure will
not be ignited.
Chapter 9. Standards and Approvals
183
Type 9 (Class II, Division 1,
Groups E, F or G, Indoor hazardous
location, Enclosure): Intended for
use in indoor locations classified as
Class II, Division 1, Groups E, F and
G as defined in the NEC and shall be
marked to show class, division, and
group. Type 9 enclosures shall be capable
of preventing the entrance of
dust.
The above two NEMA ratings are
often misunderstood. For example,
the above definition of Type 7 is essentially
the same as that for explosion–
proof. Therefore, when an approval
agency approves equipment as
explosion–proof and suitable for Class
I, Division 1, the equipment automatically
satisfies the Type 7 requirement;
however, the agency does not require
that the equipment be labeled Type 7.
Instead it is labeled as suitable for
Class I, Division 1. Similarly, Type 9
enclosures would be labeled as suitable
for Class II, Division 1.
CSA Enclosure Ratings
CSA enclosure ratings are defined in
CSA C22.2, No. 94. They are similar
to the NEMA ratings and are designated
as type numbers; for example,
Type 4. Previously they were designated
with the prefix CSA ENC (for
example, CSA ENC 4).
Intrinsically Safe Apparatus
Intrinsically safe apparatus must be
installed with barriers that limit the
electrical energy into the equipment.
Two methods determine acceptable
combinations of intrinsically safe apparatus
and connected associated apparatus
(for example, barriers) that
have not been investigated in such
combination: entity concept and system
parameter concept.
Entity Concept
The entity concept specifies four parameters:
voltage, current, capacitance,
and inductance. The length of
cable connecting intrinsically safe
equipment with associated equipment
may be limited because of the energy
storing characteristics of cable. The
entity parameters are:
Vmax = maximum voltage that may
safely be applied to the intrinsically
safe apparatus.
Imax = maximum current which may
safely be applied to the terminals of
the intrinsically safe apparatus
Ci = internal unprotected capacitance
of the intrinsically safe apparatus that
can appear at the terminals of the device
under fault conditions
Li = internal unprotected inductance
of the intrinsically safe apparatus that
can appear at the terminals of the device
under fault conditions
Barriers used with the intrinsically safe
apparatus must meet the following
conditions, which are noted on the
loop schematic (control drawing).
Vmax must be greater than Voc or Vt
Imax must be greater than Isc or It
Ca must be less than (Ci + Ccable)
La must be less than (Li + Lcable)
where:
Voc or Vt = maximum open circuit voltage,
under fault conditions, of the associated
apparatus (barrier). For multiple
associated apparatus, FM uses
the maximum combination of voltage
Vt in place of Voc.
Isc or It = maximum short circuit current
that can be delivered under fault
conditions by the associated apparatus.
For multiple associated apparatus,
FM uses the combination of current
It in place of Isc
Ca = maximum capacitance that can
safely be connected to the associated
apparatus
La = maximum inductance that can
safely be connected to the associated
apparatus
Chapter 9. Standards and Approvals
184
Ccable = capacitance of connecting
cable
Lcable = inductance of connecting
cable
The entity parameters are listed on
the loop schematic (control drawing).
The entity concept is used by FM and
UL and will be used by CSA if requested.
CSA System Parameter Concept
The parametric concept is only used
by CSA. For an intrinsically safe apparatus,
the parameters are:
The maximum hazardous location
voltage that may be connected to
the apparatus.
The minimum resistance in
ohms of the barrier that may be connected
to the apparatus.
CSA will also investigate specific
barriers, which may be listed on the
loop schematic along with the parametric
rating.
Loop Schematic (Control
Drawing)
Article 504 of the NEC specifically requires
intrinsically safe and associated
apparatus to have a control drawing
that details the allowed
interconnections between the intrinsically
safe and associated apparatus.
This drawing may also be referred to
as a loop schematic. The drawing
number is referenced on the apparatus
nameplate and is available to the
user. It must include the following information:
Wiring diagram: The drawing
shall contain a diagram of the apparatus
showing all intrinsically safe terminal
connections. For intrinsically safe
apparatus, all associated apparatus
must be defined either by specific
equipment identification or by entity
parameters.
Entity parameters: The entity
parameters (or system parameters in
case of CSA) shall be supplied in a
table showing allowable values for
each applicable Class and Group.
Hazard location identification:
A demarcation line shall be provided
on the drawing to show the equipment
in the hazardous location and the nonhazardous
location. The Class, Division,
and Group of the hazardous
location should be identified.
Equipment identification: The
equipment shall be identified by model,
part number, etc. to permit positive
identification.
Division 2: Division 2 installation
requirements for FM approved
equipment shall be shown.
Comparison of Protection
Techniques
Explosion–proof Technique:
This technique is implemented by enclosing
all electrical circuits in housings
and conduits strong enough to
contain any explosion or fires that
may take place inside the apparatus.
Advantages of this Technique
Users are familiar with this technique
and understand its principles
and applications.
Sturdy housing designs provide
protection to the internal components
of the apparatus and allow their application
in hazardous environments.
An explosion–proof housing is
usually weather–proof as well.
Disadvantages of this Technique
Circuits must be de-energized or
location rendered nonhazardous before
housing covers may be removed.
Opening of the housing in a hazardous
area voids all protection.
Chapter 9. Standards and Approvals
185
Generally this technique requires
use of heavy bolted or screwed
enclosures.
Installation Requirements
The user has responsibility for
following proper installation procedures.
(Refer to local and national
electrical codes.)
Installation requirements are
listed in Article 501 of the NEC or Article
18-106 of the CEC.
All electrical wiring leading to the
field instrument must be installed using
threaded rigid metal conduit,
threaded steel intermediate metal
conduit, or Type MI cable.
Conduit seals may be required
within 18 inches of the field instrument
to maintain the explosion–proof rating
and reduce the pressure piling effect
on the housing.
Intrinsically Safe Technique:
This technique operates by limiting
the electrical energy available in circuits
and equipment to levels that are
too low to ignite the most easily ignitable
mixtures of a hazardous area.
Advantages of this Technique
This technique offers lower cost.
No rigid metal conduit or armored
cable are required for field wiring of
the instrument.
Greater flexibility is offered since
this technique permits simple components
such as switches, contact closures,
thermocouples, RTD’s, and
other non-energy-storing instruments
to be used without certification but
with appropriate barriers.
Ease of field maintenance and
repair are advantages. There is no
need to remove power before adjustments
or calibration are performed on
the field instrument. The system remains
safe even if the instrument is
damaged, because the energy level is
too low to ignite most easily ignitable
mixtures. Diagnostic and calibration
instruments must have the appropriate
approvals for hazardous areas.
Disadvantages of this Technique
This technique requires the use
of intrinsically safe barriers to limit the
current and voltage between the hazardous
and safe areas to avoid development
of sparks or hot spots in the
circuitry of the instrument under fault
conditions.
High energy consumption applications
are not applicable to this
technique, because the energy is limited
at the source (or barrier). This
technique is limited to low-energy applications
such as DC circuits, electropneumatic
converters, etc.
Dust Ignition–proof Technique:
This technique results in an enclosure
that will exclude ignitable amounts of
dusts and will not permit arcs, sparks,
or heat otherwise generated inside the
enclosure to cause ignition of exterior
accumulations or atmospheric suspension
of a specified dust on or near
the enclosure.
Non–Incendive Technique:
This technique allows for the incorporation
of circuits in electrical instruments
that are not capable of igniting
specific flammable gases or vapor-inair
mixtures under normal operating
conditions.
Advantages of this Technique
This technique uses electronic
equipment that normally does not develop
high temperatures or produce
sparks strong enough to ignite the
hazardous environment.
There is lower cost than other
hazardous environment protection
techniques, because there is no need
Chapter 9. Standards and Approvals
186
for explosion–proof housings or energy
limiting barriers.
For non–incendive circuits, the
NEC permits any of the wiring methods
suitable for wiring in ordinary
locations.
Disadvantages of this Technique
This technique is limited to Division
2 applications only.
This technique places constraint
on control room to limit energy to field
wiring (normal operation is open, short
or grounding of field wiring) so that
arcs or sparks under normal operation
will not have enough energy to cause
ignition.
Both the field instrument and
control room device may require more
stringent labeling.
European and Asia/Pacific
Approvals
Approval Agencies
Some of the common approval agencies
in Europe and Asia/Pacific are
listed below:
Approval Agencies
Location Abbreviation Agency
United Kingdom BASEEFA British Approvals Service for Electrical Equipment in
Flammable Atmospheres
Germany PTB Physikalische-Technische Bundesanstalt
France LCIE Laboratorie Central des Industries Electriques
Australia SAA Standards Association of Australia
Japan JTIISA Japanese Technical Institution of Industry Safety Association
CENELEC Approvals
CENELEC is the acronym for European
Committee for Electrotechnical
Standardization. CENELEC standards
are applicable to all European Union
countries plus other countries that
choose to use them. A piece of equipment
that is successfully tested to the
relevant CENELEC standard has CENELEC
approval. The testing may be
performed by any recognized testing
laboratory in Europe. Approvals may
be based on national standards, but
CENELEC approvals are preferred.
Types of Protection
The types of protection commonly
used outside North America are:
Flame–proof:
A type of protection in which an
enclosure can withstand the pressure
developed during an internal explosion
of an explosive mixture and that
prevents the transmission of the explosion
to the explosive atmosphere
surrounding the enclosure and that
operates at such an external temperature
that a surrounding explosive gas
or vapor will not be ignite there. This
type of protection is similar to explosion–
proof. It is referred to by IEC as
Ex d.
Increased Safety:
A type of protection in which various
measures are applied to reduce
the probability of excessive temperatures
and the occurrence of arcs or
sparks in the interior and on the external
parts of electrical apparatus that
do not produce them in normal service.
Increased safety may be used
with the flameproof type of protection.
This type of protection is referred to
by IEC as Ex e.
Intrinsically Safe:
A type of protection in which the
electrical equipment under normal or
abnormal conditions is incapable of
releasing sufficient electrical or therChapter
9. Standards and Approvals
187
mal energy to cause ignition of a specific
hazardous atmospheric mixture in
its most easily ignitable concentration.
This type of protection is referred to
by IEC as Ex i.
Non–Incendive:
A type of protection in which the
equipment is incapable, under normal
conditions, of causing ignition of a
specified flammable gas or vapor-inair
mixture due to arcing or thermal
effect. This type of protection is referred
to by IEC as Ex n.
Nomenclature
Approval agencies that use the IEC
nomenclature (for example, BASEEFA,
LCIE, PTB, and SAA) classify
equipment to be used in hazardous
locations by specifying the type of
protection, gas group, and temperature
code as follows:
E Ex ia IIC T4
Denotes
CENELEC
Approval
Denotes
Hazardous
Area
Approval
Types of Protection
ia—Intrinsic safety (2 faults
allowed)
ib—Intrinsic safety (1 fault
allowed)
d—Flameproof
e—Increased safety
n—Type n (non–incendive)
(SAA only)
N—Type N (non–incendive)
(BASEEFA only)
Group Temperature
Code
For CENELEC approvals, the nameplate
must also include the following
symbol to indicate explosion protection:
This mark indicates compliance with
CENELEC requirements and is recognized
by all European Union member
countries.
Hazardous Location
Classification
Hazardous locations outside North
America are classified by gas group
and zone.
Group
Electrical equipment is divided into
two groups. Group I covers electrical
equipment used in mines, and Group
II covers all other electrical equipment.
Group II is further subdivided
into three subgroups: A, B, and C.
The specific hazardous materials within
each group can be found in CENELEC
EN 50014, and the automatic
ignition temperatures for some of
these materials can be found in IEC
60079-4.
Group I (Mining): Atmospheres
containing methane, or gases or vapors
of equivalent hazard.
Group IIA: Atmospheres containing
propane, or gases or vapors of
equivalent hazard.
Group IIB: Atmospheres containing
ethylene, or gases or vapors of
equivalent hazard.
Group IIC: Atmospheres containing
acetylene or hydrogen, or
gases or vapors of equivalent hazard.
Chapter 9. Standards and Approvals
188
Note
An apparatus approved
for one subgroup in
Group II may be used in
the subgroup below it;
for example, Group IIC
may be used in Group
IIB locations.
Zone
The zone defines the probability of
hazardous material being present in
an ignitable concentration in the surrounding
atmosphere:
Zone 0: Location where an explosive
concentration of a flammable
gas or vapor mixture is continuously
present or is present for long periods.
The area classified as Zone 0, although
not specifically defined, is contained
within the United States and
Canada classifications of a Division 1
location and constitutes an area with
the highest probability that an ignitable
mixture is present.
Zone 1: Location where an explosive
concentration of a flammable
or explosive gas or vapor mixture is
likely to occur in normal operation.
The area classified as Zone 1 is contained
within the United States and
Canada classifications of a Division 1
location.
Zone 2: Location in which an
explosive concentration of a flammable
or explosive gas or vapor mixture
is unlikely to occur in normal operation
and, if it does occur, will exist
only for a short time. Zone 2 is basically
equivalent to the United States
and Canadian classifications of a Division
2 location.
Temperature Code
A mixture of hazardous gases and air
may be ignited by coming into contact
with a hot surface. The conditions under
which a hot surface will ignite a
gas depends on surface area, temperature,
and the concentration of the
gas.
The approval agencies test and establish
maximum temperature ratings for
the different equipment submitted for
approval. Group II equipment that has
been tested receives a temperature
code that indicates the maximum surface
temperature attained by the
equipment. It is based on a 40 C
(104 F) ambient temperature unless
a higher ambient temperature is indicated.
IEC Temperature Codes
TEMPERATURE
MAXIMUM SURFACE
TEMPERATURE
CODE
C F
T1 450 842
T2 300 572
T3 200 392
T4 135 275
T5 100 212
T6 85 185
IEC Enclosure Rating
According to IEC 60529, the degree of
protection provided by an enclosure is
indicated by the IP Code. The code
consists of the letters IP (ingress
protection) followed by two characteristic
numerals indicating conformity
with the degree of protection desired
(for example, IP54). The first numeral
indicates the degree of protection
against the following: human contact
with or approach to live parts; human
contact with moving parts inside the
enclosure; and ingress of solid foreign
objects. The second numeral indicates
the degree of protection provided
by the enclosure against the ingress
of water. The characteristic
numerals are defined in the following
table:
NEMA and IEC Enclosure
Rating Comparison
The following table provides an equivalent
conversion from NEMA type
numbers to IEC IP designations. The
Chapter 9. Standards and Approvals
189
NEMA types meet or exceed the test
requirements for the associated IEC
classifications; for
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