Industrial Weighing
Safe
Accurate
Smart
Guide Book
Edition 2
Hazardous Area
Industrial Weighing Solutions
Hazardous Area Guide
Table of Contents
METTLER TOLEDO provides this practical guide as a service to help customers. In reading or making any
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claims arising out of or in connection with your use of this document.
Chapter 1
Introduction: Safe Weighing Solutions
4-5
Chapter 2
Explosion Protection Basics
6-9
2.1 Flammable or Explosive Substances
2.2 Prevention of Explosion
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PROVIDED IN THIS DOCUMENT. THIS DOCUMENT IS NOT INTENDED AS LEGAL ADVICE. BEFORE MAKING ANY
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FOR A PARTICULAR PURPOSE, OR NON-INFRINGEMENT, WITH THE SOLE EXCEPTION BEING WARRANTIES (IF
ANY) WHICH CANNOT BE EXPRESSLY EXCLUDED UNDER APPLICABLE LAW.
Chapter 3
International Standards and Regulations
10-19
3.1 International Overview
3.2 Classification at a Glance
Chapter 4
Ignition Protection Methods
20-23
4.1 Intrinsic Safety and Protection
4.2 Marking of Electrical Equipment
4.3 Equipment Installation and Maintenance
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and accounting fees and expenses).
Chapter 5
Weighing in Hazardous Areas – System Examples
24-37
5.1 Basic System
5.2 Advanced System
5.3 Active Control System
5.4 Active/Passive Control System
5.5 Passive Input/Output System
5.6 Fully Integrated Process Control
No part of this publication may be reproduced or distributed in any form without written permission from
METTLER TOLEDO
Chapter 6
Summary
38-39
"How can I ensure safety and
accuracy in hazardous zones?"
Production Manager
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Introduction
1. Safe Weighing Solutions
Careful selection of weighing solutions to meet specific process requirements and
regulations can be a challenging task for manufacturers that work within hazardous areas.
However, it is essential to workplace safety as well as measurement accuracy.
Explosion Protection Basics
This guide illustrates how to avoid dangerous situation in hazardous areas with compliant
equipment. It also provides illustrated recommendations for correct installation and maintenance
on equipment in these areas.
When electrical equipment is used in areas where flammable or explosive
substances are present there is always a possibility or risk of fire or explosion. These are classified as hazardous locations or hazardous areas.
Standards and Regulations
Globally, explosion protection is regulated by the legislatures of the individual countries. Standards and mandatory regulations facilitate the free
movement of goods by providing a uniformly recognized framework. Classification varies but generally, there are two types: ATEX and NEC.
Ignition Protection
The basic safety concept is to eliminate the simultaneous existence of possible ignition sources. The method of equipment protection will depend
on the degree of safety needed for the type of hazardous area. Marking of
electrical equipment is mandatory and must be placed on equipment before it is distributed.
Weighing in Hazardous Areas
Weighing is one of the most basic and important process variables in
most manufacturing processes. However, it can be one of the most challenging parameters to control. To achieve reliable quality and repeatable
results, you require efficient capture and communication of data to the existing plant network, which can be challenging in hazardous areas.
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Flammable Substances Characteristics
Flammable Gas
As the percentage of gas increases, the risk is
greater. When the concentration exceeds a certain
limit, the air becomes saturated with the gas and
ignition becomes less likely.
Most manufacturing and processing industries generate potentially explosive
atmospheres using flammable or explosive substances, such as flammable gases
or vapors, flammable liquids, combustible dusts, ignitable fibers or debris. These
substances can form an explosive atmosphere with oxygen normally present in
air. When electrical equipment is used in or around such areas, there is always a
possibility of fire or explosion. These areas are classified as hazardous.
Flammable Liquids / Vapors
Flash point is the minimum temperature at which the
vapor near the surface is high enough to form an ignitable mixture. The higher the flash point of the liquid, the less the danger of ignition.
Flammable Solids
For dusts, information on particle size and density
will be needed, once it has been shown that a particular dust can form an explosive atmosphere.
2.1 Flammable or explosive substances
Explosive Limits
Flammable or explosive substances may be present in the form of gases, vapors, mist, or dust clouds. Each
material is present in a defined concentration and for a certain period. The properties of a dangerous substance that need to be known include the boiling point and flash point and whether any flammable gas or
vapor involved is lighter or heavier than air.
Mixture is too lean to
cause
an explosion
anc
e
1 Flammable or explosive substance
n
yge
Ox
Fla
mm
abl
es
ubs
t
0% Vol.
LEL

3 Components of Explosions
Mixture is too
rich to cause
an explosion
Explosive
Mixture
Explosion Limit
Concentration of
flammable substance

UEL
Explosion Protection - Basics
2. Explosion Protection – The Basics
100% Vol.
Figure 2: Explosive limits diagram
Lower Explosive Limit (LEL)
The minimum concentration of explosive liquid vapor in air that will support the propagation of flame, or flame
spread, through the entire volume of vapor-air mixture upon contact with an ignition source.
Upper Explosive Limit (UEL)
The maximum concentration of vapor in air that will support the propagation of flame.
2 Oxygen
Explosion
3 Source of ignition
Ignition source
Figure 1: Triangle of fire
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anc
e
ub
st
Fla
mm
abl
es
anc
e
ub
st
Triangle
of fire
anc
e
Ignition source
ub
st
Secondary Explosion Protection
Secondary explosion protection is related to preventing the ignition of
potentially explosive atmospheres. This means avoiding sparks (mechanical, electrical, static), flames, hot gases or hot surfaces, as well
as eliminating other possible ignition sources such as electromagnetic, ultrasonic etc. Equipment designers are searching for different
ways to eliminate those sources.
This can be done by:
• Using appropriate housing materials, such as stainless steel,
which prevents formation of sparks
• Using proper earthing and conducting materials
• Avoiding hot surfaces, e.g. by means of applying intrinsically safe
circuits in the design of electrical equipment.
Ignition source
Fla
mm
abl
es
In the absence of flammable substances, no ignitable mixture will be
created. As a result, no explosion risk will exist. This approach can, of
course, only be applied to a limited extent. After all, the flammability of
many substances is a desired product property that is either indispensable or which cannot be controlled, for example, gases and their release in the field of mining. In such cases, secondary explosion protection measures must be taken.
Triangle
of fire
Triangle
of fire
n
yge
Ox
8
Primary Explosion Protection
Primary explosion protection is based on the concept of preventing
the formation of a potentially explosive atmosphere. These might include:
• Using substitutes for flammable substances, for example, in-flammable organic solvents can be substituted with hydrous solutions
• Using gas detectors
• Preventing the formation of explosive atmospheres in hazardous
areas, for example, by means of ventilation.
n
yge
Ox
• Hot surfaces are a result of energy loss from systems, equipment and components during normal operation.
• Flames and hot gases (including hot particles) can occur inside combustion engines or analysis devices during
normal operation and when a fault has occurred.
• Mechanically generated sparks are produced, for example, by grinding and cutting devices during normal operation and are not permitted in a potentially explosive atmosphere.
• Electrical apparatuses must normally be regarded as a sufficient ignition sources. Only very low energy sparks
with energies of only micro Joules may be regarded as too weak to start an explosion. For this rea-son, suitable
measures must be adopted to prevent these ignition sources.
• Static electricity. The stored energy can be released in the form of sparks and function as an ignition source. Because this ignition source can arise independently of an electrical voltage supply, it must also be considered
with non-electrical devices and components. It is connected with separation processes; therefore, these cases
must be assessed where this ignition source needs to be taken into account.
• Electromagnetic fields: Frequency ranges from 9 x 1000 to 3 x 1011 Hz. These include high-frequency equipment such as radio equipment or high-frequency generators.
• Electromagnetic radiation: Frequency ranges from 3 x 1011 to 3 x 1015 Hz and wavelengths of 1000 to 0.1
μm. This includes optical radiation such as sunlight, lasers, lightning strikes, electric arcs.
• Ionizing radiation: Ignition due to energy absorption, with causes such as short-wave UV rays, X-rays or radioactive materials.
• Adiabatic compression and streaming gases: Due to the high temperatures that occur due to shock waves and
in instances of adiabatic compression, an atmosphere subject to explosion can ignite.
• Chemical reactions: Due to chemical reactions that cause heat development (exothermic reactions), materials
heat up and can cause an explosion.
To eliminate the risk of explosion, one of the three elements of the “Triangle of Fire” must be removed.
en
Ignition Sources
This section discusses ignition sources related to equipment. These can be hot surfaces, mechanically generated
sparks, electrical apparatuses and static electricity. Equipment suppliers reduce the risk of explosion by eliminating ignition sources and by keeping the system’s active ignition energy at the lowest possible level – lower than
the minimum ignition energy. The minimum ignition energy is the smallest amount of energy required to ignite a
combustible vapor, gas or dust cloud. The minimum ignition energy is measured in µ Joules.
2.2 Prevention of Explosion
Fla
mm
abl
es
Oxygen
Although an explosion usually occurs because of direct presence of oxygen in the mixture, this is not always true.
For example a mixture of the (now seldom used) anesthetic gases, ethyl ether and nitrous oxide, can cause violent explosions because oxygen is formed from the nitrous oxide. If the oxygen concentration is above the percentage normally found in the air (21% by volume) flammable limits are normally exceeded, and the ignition energy
is decreased. In addition, the explosion is often considerably more violent than at normal oxygen concentration in
the air. Please note: The IEC/EN60079 series and products certified according to those approval standards only
covers the standard concentration of oxygen in air and normal atmospheric pressure.
yg
Ox
Explosion Protection - Basics
Combustible Dusts
When solids are processed in industrial environments, such as chemical plants and flour mills, often small particles are present in the environment in the form of dust or dust clouds. Dust is defined in IEC 60079-10-2 (Classification of areas - Explosive dust atmospheres) as finely divided solid particles no larger than 500 µm or fibers
greater than 500 μm in nominal size in the atmosphere that are deposited due to their own weight but which remain in the atmosphere for some period of time in the form of a dust-air mixture.
Ignition source
Figure 3: Explosion protection concepts
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International Standards and Regulations
3. International Standards and Regulations
3.1 Global Map of Guidelines and Standards
Safety problems related to the design and use of electrical equipment in hazardous areas have led authorities to
impose strict rules. It has also elicited awareness of safe equipment design.
Globally, explosion protection is regulated by the legislatures of the individual countries. National differences in
technical requirements and the required approvals for explosion protected equipment make significant demands,
primarily on global players, and require considerable transparency in development and approval testing. Standards and mandatory regulations facilitate the free movement of goods by providing a uniformly recognized
framework.
They cover everything from product certification requirements to protective measures for employees who work
with the products. Recognized authorities work to develop uniform standards on both a national and international
scale. However, historical and country-specific developments have the result that many areas – including explosion protection – do not yet have a global standard in place.
A basic map of global standards and regulations shows how hazardous-area guidelines and certifications
are distributed around the world (Figure 4).
Local hazardous location approval standards are based off IEC standards with national / regional differences applied. In Europe, for example, CENELEC is responsible for either creating or adapting IEC standards
so that the standard is harmonized for all the members of the European Union. Notified Bodies use these
harmonized standards to evaluate product to ensure that they meet the Essential Health and Safety Requirements of the ATEX directive.
Regional Regulation
China, Korea and Russia are all examples of regions that follow their own local laws regarding electrical
equipment regulations. Based on requirements some accept IECEx test reports and ATEX reports and may issue approvals on their merit. If you’re located in one of these regions, be sure to review local requirements
for equipment in hazardous areas.
Global Regulations for Electrical Equipment
There are two major global organizations that set hazardous-area standards globally. One is the International Electro-Technical Commission (IEC). This is the premier international standardization organization for
electric, electronic and related technologies. The aim of the IEC is to harmonize the many different standards
and regulations throughout the world and to remove trade barriers for related products. For example, the IEC
60079 standards are related to the general requirements for hazardous areas. The IEC system is followed in
Europe, Asia, Australia, Africa and some other regions.
The second system is the North American system with the National Electrical Code (NEC), which are published by the National Fire Protection Association (NFPA).The requirements for hazardous areas and safety
in the workplace are defined in ATEX directives in the European Union and in NEC articles in the USA.
Country
Canada USA
Brazil
Guideline
CEC
NECTM
INMETRO ATEX
Europa
Standard
CSA
FMUL
INMETRO CENELEC
Testing marks
(nationally accredibid Testing
abs or official
mark)
Examples: Examples:
Russia
China
Japan
Australia Korea
EACEx/TR CU
012/2011
GOST 31xxx
series
CCC
MHLW
IEC
KOHSA
GB 3836.x
JNOSHI
IECEx
KCs
IEC
ATEX
CENELEC
UL and FM
OSHA/
NEC
JPEX / TIIS
PTB
DEKRA
NRTL'S
Figure 5: Global view of hazardous standards and regulations
Figure 4: Global map of guidelines and standards
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International Standards and Regulations
IECEx, ATEX and NEC at a glance
A common route to compliance is to use pre-approved products in a specified manner and, if required, a
notified body (for example DEKRA, Baseefa, Sira, TUV) to provide a conformity assessment to gain additional certification. Unfortunately, for global export, this is not always sufficient. An internationally acceptable
solution was required to allow equipment designs to be used in any part of the world without significant
modification. The approach would use recognized practice to achieve a common acceptable level of safety,
with the goal of facilitating free trade of hazardous-area use products across all major markets.
ATEX
NEC/CEC
IECEx
Regulatory status
Mandatory system
Mandatory system
Voluntary system
Conformity basis
EU-directives (EHSR)
US/CAN standards
e.g. ISA, UL, FM, CSA
IEC standards
Standards
EN standards
US/CAN standards
IEC standards
Area classification
Zone
Zone, Class and Division
Zone
Conformity assessor
Ex notified bodies (Ex NB)
manufacturer
Nationaly Recognized Testing Laboratories (NRTL) /
Standards Council of
Canada Accredited Testing
Laboratories (SCC)
Ex certifying bodies (ExCB)
Ex test labs
- E C-type examination
certificate
- T est report (on request)
-Q
M certificate / report
-Listing and Marking
-Certificate of conformity
-Test report (not public)
-Audit report (not public)
- Certificate of conformity
- Test report (ExTR)
- Quality assessment report
- Full public access to all issued IECEx certificates
European Union, some
Latin American countries
North America, Canada,
some Latin American
countries
Australia, New Zealand
Issued documents
Acceptance
Conformity mark
Regional acceptance of
Technical Report (TR)
Level of
ignition
protection
Conditions of operation
Performance of
protection
M1
Methane,
dust
Very high
Equipment remains energized and functioning when
explosive atmosphere is
present
2 independent protection
methods or safe with 2
faults
High
Equipment is de-energized
in the event of an explosive
atmosphere
Sufficient level of safety
during the normal operating conditions
Equipment
Group II,
(e.g. processing
industries)
Very high
Equipment remains energized and functioning
in Zones 0,1,2 (G) and/or
20,21,22 (D)
2 independent protection
methods, or safe with 2
faults
Cat.2
High
Equipment remains energized and functioning in
Zones 1,2 (G) and/or 21,22
(D)
Suitable for normal operation and frequently
occurring disturbances,
or safe with 1 fault
Cat.3
Normal
Equipment remains energized and functioning in
Zones 2 (G) and/or 22 (D)
Suitable for normal operation
Cat.1
Gas, vapor,
mist, dust
Examples for marks:
Europe, North America,
Japan, Brazil, China
North America,
Canada, Mexico
36 IECEx members states
(status 2023)
To prevent any risk of explosion or fire and improve safety of the process and employees, all potentially hazardous areas must be classified according to the processes conducted in these areas. It is in responsibility of the end
user to define the hazardous areas within the operations. Area classification is the method of analyzing and classifying the environment where explosive gas atmospheres may occur so as to facilitate the proper selection of the
electrical equipment. The concept of assessing the potentially explosive areas is crucial to limiting the risk associated with the installation of electrical equipment in potentially explosive environment.
The assessment tests and appropriate areas classification allows preparation of safety procedures for plant operation and maintenance. Classification varies across the world, but generally, there are two types of classification:
• European Classification System described by ATEX Directive
• American Classification System described by National Electrical Code (NEC)
Area Guide
Hazardous
atmosphere
Table 3: Equipment group and category classification according to ATEX 2014/34/EU
3.2 Classification at a Glance
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Equipment
Group I,
(e.g. mines)
Category of
equipment
M2
Table 2: Comparison of ATEX, NEC/CEC and IECEx
12
ATEX Group Classifications
Group I equipment applies to equipment used in underground operations such as mines. Group II equipment applies to surface-processing industries. The petrochemical, chemical, pharmaceutical as well as food industries are
typical processing industries. The equipment groups are further sub-divided into categories as shown in Table 3.
Group I is divided into categories M1 and M2. Group II is divided into equipment categories 1, 2 and 3.
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ATEX 1999/92/EC Zone Classification
According to the ATEX 1999/92/EC Directive, Equipment Group II, which is intended to be used in hazardous areas, is divided into three zones for gases and three zones for dust substances. The classification given to a particular zone, is made based on the frequency and duration of the occurrence of the explosive atmosphere (Table 4).
This concept has been used successfully for many years for specification and selection of electrical equipment for
explosive gas and dust atmospheres. The zones 0, 1, and 2 are used to denote explosive atmospheres containing
gases and vapors. The zones 20, 21, and 22 are the zones containing explosive and flammable dusts.
Zone classification
Gas
Dust
Zone 0
An explosive atmosphere is present continuously or for long periods of time.
Zone 1
An explosive atmosphere is likely to occur occasionally during normal operation.
Zone 2
An explosive atmosphere is likely to occur infrequently or for short periods of time.
Zone 20
An explosive atmosphere is present continuously or for long periods of time.
Zone 21
An explosive atmosphere is likely to occur occasionally during normal operation.
Zone 22
An explosive atmosphere is likely to occur infrequently or for short periods of time.
Table 4: ATEX Zone classifiication
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Hazardous Area Classification
Gas and Dust Substance Group
Up to this point, explosion-proof equipment has been divided into Equipment Group I (underground, for example working mines) and equipment Group II (surface equipment, for areas at risk of explosion excluding working
mines). Equipment Group II, which is the focus of this guide, draws a further distinction by differentiating between
areas at risk due to gases, vapors, mists and dust. The information in the table below shows the relationships between the gas/dust explosion group and the protection type required in each case (Table 5).
Explosion group
Type of group
Characteristics of group
Gas explosion group
IIC
Easily ignitable (e.g. hydrogen, acetylene)
IIB
Ignitable (e.g. coalgas, ethylene, ethylene glycol)
IIA
Less ignitable (e.g. acetone, benzene, toluene)
IIIC
Conductive dusts (resistivity ≤103Ωm)
IIIB
Non-conductive dusts (resistivity >103Ωm)
IIIA
Flammable fibers (length >500µm)
Dust explosion group
Table 5: ATEX substance group classification
Div 1
Zone 1
Div 2
Zone 2
Div 1
Zone 0
Div 1, Zone 0 or 1
Figure 6: Zone classification according to IEC EN 60079-10 -1 (Gas) and ATEX 1999/92/EC, respectively NEC500
ATEX 2014/34/EU Equipment Categories & IECEx Equipment Protection Level
Equipment to be used must correspond to the assigned equipment categories and equipment protection levels
(EPL) as prescribed by the zone classification. These equipment requirements are categorized in relation to the
European equipment categories based on EU Directive 2014/34/EU; the EPL, introduced by the IEC, applies on an
international level. Both classifications can be used when marking equipment. This classification tells us about the
probability of ignition, considering potentially explosive gas and dust atmospheres. Table 6 presents the definitions of equipment categories and equipment protection levels (EPL) and their differences. The definition of EPL is
usually used in the IECEx marking system (see chapter 4).
ATEX 2014/34/EU (formerly ATEX 95) - equipment
category
Classification of equipment for use in surface areas at
risk of explosion within Europe divided into three equipment categories for areas at risk of explosion due to gas
(G) or for areas with flammable dust (D)
IECEx (IEC 60079-0) - equipment protection level
Category 1: 1G or 1D
Very high level of safety. Safe even when rare equipment
faults occur. Two independent explosion protection measures, even safe when two faults occur independently of
one another.
Category 2: 2G or 2D
High level of safety. Safe even in the case of equipment
faults, which occur frequently or which are usually to be
expected. Even safe when a fault occurs.
Category 3: 3G or 3D
Normal level of safety. Safe during normal operation
EPL Ga or Da
Equipment with "very high" protection level for use in areas
at risk of explosion where there is no ignition risk during
normal operation, or in the case of predictable or rare faults/
malfunctions
EPL Gb or Db
Equipment with "high" protection level for use in areas at risk
of explosion where there is no ignition risk during normal operation, or in the case of predictable faults/malfunctions.
EPL Gc or Dc
Equipment with "extended" protection level for use in areas at
risk of explosion where there is no ignition risk during normal
operation, and which has some additional protective measures, which ensure that there is no ignition risk in the case
of predictable equipment faults.
Classification of equipment for use in surface areas at risk of
explosion divided into three protection levels for areas at risk of
explosion due to gas (G) or for areas with flammable dust (D)
Table 6: Definition of ATEX 2014/34/EU equipment categories and IECEx equipment protection level
Comparison of ATEX 2014/34/EU & ATEX 1999/92/EC
Table 7 shows the comparison between two directives for equipment manufacturers and for users. There is a direct link between the two directives in that the three equipment categories specified in ATEX 2014/34/EU correspond to the three Zones used in ATEX 1999/92/EC for the classification of hazardous areas. Therefore, in Zone
2/22, equipment category 3 may be used, whereas in Zone 0/20 (explosive atmosphere can be present continuously), equipment category 1 must be used.
Manufacturer requirements ATEX 2014/34/EU
Definition of area of use of equipment, specification of equipment group / category
Equipment Category 1
Equipment Category 2
Equipment Category 3
Comply with essential safety and health requirements
or relevant standards
Carry out a risk / ignition-hazard assessment of equipment
User requirements ATEX 1999/92/EC
Risk assessment of hazardous area in working places,
employee safety
Zone 0/20
Zone 1/21
Zone 2/22
Comply with installation and maintenance requirements
Prepare conformity documentation
Carry out a risk assessment of the work place, duty of coordination
Prepare an explosion document
Appropriate quality control
Regular updates
Table 7: Comparison ATEX 2014/34/EU and ATEX 1999/92/EC
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Hazardous Area Classification
North American NEC Class / Division Classification
In the USA, all regulations related to manufacturing facilities at risk are found in the National Electrical Code (NEC)
Handbook. Articles 500, 501, 502, 503 and 505, and 506 define the requirements for classification of hazardous
locations into Classes, Groups, Divisions and Zones. According to NEC 500, hazardous locations are divided into
Classes I, II, and III depending on the type of material present. Table 8 shows the classification of the hazardous
locations according to NEC 500 - 505 articles.
Substance
Gases /
vapors
Substance
Area Classification
Class
NEC500
NEC505
Class I
(NEC 501)
Division
1
Zone 1
Division
2
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Area Guide
Zone 0
Zone 2
Sub-
Substance
Area Classification
stance
Class
NEC500
NEC505/506
Dusts
Class II
(NEC 502)
Division
1
Zone 20
Division
2
Zone 22
Division
1
Zone 20 or
21 Group IIIA
Division
2
Zone 22
Group IIIA
Zone 21
Hazardous location characteristics
Explosion hazard present continuously or occasionally under normal operating conditions
(1) In which ignitible concentrations of flammable gases, flammable liquid–produced vapors, or combustible liquid–produced
vapors can exist under normal operating
conditions, or
(2) In which ignitible concentrations of such flammable gases,
flammable liquid–produced vapors, or combustible liquids above
their flash points may exist frequently because of repair or maintenance operations or because of leakage, or
(3) In which breakdown or faulty operation of equipment or processes might release ignitible concentrations of flammable gases,
flammable liquid–produced vapors, or combustible liquid–produced vapors and might also cause simultaneous failure of electrical equipment in such a way as to directly cause the electrical
equipment to become a source of ignition
Ignitable concentrations of flammable gases or vapors are not normally present, but could be present in the case of a fault
(1) In which volatile flammable gases, flammable liquid– produced
vapors, or combustible liquid–produced vapors are handled, processed, or used, but in which the liquids, vapors, or gases will
normally be confined within closed containers or closed systems
from which they can escape only in case of accidental rupture or
breakdown of such containers or systems or in case of abnormal
operation of equipment, or
(2) In which ignitible concentrations of flammable gases, flammable liquid–produced vapors, or combustible liquid–produced
vapors are normally prevented by positive mechanical ventilation
and which might become hazardous through failure or abnormal
operation of the ventilating equipment, or
(3) That is adjacent to a Class I, Division 1 location, and to which
ignitible concentrations of flammable gases, flammable liquid–produced vapors, or combustible liquid– produced vapors above their
flash points might occasionally be communicated unless such
communication is prevented by adequate positive-pressure ventilation from a source of clean air and effective safeguards against
ventilation failure are provided.
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Fibers
Class III
(NEC 503)
Hazardous location characteristics
Combustible dusts are present in quantities sufficient to produce
explosive and ignitable mixtures
(1) In which combustible dust is in the air under normal operating
conditions in quantities sufficient to produce explosive or ignitible
mixtures, or
(2) Where mechanical failure or abnormal operation of machinery
or equipment might cause such explosive or ignitible mixtures to
be produced, and might also provide a source of ignition through
simultaneous failure of electrical equipment, through operation of
protection devices, or from other causes, or
(3) In which Group E combustible dusts may be present in quantities sufficient to be hazardous.
Combustible dust due to abnormal operations may be present in
quantities sufficient to produce explosive or ignitable mixtures (1) In
which combustible dust due to abnormal operations may be present in the air in quantities sufficient to produce explosive or ignitible
mixtures; or
(2) Where combustible dust accumulations are present but are normally insufficient to interfere with the normal operation of electrical
equipment or other apparatus, but could as a result of infrequent
malfunctioning of handling or processing equipment become suspended in the air; or
(3) In which combustible dust accumulations on, in, or in the vicinity of the electrical equipment could be sufficient to interfere with
the safe dissipation of heat from electrical equipment, or could be
ignitible by abnormal operation or failure of electrical equipment.
Easily ignitable fibers / flyings are handled or manufactured
A Class III, Division 1 location is a location in which easily ignitible
fibers/flyings are handled, manufactured, or used.
Easily ignitable fibers / flyings are stored or handled
A Class III, Division 2 location is a location in which easily ignitible
fibers/flyings are stored or handled other than in the process of
manufacture.
Table 8: Hazardous locations classification system according to NEC 500 – 506
NEC Material Group Classification
Each class is also divided into the material groups A, B, C, D, E, F and G. Article 500 defines the classification of
the substance classes into substance groups according to their properties and ignitability. Table 9 presents this
classification. The basis for definition of the substance group according to Article 500 is the degree of risk. In this
case, it is a factor of the maximum experimental safety gap or minimum igniting current.
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Hazardous Area Classification
Substance
Class I
Material Group
(NEC500)
A
Material Group
(NEC 505/506)
IIC
T1
Acetylene
T2
T3
T4
Acetylaldehyde
Ethyl ether
Hydrogen
I
Not relevant for weighing equipment
IIB
Ethylene
IIA
D
IIA
Propane
E
IIIC
Methanol
n-Amyl acetate
n-Butanol
Acetic Acid
Ethyl acetate
F
IIIB
Combustible metal dust
Conductive dusts
Carbonaceous dust
Diesel fuel
Aviation fuel
Domestic fuel
Oils
n-Hexane
n-Butane
IIB
Acetone
Ethane
Ammonia
Benzene (pure)
Carbon monoxide
Methane
Propane
Toluene
Ethylene
IIC
Hydrogen
Acetylene
Hydrogen Disulfide
G
Class III
Temperature code
C
B
Class II
Substance
Group
Substance
N/A
Combustible dusts not in Group E of F
IIIA
Fibers and flyings
Table 9: NEC 500 and NEC505/506 substance group classification
Ignition Temperature and Temperature Code
Flammable gases and vapors are classified into temperature codes according to their autoignition temperature
(AIT). The ignition temperature of a flammable gas is the lowest temperature of a surface required to ignite a combustible vapor, gas or dust cloud. The equipment manufacturers are obliged to classify the electrical equipment in
the temperature codes depending on the flammable material used depending on the maximum surface temperature the equipment generates. Temperature codes from T1 to T6 are defined for flammable gases and vapors as a
means of ensuring safety and protection. In practice, this means that the maximum surface temperature of a piece
of equipment must always be lower than the ignition temperature of the gas/air or vapor/air mixture. Increasing
temperature code numbers correspond to lower surface temperature of the equipment.
Ignition temperature
gases and vapors
>450°C
Maximum surface
temperature
450°C
NEC 500
IEC EN / NEC 505
T1
T1
>300
300°C
T2
T2
>280°C
280°C
T2A
-
>260°C
260°C
T2B
-
>230°C
230°C
T2C
-
>215°C
215°C
T2D
-
>200°C
200°C
T3
T3
>180°C
180°C
T3A
-
>165°C
165°C
T3B
-
>160°C
160°C
T3C
-
>135°C
135°C
T4
T4
>120°C
120°C
T4A
-
>100°C
100°C
T5
T5
>85°C
85°C
T6
T6
T5
T6
Carbondisulphid
Table 11. Temperature codes vs. substance group system
What type of hazardous area classifications
and protection does your company use?
NEC CEC standards
ATEX /IECEx standards
What type of hazardous substances
may be present?
Class I
Gas and
vapors
Class II
Dust
Class III
Fibers and
flyings
How often may hazardous substances
be present in the atmosphere?
What type of hazardous
substances may be present?
Zone 0, 1, 2
Gas and vapors
Zone 20, 21, 22
Dust
How often may hazardous substances
be present in the atmosphere?
Are likely to exist
under normal
operating conditions
Are NOT likely to exist
under normal
operating conditions
Continuously present
for long periods of
time under normal
operating conditions
Class I
Gas and vapors
Class, II, III
Gas and vapors
Zone 0: Gas & vapors
Zone 20: Dust
Are likely to exist
under normal
operating conditions
Are NOT likely to
exist under normal
operating conditions
Table 10: Ignition temperature classes
American NEC and Canadian CEC Zone Classification
Zone 1: Gas & vapors Zone 2: Gas & vapors
Zone 21: Dust
Zone 22: Dust
Figure 7: Comparison NEC and ATEX /IECEx class and zone classification system
The NEC allows hazardous locations to be classified using the Class/Division scheme per Articles 501-504 or using the American Zone scheme of Articles 505 and 506. The use of American zones is not as popular as the legacy Class/Div. classification scheme, but it is becoming more popular with global companies looking for uniformity between international jurisdictions. Canada requires all new hazardous location installations be classified using the zone
scheme. Class / Division schemes are only for existing and expansions of existing Class / Division areas.
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Ignition Protection Methods
4. Ignition Protection Methods
The basic safety concept is to eliminate the simultaneous existence of possible ignition sources. The method of
equipment protection will likely depend on the degree of safety needed for the type of hazardous location. Besides
the degree of safety required for the classified area, other considerations must be made, such as the size of the
equipment, its normal function, power requirements, installation costs and flexibility of the protection method for
maintenance. Table 12 shows an overview of the standardized types of protection. It describes the basic principle
of each protection method as well as the applicable standard and the classified area. The protection methods are
standardized and the standards vary in different countries. However, the principles of protection are the same regardless of the country. When it comes to designing and developing weighing equipment for hazardous areas, the
intrinsic safety and flameproof methods are mainly applied.
Types of Protection (Divisions)
Types of Protection (Zones)
Electrical Equipment for Flammable Gases, Vapors and Mists
Electrical Equipment
Type of
Protection
Concept
General
Requirement
General requirements
for all
Ex equipment
Flameproof
Contain explosion;
extinguish the flame
Purge and
Pressurization
Prevent ingress of explosive atmosphere; limit
surface temperature
Powder
Filled
Contain explosion;
extinguish the flame
Liquid
Immersion
Prevent ingress of explosive atmosphere; limit
surface temperature
Increased
Safety
No arcs, sparks or hot
surfaces
Intrinsic
Safety
Limit energy of sparks and
surface temperature
Type of
Protection "n"
Non-sparking devices
Sparking devices
Restricted breathing
Encapsulation
Ex
Code
Zone
–
–
da
0
db
1
dc
2
pxb
1, 21
pyc
1, 21
pzc
2
q
1
ob
1
oc
2
eb
1
ec
2
ia
0, 20
ib
1, 21
ic
2, 22
nA
2
nC
2
nR
2
Prevent ingress of explosive atmosphere; limit
surface temperature
ma
0, 20
mb
1, 21
mc
2, 22
Optical
Radiation
Protected by shutdown,
enclosure or inherently
safe
op pr
1, 21
op is
0, 20
op sh
0, 20
Protection by
Enclosure
Prevent ingress of explosive atmosphere; limit
surface temperature
ta
20
tb
21
tc
22
Standards
Standards
(ATEX/IECEx) (NEC/CEC)
Type of
Protection
EN/IEC
60079-0
ANSI 60079-0
C22.2 No. 60079-0
Explosionproof
Contain explosion and
extinguish the
flame
EN/IEC
60079-1
ANSI 60079-1
C22.2 No.60079-1
Purge and
Pressurization
EN/IEC
60079-2
ISA 60079-2
C22.2 No.60079-2
Keep the
explosive
atmosphere
out
EN/IEC
60079-5
ANSI 60079-5
C22.2 No. 60079-5
EN/IEC
60079-6
ANSI 60079-6
C22.2 No.60079-6
EN/IEC
60079-7
ANSI 60079-7
C22.2 No.60079-7
EN/IEC
60079-11
EN/IEC
60079-15
–
ANSI 60079-15
C22.2 No.60079-15
Intrinsic
Safety
Non-incendive
Type of
Protection
Dust Ignition
Protection
Fiber and
Flying
Protection
ANSI 60079-18
C22.2 No.60079-18
EN/IEC
60079-28
ANSI 60079-28
C22.2 No.60079-28
EN/IEC
60079-31
ANSI 60079-31
C22.2 No.60079-31
Class I Division
Standards
1
C22.2 No. 30
UL 1203
FM 3615
2
1, 2
ISA 60079-2
C22.2 No. 60079-2
NFPA 496 (FM
3620)
1, 2
2
Limit energy
of sparks and
surface
temperature
1
FM 3610
UL 913
2
UL 12.12.01
C22.2 No. 213
FM 3611
Electrical Equipment for Combustible Dusts
Non-Incendive
EN/IEC
60079-18
Concept
Intrinsic Safety
Pressurization
Concept
Class II
Division
ClassIII
Division
1, 2
1, 2
C22.2 No. 25
FM 3616
UL 1203
–
1, 2
C22.2 No. 25
FM 3616
UL 1203
2
1, 2
ISA 12.12.01 C22.2
No. 213
FM 3611
1, 2
1, 2
ISA 60079-11
C22.2 No. 60079-11
FM 3610
UL 913
1, 2
1, 2
1, 2
1, 2
2
1, 2
Keep combustible dust out
Limit energy
of sparks and
surface temperature
Keep the
explosive atmosphere out
Standards
4.1 Intrinsic Safety Classification and Protection Levels
Intrinsic safety offers three classification levels, "ia," "ib" or "ic," which are based on the safety level and number of faults possible. Each classification attempts to balance the probability of an explosive atmosphere being
present against the probability of an ignition occurring. The level of protection "ia" is a prerequisite for Category
1 equipment and is suitable for use in Zone 0/Division 1. The level of protection “ib” for Category 2 equipment is
suitable for use in Zone 1. The level of protection "ic" for Category 3 is suitable for use in Zone 2/Division 2. The
classifications ensure that the equipment is suitable for an appropriate hazardous application. For example, having equipment classified as "Ex ib" means that the equipment is designed containing an intrinsically safe circuit
and can be installed in the certified hazardous areas Zone 1. Moreover, the "ib" classification indicates that one
fault is possible.
ai
ib
ic
Hazardous area
Zone 0,1,2 / Division 1
Zone 1,2 / Division 2
Zone 2 / Division 2
Faults possible
2
1
Normal operation
Table 13: Intrinsically safe protection levels
NFPA 496
(FM3620)
Table 12. Protection methods and related standards (current revision available on standardization bodies websites)
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Ignition Protection Methods
Terms to Know
Intrinsically
Safe Circuit
A circuit in which no spark
and no thermal effect can
cause the ignition of a potentially explosive atmosphere.
Minimum Ignition Energy
The minimum ignition energy
is the smallest possible electrical energy required to ignite
a combustible vapor, gas or
dust cloud. The minimum ignition energy is measured in
µJoules.
Intrinsically Safe
Electrical Equipment
All circuits of electrical equipment are intrinsically safe. The
Voltage and the current in the
intrinsically safe circuit are
low enough such that a short
– circuit, interruption or short
– circuit to ground will not ignite the potentially ex-plosive
atmosphere. Intrinsically safe
electrical equipment is suitable for operation direct in
hazardous area Zone 0, 1, 2 /
Zone 20, 21, 22 and Division
1, 2.Typical marking: Ex ib IIC
Flame Proof – Ex d
The flameproof protection method is based on the explosion-containment concept and is in accordance with IEC EN60079-1 classified as
"Ex d." This concept relies on equipment and wiring enclosures to prevent an internal ignition from escaping to the surrounding atmosphere.
In other words, the explosion is allowed to take place, but it must remain
confined in the enclosure that is designed to resist the excess pressure
the internal explosion causes. The theory supporting this method is that
the resultant gas jet coming from the enclosure is cooled rapidly through
the enclosure’s heat conduction and the expansion of the gas. The hot
gas is then diluted in the colder external atmosphere. That is only possible if the enclosure openings or interstices have sufficiently small and
well-controlled dimensions. A flameproof system is generally considered somewhat simpler to design than an intrinsically safe system, as
it doesn’t require completely new equipment design. However, it is generally more expensive to install because of the high cost of running field
wiring inside a conduit, which must be sealed between the safe and hazardous areas. It is also often physically larger and much heavier than
an intrinsically safe solution. Flameproof equipment is also more difficult and time-consuming to maintain because either the area must be
known to be non-hazardous, or the equipment must have the energy
drained before covers can be removed. Hot permits are required to perform maintenance work on these systems. Further, when covers are reinstalled, extra care must be taken that fasteners are precisely torqued
to specified values.
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Associated Electrical
Equipment
At least one circuit of the associated electrical equipment
is intrinsically safe. Sensors
connected to the intrinsically
safe circuit can be in the hazardous area. How-ever the associated electrical equipment
must not be in the hazardous
area without further protection
types. The type of protection is
placed in the square brackets.
Typical marking: [Ex ib] IIC.
Ex d
Figure 8: Explosion proof enclosure
Increased Safety – Ex e
The principal of increased safety is to ensure reliable prevention of unacceptably high temperatures and sparks or electrical arcs. The equipment usually has a maximum voltage,
which is rating to 11 kV. The basic design requirements to construction and protection principles are described in the IEC 60079-7 standard. An enclosure must be constructed to withstand mechanical impact and pro-vide a specified degree of ingress protection (IP Rating).
Two fundamental requirements of increased safety protection are that the equipment shall
be protected to IP54 minimum for gas / vapor and IP6X for dust hazards. This method of
protection can be used in both Zone 1 and Zone 2/Division 2 hazardous areas. Therefore,
it is also often preferred to flameproof protection method “Ex d” due to the need for reduced
levels of maintenance and inspection. Another major consideration is that increased safety
equipment is often constructed from light-weight materials, which often leads to lower cost.
Non-Sparking – Ex n
Non–sparking protection is regulated by the IEC 60079-15 European standard. The nonsparking enclosure is permitted for use only in Zone 2/Division 2 hazardous areas. It is
considered incendive so that it does not generate arcs or sparks or dangerous tem-peratures in normal operation. Internal component temperatures must be controlled and wiring
connections must be selected with “non-sparking” in mind. The concept has simi-larities
with the “increased safety – Ex e” philosophy but it is suitable only for Zone 2/Di-vision 2
hazardous areas. The equipment approved to “non-sparking” is not designed to withstand
explosion and will usually employ a light enclosure metallic construction with a high ingress protection level.
Encapsulation – Ex m
IEC 60079-18 gives the specific requirements for the construction, testing and marking
of electrical equipment, parts of electrical equipment and Ex components with the type of
protection encapsulation "m" intended for use in explosive gas atmospheres or explosive
dust atmospheres. This part applies only for encapsulated electrical equipment, encapsulated parts of electrical equipment and encapsulat ed Ex components, always referred to
as "m" equipment.
Pressurized – Ex p
Pressurized or purged equipment, which is type “p,” relies on a combination of a positive
static pressure applied inside of enclosure and a continuous flow of air or inert gas to expel
any explosive mixture that may have entered inside. The system reliesa on purging schedules and monitoring systems to ensure the reliability and effectiveness of the overall protection. The IEC EN 60079-2 standard describes the requirements and design considerations.
Figure 9. Increased safety enclosure
Ex n
Figure 10. Non-sparking enclosure
Ex m
Figure 11: Encapsulation enclosure
Ex p
Figure 12: Pressurized enclosure
Oil Immersion – Ex o
Permitted only in Zone 2/Division 2 areas where the likelihood of a flammable atmosphere
is remote. The type “o” equipment or enclosures use the concept where the sparking components are inserted in oil and controlled venting is also a feature.
Ex o
Figure 13: Oil immersion enclosure
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Electrical Equipment Marking
4.2 Marking of Electrical Equipment
Solutions to automate the truck scale enable faster processing time,
improve driver safety and help to reduce emissions from idling trucks.
We have solutions from weigh-in-motion scales to transaction management software. Streamlining processes now can ensure readiness to
meet increased demand while saving time and money in the process.
ATEX CE Marking
Marking is mandatory and must be placed on equipment before it
is assigned to be distributed in the market or put into service. The
intention is to facilitate the free movement of equipment within
the European Union by signifying that essential health and safety
standards have been met. It is a declaration that the product was
produced in conformity with all applicable provisions and requirements of the Directive 2014/34/EU and that the product has been
the subject of the appropriate conformity assessment procedures.
The ATEX 2014/34/EU Directive specifies the minimum requirements for marking that must be implemented. Further information
and requirements to marking of equipment for use in hazardous
areas are given in the European standards. The IEC EN 60079-0
standard defines the requirements on electrical equipment for use
in explosive atmospheres.
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Marking
Description
II (2)
[Ex ib]
The brackets show that the product must be installed in the safe area but it
can be connected to the equipment installed in the hazardous area
The brackets indicate that the device must be installed in the safe area
nA
Non-sparking equipment; does not generate a spark during normal operation
ic
Energy limited, intrinsically safe during normal operation
Table 14: Additional markings for electrical equipment (ATEX 2014/34/EU)
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Electrical Equipment Marking
NEC Marking
Essentially, the NEC markings contain similar sets of information as ATEX markings; however, it is important to
note the differences and ensure that equipment in use in any hazardous environment conforms to all appropriate
safety standards. The equipment classified for Division area are marked in accordance to the area that it is classified.
Class
I, II, III
Div. 1
Group
A-F
T4
Ta
Temperature range
(if outside of default range in standard)
Temperature Code (Chapter 3.4)
Hazardous substance group (Chapter 3.3.1)
Division classification (Chapter 3.3)
Equipment class (Chaper 3.3)
Equipment approved by notified body
Figure 16: Example of electrical equipment marking according to NEC500
Alternative marking is used by classification of areas according to Zoning methodology. An example of the. equipment marking to NEC505 is shown in Figure 17. The equipment within the Zone method is marked in accordance
with the type of protection used similar to ATEX methodology. It is then the responsibility of the user to apply the
proper method of protection in each Zone
ATEX CE Marking
II
2G
Ex
ib
IIC T4 Gb
Equipment protection level
Class I
Temperature Code
Ta
Type of ignition protection method (Chapter 4)
Equipment category and hazardous atmosphere - gas
Symbol of explosion protection, American Standards
Equipment group
Hazardous area classification (Chapter 3.3)
Indication that this is an explosion-protected device
Equipment class (Chapter 3.3)
Figure 14: Typical electrical equipment marking (gas) according to ATEX 2014/34/EU
IIIC
IIB T4
Hazardous substance group (Chapter 3.2)
'Ex' indicates compliance with explosion proof standards
ib
ib
Temperature Code (Chapter 3.4)
Type of ignition protection method
2D Ex
AEx
Ambient temperature
Hazardous substance group
II
Zone 1
Equipment approved by notified body
Figure 17: Example of electrical equipment marking according to NEC505
T55°C Db - Equipment protection level
Maximum surface temperature
Material Groups
Type of ignition protection method
'Ex' indicates compliance with explosion proof standards
Equipment category and hazardous atmosphere - gas
Equipment group
Indication that this is an explosion-protected device
Figure 15: Typical electrical equipment marking (dust) according to ATEX 2014/34/EU
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Installation and Maintenance
4.3 Electrical Equipment – Installation and Maintenance
Hazardous Area Inspections
When electrical equipment is installed in a hazardous environment, there are different inspection stages that must
occur to ensure equipment does not become the ignition source in the “Triangle of Fire.” The Triangle of Fire refers
to the three elements required for a fire to ignite: flammable substance ignition source and oxidizing agent (chapter 1). The first stage of inspection happens prior to equipment being brought into service. An initial inspection is
performed to validate that equipment meets the requirements of the hazardous area. Those requirements are defined by the customer in accordance with specifications defined by the equipment manufacturer. Once hazardousarea equipment is installed, a periodic inspection schedule should be set to ensure the equipment still meets the
initial inspection requirements later.
Types of Inspection
According to IEC 60079-17:2013 [ed5], there are five different inspection methods that ensure the continued reliability
of plant equipment used in a hazardous environment: initial, visual, close, detailed and continuous supervision.
Initial Inspection
The initial inspection provides a thorough assessment that the selected type of protection and its installation are
appropriate. This inspection is validated using the control diagram produced by the equipment manufacturer. This
validation should be performed at installation, prior to equipment use.
Deterioration of Equipment
When determining periodic inspection intervals, additional consideration should be taken for the environment in
which the equipment is used and potential equipment deterioration or degradation over time.
Intrinsically safe equipment inspection schedule
A
Equipment
Detailed
Close
Visual
1
P
P
P
P
P
3
Circuit and/or equipment documentation is appropriate to the
EPL/zone requirements of the location
Equipment installed is that specified in the documentation –
fixed equipment only
Circuit and/or equipment category and group correct
P
P
4
IP rating of equipment is appropriate to the Group III material present
P
P
5
Equipment temperature class is correct
P
P
6
Ambient temperature range of the apparatus is correct for the installation
P
P
7
Service temperature range of the apparatus is correct for the installation
P
P
8
Installation is clearly labeled
P
P
9
Enclosure, glass parts and glass-to-metal sealing gaskets and/or compounds are
satisfactory
Cable glands and blanking elements are the correct type, complete and tight –
physical check– visual check
There are no unauthorized modifications
P
2
10
11
Visual Inspection (periodic)
The least invasive assessment of hazardous-area equipment is the visual inspection. This inspection is performed while the equipment is energized and does not require the equipment to be isolated or any special tools. It
evaluates potential defects that are obvious by sight.
Close Inspection (periodic)
Building upon the visual inspection, the close inspection is also performed while the equipment is energized.
However, the close inspection reveals defects that would not be apparent by sight. This inspection requires the
use of additional tools and, for example, identifies loose bolts. A close inspection is appropriate whenever regular
maintenance is performed.
Factors to Determine Periodic Inspection Intervals
Accurately determining an appropriate periodic inspection interval is a complex issue. The type of installation application helps to determine the periodic inspection interval, along with manufacturer’s guidance and factors impacting equipment deterioration. There are two main types of installation applications hazardous-area equipment
falls under: fixed installations and moveable (portable) equipment.
Fixed Installations
For intrinsically safe equipment installed in a fixed location, the maximum interval between periodic inspections
should not exceed three years without seeking expert advice. Once an interval is determined, additional periodic
inspections should be performed to support or modify the proposed interval.
Moveable (Portable) Equipment
Moveable or portable intrinsically safe equipment is more prone to damage or misuse than fixed installations and
therefore the interval between periodic close inspections should be a maximum of every 12 months. Enclosures
that are frequently opened, such as battery housings, should have a detailed inspection at least every six months.
Additionally, all equipment should be visually inspected by a trained operator before use to ensure that it is not
obviously damaged.
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P
P
P
P
P
P
P
P
12
There is no evidence of unauthorized modifications
13
P
14
Diode safety barriers, galvanic isolators, relays and other energy limiting devices
are of the approved type, installed in accordance with the certification requirements
and securely earthed where required
Condition of enclosure gaskets is satisfactory
15
Electrical connections are tight
P
16
Printed circuit boards are clean and undamaged
P
17
The maximum voltage Um of the associated apparatus is not exceeded
P
P
Detailed
Close
Visual
P
B
Installation
1
Cables are installed in accordance with the documentation
P
2
Cable screens are earthed in accordance with the documentation
P
3
There is no obvious damage to cables
P
P
P
4
Sealing of trunking, ducts, pipes and/or conduits is satisfactory
P
P
P
5
Point-to-point connections are all correct (initial inspection only)
P
6
P
7
Earth continuity is satisfactory (e.g. connections are tight, conductors are of sufficient cross-section) for non-galvanically isolated circuits
Earth connections maintain the integrity of the type of protection
8
Intrinsically safe circuit earthing is satisfactory
P
9
Insulation resistance is satisfactory
P
10
P
12
Separation is maintained between intrinsically safe and non-intrinsically safe circuits in common distribution boxes or relay cubicles
Short-circuit protection of the power supply is in accordance with the documentation
Specific conditions of use (if applicable) are complied with
13
Cables not in use are correctly terminated
P
Detailed
Close
Visual
P
P
P
P
P
P
11
C
Environment
1
Equipment is adequately protected against corrosion, weather, vibration and other
adverse factors
No undue external accumulation of dust and dirt
2
P
P
P
Table 15: Inspection table for intrinsically safe installations
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Weighing in Hazardous Areas
5. Weighing in Hazardous Areas
Weighing is one of the most basic and important process variables in a vast majority of manufacturing processes.
However, it can be one of the most challenging parameters to control. Accurate and consistent filling, dosing and
batching reduce variability in the product, which ensures consistently high quality. To achieve reliable quality and
reproducible results requires efficient capture and communication of weighing data to the existing plant network,
which can be challenging due to hazardous-area requirements. To prevent any ignition and provide safe operation
of electrical weighing systems in hazardous areas, one possibility is limiting energy to safe levels. To achieve low
energy and prevent ignition, the main components of weighing systems, such as load cells, junction boxes and
weighing indicators are designed for intrinsic safety. Intrinsically safe technology prevents explosions by ensuring
that the energy in intrinsically safe circuits is well below the energy required to initiate an explosion. Intrinsically
safe electrical equipment and wiring is designed and certified mostly for use in Zone 1/Division 1 hazardous areas if they are approved for the location. Intrinsically safe circuits often combine elements with the various safety
levels. Depending on functionality and the classification of the safety level, the circuit elements can be applied either in hazardous or non-hazardous areas.
5.1 Basic System
In hazardous production areas, there are many processes which require simple standalone weighing applications.
Filling tanks, drums or bags with hazardous powders or liquids is one example.
A simple weighing system usually consists of strain gauge (analog) or digital weighing platforms or load cells
controlled and monitored directly through a PC installed in a safe area. The weighing signal is interpreted by the
hazardous-area indicator and transferred to the safe area computer or printer. As all components of the weighing
system are intrinsically safe, the weighing system is powered by an intrinsically safe power supply. Communicating the weighing signal from the Zone 1/21, Division 1 to the safe area requires energy-limiting devices referred to
as intrinsically safe barriers. These are barriers installed in the safe area that interface with the communication device to prevent excess energy from a fault occurring on the safe side from crossing over to the hazardous area.
Entity parameters of intrinsically safe equipment
This determines if the peripheral device is safe for connection to the intrinsically safe equipment. Entity parameters
are usually found on the control drawing of the intrinsically safe device supplied by the manufacturer or on the
Examination Certificate (see Table 16). There are some differences in the abbreviations of US Class/Division and
Europe Zone classification. The US Class / Division system uses the abbreviation of entity parameters like VOC,
ISC, and LA. In Europe, the safety parameters are referred to as VO, IO, CO, etc.
Intrinsically safe weighing terminal
(Intrinsically safe apparatus)
Intrinsically safe barrier or communication
module (associated apparatus)
Open circuit voltage (Voc)
≤
Vmax
Short circuit current (Isc)
≤
Imax
Allowed capacitance (Ca)
≥
Ci + Ccable
Allowed inductance (La)
≥
Li + Lcable
Maximum Power (Po)
≤
Pi
Table 16: Entity parameters of intrinsically safe equipment and associated equipment
Benefits
Limitations
- Simple application through RS232 interface
- Cost efficiency
- Precise signal response
- Small barrier footprint
- Short signal distance (15 – 20 m max)
- Safety barrier requires securely implemented
earthing system.
Basic System Example
Hazardous Area
Non-Hazardous Area
Under normal operating conditions, intrinsically safe barriers have no arcing or heat-producing contacts, and if
specially marked, they can be installed in the Zone 2/22, Division 2. In fault conditions, the barriers limit voltage
and current to levels that are not sufficient to ignite the hazardous atmosphere
Weighing Terminal - IDNet
3 components of these barriers
PC connection
SICSPro Interface
Power Supply
COM 1 - Intr. Safe RS232
1 The Zener diode, which limits the voltage to a value referred to as an open circuit voltage (VOC)
Intr. safe RS232
2 A resistor, which limits the current to a specific value known as a short circuit current (ISC)
Serial RS232 Zener barrier
Digital platform IDNet
3 The fuse, which limits the maximum current that can flow through the diodes. When the
Power supply
current flows through diode, the fuse will blow. This interrupts the circuit and prevents the
All apparatuses require equipotential bonding.
All earthing points to the same location.
diode from failing. As a result, the excess voltage does not reach the hazardous area.
Figure 18: Direct RS232 communication in the safe area
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5.2 Advanced System
In the case of advanced data communication capabilities, such as communicating via Ethernet or programmable logic controller (PLC), the amount of transferred data can be increased. This requires a more powerful and sophisticated interface between the hazardous and safe area, as well as more powerful and sophisticated communication modules themselves. The intrinsically safe current loop interface performs best when large amounts of data
must be communicated from the hazardous to the safe area. Figure 19 shows an example of advanced system
set-up. Safe area communication is achieved by the intrinsically safe communication module, which has a functional principal that is based on a current loop interface. The current loop interface provides one or two full channels of bidirectional communication and is designed to use a copper-wire cable. High speed transmitters and receivers are used to increase data throughput. When combined with the communication module and its options,
this permits remote operation in the safe area with Ethernet and PLC interfaces at distances up to 300 meters
(1,000 ft.) from the intrinsically safe weighing indicator.
Benefits
Limitations
- Long signal distance (up to 300M)
- Enhanced high-speed data communication
- Larger barrier footprint
- Additional wiring
Process Control
Handling, filling, dosing, blending or batching of hazardous liquids or solids requires precise control. Small process changes can have a big impact on end-product quality. Variations in proportions, speed, flow, turbulence
and many other factors must be carefully and consistently controlled to produce the desired end-product with a
minimum of raw materials and energy. Process control through discrete internal Inputs/Outputs (I/Os) keeps the
weighing process running within specified limits and allows more precise target limits to be set to maximize profitability and ensure quality and safety.
Remote I/O technology can be a cost-efficient and flexible solution for data control in processing plants. However,
good management of this type of system is particularly critical for hazardous areas where explosion protection
measures for all system components are generally required
5.3 Active / Active Control
In the case of active/active control (Figure 20), both active inputs and active outputs are installed in the hazardous area. Signal inputs are powered internally by the weighing terminal and are designed to be used with simple
switches housed within the hazardous area. Signal outputs are also powered by the weighing terminal and provide 12V switching at 50mA total. These outputs are intended for use with extremely low power, intrinsically safe
solenoids or piezo fluid control valves.
Advanced System Example
Active / Active Control Example
Non-Hazardous Area
Hazardous Area
Non-Hazardous Area
Hazardous Area
Intrinsically safe
control valve
Up to 8 x active
outputs / 12VDC
max. 50mA total
ACM500 - CL
Weighing Indicator
Power Supply
Analog Interface
COM 4 -CL
Power Supply
COM 5 -CL
ACM500 - CL
Weighing Indicator
Power Supply
Power Supply
COM 4 -CL
Analog Interface
COM 5 -CL
COM 1 - RS232
Analog platform
COM 4 -CL
COM 5 -CL
ETHERNET
COM 4 -CL
COM 2 - RS232
COM 5 -CL
COM 3 - RS232/422/485
ETHERNET
COM 2 - RS232
COM 3 - RS232/422/485
I/O
RS232
Printer
RS232
RS232
PC connection
Switch
Remote terminal
(Tare, Print, Zero, Clear)
RS232
Power supply
Junction box
PC connection
Remote display
Power supply
All apparatuses require equipotential bonding.
All earthing points to the same location.
All apparatuses require equipotential bonding.
All earthing points to the same location.
Figure 19: Current loop communication
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Figure 20: Active / active control with internal discrete I/O module
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5.4 Active / Passive Control
5.5 Passive Input / Passive Output Control
In the active input/passive output set-up (Figure 21), active input controls are installed in the hazardous area. The
passive outputs are usually high-voltage solenoids, which are not approved for hazardous-location use and must
therefore be considered for installation in the safe area. Passive inputs allow connection of an external intrinsically
safe voltage supply to power switches or other simple devices to trigger the input.
Passive inputs can be used when the input signal is coming from the safe area or some other type of active device, such as a PLC. The protective switch amplifier provides controls passive outputs and high voltage solenoids
or other controls in the safe area. The external switch amplifier receives a high voltage and converts it into an intrinsically safe voltage to send to the weighing indicator in the hazardous area. Passive outputs remain isolated
while signaling to switch the higher AC or DC voltage through the switch amplifier.
Both active inputs and passive outputs are powered by the intrinsically safe weighing indicator, which is installed
in the hazardous area. The indicator, in turn, is powered through the external intrinsically safe power supply, which
meets hazardous-area requirements.
Passive Input / Passive Output Control Example
Active / Passive Control Example
Non-Hazardous Area
Hazardous Area
Non-Hazardous Area
Hazardous Area
ACM500 - CL
ACM500 - CL
Weighing Indicator
Power Supply
Weighing Indicator
Power Supply
Power Supply
COM 4 -CL
Power Supply
COM 4 -CL
Analog Interface
COM 5 -CL
Analog Interface
COM 5 -CL
COM 1 - RS232
COM 4 -CL
COM 5 -CL
Simple switch
Up to 8 active inputs
(3 Standard + 5 Optional)
active inputs
The simple switch can be protected by the galvanically isolated type of barrier. The barrier provides complete isolation and limits the high voltage coming from the switch, converting it to intrinsically safe voltage before sending it
back to the I/O module in the hazardous area (Figure 22).The entity values of the safety barrier or the external power
supply must be compared to the entity parameters of the intrinsically safe weighing indicator passive inputs.
Slot 1
COM 1 - RS232
ETHERNET
COM 2 - RS232
COM 3 - RS232/422/485
Slot 1
COM 4 -CL
ETHERNET
COM 5 -CL
COM 2 - RS232
COM 3 - RS232/422/485
I/O
I/O
Switching amplifier
Switching amplifier
PLC
Control valve
Up to 11 passive outputs (3 standard
+ 8 optional) to external switching
amplifiers
Control valve
Up to 11 passive outputs (3 standard
+ 8 optional) to external switching amplifiers
PLC
Galvanically
isolated barrier
Junction box
Simple switch
Junction box
Power supply
All apparatuses require equipotential bonding.
All earthing points to the same location.
Figure 21: Active input / passive output valve control
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Power supply
All apparatuses require equipotential bonding.
All earthing points to the same location.
Figure 22: Passsive input / passive output valve control
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5.6 Fully Integrated Process Control
To increase efficiency, reduce variability and ensure maximum safety, fully integrated process control is the
method of choice. For example, in multipurpose plants where several components are filled, mixed, blended and
dispensed at the same time, an internal I/O system supporting 12 inputs and 18 outputs can be the right choice.
To achieve the maximum number of I/Os, safe-area active remote modules can be used. These mod-ules are not
approved for installation in the hazardous area. Active remote modules can switch the high voltage to control the
energy in the hazardous area and require advanced intrinsically safe communication modules, which serve both
as a safety barrier and provide enhanced Ethernet communication. The current loop interface provides two-channel bidirectional communications to the hazardous area. Weighing data can be stored on the PC through the Ethernet communication or the system can be connected to the PLC using Profibus (Figure 23).
Optional Safe-Area Peripheral Communications
Depending on weighing process requirements and the degree of automation required, different communication
possibilities allow efficient communication to the safe area.
When defining data transfer requirements to efficiently communicate weighing results to higher level manufacturing execution systems (MES) or enterprise resource planning (ERP) systems, several points must be considered:
• What type of information will be communicated between the weighing indicator and automation system?
• What triggers initiate data transfer and how frequently will communication be made?
• What are the present data format requirements? Are those requirements flexible?
• What is the current communication medium? Are there other viable options?
• How might data format requirements or the communication medium evolve in the foreseeable future?
The key to data integration is having the correct connection to your wider control system. Whether it is a PLC, MES
or an ERP system, connection requirements for hardware and software differs. Options include fieldbus interfaces,
such as analog output, Profibus, DeviceNet or EtherNet/IP; and data interfaces such as Ethernet TCP/IP or serial
interface communication.
When purchasing a new weighing system, the data integration capabilities must match the data requirements of
the wider manufacturing system. METTLER TOLEDO offers two different possibilities of communication in the safe
area that can meet these system requirements.
Fully Integrated Process Control Example
Non-Hazardous Area
Hazardous Area
ACM500 - CL
Weighing Terminal
Power Supply
Power Supply
COM 4 -CL
SICSPro Interface
COM 5 -CL
COM 1 - RS232
Slot 1
ETHERNET
COM 4 -CL
Advanced weighing applications require enhanced control as well as enhanced safe area communication. The
communication module ACM500 plays an important role for such application types providing several optional interfaces from RS232 to PLC data integration.
COM 2 - RS232
COM 5 -CL
Intrinsically safe
control valve
Up to 8 x active
outputs / 12VDC
max. 50mA total
The basic ACM 200 communication module approved for the safe area installation provides the connective link
between the weighing system installed in the certified hazardous area and the computer, printer or remote-control
indicator in the safe area. This module provides a flexible choice between different communication interfaces such
as RS232, RS422, RS485 and CL20mA.
COM 3 - RS232/422/485
If PLC data integration is not required, PC-based serial or Ethernet TCP/IP data communication options become viable.
I/O
Ex Control valve
Simple switch
Simple switch
Up to 8 active
inputs
Up to 3 x ARM 100 modules
Maximum 12 inputs & 18 outputs
PC connection
Zener barrier
Digital platform IDNet
Power supply
All apparatuses require equipotential bonding.
All earthing points to the same location.
Figure 24: Communication modules: ACM200 (left) and ACM500 (right)
Figure 23: Fully integrated process control
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Summary and References
6 Summary
References
Safety is crucial for businesses that operate in potentially explosive environments. Standards and regulations play
an important role in these hazardous manufacturing venues by specifying the framework of conditions that guide
both equipment manufacturers and operators to help ensure safety in manufacturing.
• C22.1-12 - Canadian electrical code, part I (22nd edition): Safety standard for electrical installations
• Electrical apparatus for use in the presence of combustible dust
• Directive 2014/34/EU on equipment and protective systems intended for use in potentially explosive atmospheres (ATEX); European Parliament and the Council
• Directive 1999/92/EC on minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmosphere (ATEX); European Parliament and of the Council
• IEC 60079-0:2017 Ed. 7.0; Explosive atmospheres - Part 0: Equipment - General requirements
• IEC 60079-10-1:2015 Ed. 2.0; Explosive atmospheres - Part 10-1: Classification of areas - Explosive gas
atmospheres
• IEC 60079-10-2:2015 Ed. 2.0; Explosive atmospheres - Part 10-2: Classification of areas - Combustible dust
atmospheres
• EC 60079-11 Ed. 6.0 b:2011; Explosive atmospheres - Part 11: Equipment protection by intrinsic safety "i"
• IEC 60079-17 Ed. 5.0 b:2013; Explosive atmospheres - Part 17: Electrical installations inspection and maintenance
• IEC 60079-26:2014 Ed. 3.0; Explosive atmospheres - Part 26: Equipment with equipment protection level
(EPL) Ga
• National Electrical Code, Article 500, NFPA 70, 2011, Delmar: National Electric Code
• National Electrical Code, Article 505, NFPA 70, 2011, Delmar: National Electric Code
Many standards that are applied worldwide are based on other standards. While standards are similar throughout
the world, there is still no uniform global standard. Furthermore, symbols on the respective labels differ. Therefore,
products sold globally also must have various certifications for different explosion-risk environments.
Many countries in Southeast Asia and Latin America have no local standards of their own and accept international
and national IECEx, ATEX and FM approvals. However, locations such as China, Korea, Japan and Russia have
local certification requirements to which equipment must be adhered, although primary certification schemes may
be accepted for most purposes, depending on the nation.
Weighing is an important component of many manufacturing processes, and it requires special attention when
conducted in hazardous areas. Though weighing system components may be both intrinsically safe and nonintrinsically safe depending on where and how they are used, it is crucial to ensure they have an appropriate level
of safety and provide required communication possibilities.
There are several options when it comes to ignition protection in hazardous environments. Installing intrinsically
safe weighing equipment is the safest method. It safely facilitates activities in the hazardous area and is low maintenance. When fire or explosive incidents occur, the units can be serviced without halting production, and it eliminates heat and sparks in the production area.
METTLER TOLEDO focuses on development of intrinsically safe weighing systems. The intrinsically safe weighing
solutions provide the benefits of modularity for a wide range of weighing platforms, weighing modules and control
terminals. It also offers flexibility with various communication interfaces, such as serial interfaces and wide range
of Fieldbuses.
Additional METTLER TOLEDO Resources
• Hazardous Area Resource Portal:
} www.mt.com/ind-hazardous-compliance
• Poster: Hazardous-Area Equipment Marking
• eBook: How to Avoid Dust Explosions
• On Demand Webinars: "Standards and Regulations" and "Hazardous-Area Protection Measures
• Catalog: Weighing Solutions for Hazardous Areas
Moreover, the weighing equipment is developed for use in hazardous areas and has obtained approvals on the
global level, including IECEx, ATEX, and FM, which are accepted in most countries.
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Service Offering
Our extensive service network is among the best in the
world and ensures maximum availability and service life
of your product.
Explore Our Service Solutions
Tailored to Fit Your Equipment Needs
METTLER TOLEDO Service delivers resources to enhance your efficiency, performance, and
productivity by providing service packages that fit your operational needs, maximize your
equipment lifetime, and protect your investment.
` www.mt.com/IND-Service
Start with professional installation
Installation services include support for your unique production
situation:
• Professional IQ/OQ/PQ/MQ documentation
• Initial calibration and confirmation of fit-for-purpose
• Hazardous area installations
Calibrate for quality and compliance
The professional Accuracy Calibration Certificate (ACC) determines measurement uncertainty in use over the entire weighing
range. Corresponding annexes gives a clear pass/fail statement
for specific tolerances applied, such as fit-for-purpose (GWP®),
OIML R76, NTEP HB44, or further regulations.
Extend your warranty coverage
Add two years of preventive maintenance and repair coverage to
protect your equipment purchase and achieve maximum productivity and budget control.
Schedule maintenance
Full preventative maintenance plans offer inspection, functional
testing, and proactive replacement of worn parts.
Health inspections offer a full assessment of current equipment condition with professional maintenance recommendations.
Maintain accuracy over time
Receive professional guidance (GWP® VerificationTM), including
a routine testing plan that specifies four key factors to maximize
your efficiency and ensure quality:
• Tests to perform
• Weights to use
• Testing frequency
• Tolerances to apply
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Additional Resources for Success
Industrial Weighing YouTube Channel
METTLER TOLEDO's Industrial Solutions YouTube
Channel is dedicated entirely to helping you maximize
yield and improve your weighing process.
www.youtube.com/MTindustrial
Truck Scale Buying Guide
This comprehensive guide has everything you need to
know when designing your next weighbridge.
www.mt.com/TruckScaleGuide
Raw Materials Resource Library
Discover in-depth white papers, guides, brochures and more
to help you streamline your weighing operation
www.mt.com/ind-raw-materials-library
Tank Weighing Handbook
This handbook offers more than 150 fact-filled pages to help
you create an ideal weighing and inventory control system.
www.mt.com/ind-system-handbook
www.mt.com/ind-hazcat
For more information
METTLER TOLEDO Group
Industrial Division
Local contact: www.mt.com/contacts
Subject to technical changes
© 06/2023 METTLER TOLEDO. All rights reserved
Document No. 30220321 C
MarCom Industrial