Electrical
equipment in hazardous areas
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Wikipedia, the free encyclopedia
This inspection lamp is constructed so that it cannot set off an explosion
when surrounded by specified flammable gases or dust.
In electrical engineering, a hazardous location is defined as a place where
concentrations of flammable gases, vapors, or dusts occur. Electrical equipment
that must be installed in such locations is especially designed and tested to
ensure it does not initiate an explosion, due to arcing contacts or high surface temperature
of equipment.
For example a household light switch may emit a small, harmless visible
spark when switching; in an ordinary atmosphere this arc is of no concern, but
if a flammable vapor is present, the arc might start an explosion. Electrical
equipment intended for use in a chemical factory or refinery is designed either to contain any
explosion within the device, or is designed not to produce sparks with
sufficient energy to trigger an explosion.
Many strategies exist for safety in
electrical installations. The simplest strategy is to
minimize the amount of electrical equipment installed in a hazardous area,
either by keeping the equipment out of the area altogether or by making the
area less hazardous by process improvements or ventilation with clean air. Intrinsic safety and non-incendiveequipment and wiring methods
are practices where apparatus is designed with low power levels and low stored
energy, so that an arc produced during normal functioning of the equipment or
as the result of equipment failure has insufficient energy to initiate ignition
of the explosive mixture. Equipment enclosures can be pressurized with clean
air or inert gas and designed with various controls to remove power or provide
notification in case of supply or pressure loss of such gases. Arc-producing
elements of the equipment can also be isolated from the surrounding atmosphere
by encapsulation, immersion in oil, sand, etc. Heat producing elements such as
motor winding, electrical heaters, including heat tracing and lighting fixtures
are often designed to limit their maximum temperature below the autoignition
temperature of the material involved. Both external and internal temperatures
are taken into consideration.
As in most fields of electrical installation,
different countries have approached the standardization and testing of
equipment for hazardous areas in different ways. As world trade becomes more
important in distribution of electrical products, international standards are
slowly converging so that a wider range of acceptable techniques can be
approved by national regulatory agencies.
Area classification is required by
governmental bodies, for example the U.S. Occupational Safety and Health
Administration and
compliance is enforced.
Documentation requirements are varied. Often
an area classification plan-view is provided to identify equipment ratings and
installation techniques to be used for each classified plant area. The plan may
contain the list of chemicals with their group and temperature rating, and
elevation details shaded to indicate Class, Division(Zone) and group
combination. The area classification process would require the participation of
operations, maintenance, safety, electrical and instrumentation professionals,
the use of process diagrams and material flows, MSDS and any pertinent documents,
information and knowledge to determine the hazards and their extent and the
countermeasures to be employed. Area classification documentations are reviewed
and updated to reflect process changes.
[edit]History.
Soon after the introduction of electric power
into coal mines,
it was discovered that lethal explosions could be initiated by electrical
equipment such as lighting, signals, or motors. The hazard of fire damp or methane accumulation in mines was
well known by the time electricity was introduced, along with the danger of
suspended coal dust. At least two British mine explosions were attributed to an
electric bell signal system. In this system, two bare wires were run along the
length of a drift, and any miner desiring to signal the surface would
momentarily touch the wires to each other or bridge the wires with a metal
tool. Theinductance of the signal bell coils, combined
with breaking of contacts by exposed metal surfaces, resulted in sparks which
could ignite methane, causing an explosion.[1]
[edit]Divisions or zones (gases, vapors and
dust.)
In an industrial plant such as a refinery or chemical process plant,
handling of large quantities of flammable liquids and gases creates a risk of
leaks. In some cases the gas, ignitable vapor or dust is present all the time
or for long periods. Other areas would have a dangerous concentration of flammable
substances only during process upsets, equipment deterioration between
maintenance periods, or during an incident. Refineries and chemical plants are
then divided into areas of risk of release of gas, vapor or dust known as
divisions or zones. The process of determining the type and size of these
hazardous areas is called area classification. Guidance on assessing the extent
of the hazard is given in the NFPA 497 Standard, or API 500 and according to
their adaptation by other areas gas zones is given in the current edition of
IEC 60079.10. For hazardous dusts, the guiding standard is IEC 61421.10.
Typical gas hazards are from hydrocarbon
compounds, but hydrogen and ammonia are common industrial gases that are
flammable.
Non-Hazardous Area
An area such as a residence or office would be classed as
Non Hazardous (safe area), where the only risk of a release of explosive or
flammable gas would be such things as the propellant in an aerosol spray.
The only explosive or flammable liquid would be paint and brush cleaner. These
are classed as very low risk of causing an explosion and are more of a fire
risk (although gas explosions in residential buildings do occur). Non hazardous
areas on chemical and other plant are present where the hazardous gas is
diluted to a concentration below 25% of its lower flammability limit (or lower explosive limit (LEL)).
Division 2 or Zone 2 area
This is a step up from the safe area. In this zone the gas, vapor or mist
would only be present under abnormal conditions (most often leaks under
abnormal conditions). As a general guide for Zone 2, unwanted substances should
only be present under 10 hours/year or 0–0.1% of the time.[2]
Division 1 or Zone 1 area
Gas, vapor or mist will be present or expected to be present for long
periods of time under normal operating conditions. As a guide for Zone 1, this
can be defined as 10–1000 hours/year or 0.1–10% of the time.[2]
Zone 0 area
Gas or vapor is present all of the time. An example of this would be the
vapor space above the liquid in the top of a tank or drum. The ANSI/NEC
classification method consider this environment a Division 1 area. As a guide
for Zone 0, this can be defined as over 1000 hours/year or >10% of the time.[2]
[edit]Zones (dusts)
An explosion of dust at this grain elevatorin
Kansas killed five workers in 1998.
Flammable dusts when suspended in air can explode. An
old system of area classification to a British standard used a system of
letters to designate the zones. This has been replaced by a European numerical
system, as set out in directive 1999/92/EU implemented in the UK as the
Dangerous Substances and Explosives Atmospheres Regulations 2002
The boundaries and extent of these
three dimensional zones should be decided by a competent person. There must be
a site plan drawn up of the factory with the zones marked on.
The zone definitions are:
Zone 20
A place in which an
explosive atmosphere in the form of a cloud of combustible dust in air is
present continuously, or for long periods or frequently.
Zone 21
A place in which an
explosive atmosphere in the form of a cloud of combustible dust in air is
likely to occur, occasionally, in normal operation.
Zone 22
A place in which an
explosive atmosphere in the form of a cloud of combustible dust in air is not
likely to occur in normal operation but, if it does occur, will persist for a
short period only
[edit]Gas groups
Explosive gases, vapors and dusts have
different chemical properties that affect the likelihood and severity of an
explosion. Such properties include flame temperature, minimum ignition energy,
upper and lower explosive limits, and molecular weight. Empirical testing is
done to determine parameters such as the maximum experimental safe gap, minimum
ignition current, explosion pressure and time to peak pressure, spontaneous
ignition temperature, and maximum rate of pressure rise. Every substance has a
differing combination of properties but it is found that they can be ranked
into similar ranges, simplifying the selection of equipment for hazardous areas.[3]
Flammability of combustible liquids are
defined by their flash-point. The flash-point is the temperature at which the
material will generate sufficient quantity of vapor to form an ignitable
mixture. The flash point determines if an area needs to be classified. A
material may have a relatively low autoignition temperature yet if its
flash-point is above the ambient temperature, then the area may not need to be
classified. Conversely if the same material is heated and handled above its
flash-point, the area must be classified.[citation needed]
Each chemical gas or vapour used in
industry is classified into a gas group.
|
Group |
Representative Gases |
|
I |
All Underground Coal Mining. Firedamp (methane) |
|
IIA |
Industrial methane, propane, petrol and the majority
of industrial |
|
IIB |
Ethylene, coke oven gas and other industrial
gases |
|
IIC |
Apparatus marked IIB can also be used
for IIA gases. IIC marked equipment can be used for both IIA and IIB. If a
piece of equipment has just II and no A, B, or C after then it is suitable for
any gas group.
A list must be drawn up of every
chemical gas or vapor that is on the refinery/chemical complex and included in
the site plan of the classified areas. The above groups are formed in order of
how volatile the gas or vapor would be if it was ignited, IIC being the most
volatile and IIA being the least. The groups also indicate how much energy is
required to ignite the gas by spark ignition, Group IIA requiring the most
energy and IIC the least.
[edit]Equipment Protection Level (EPL)
In recent years also the Equipment
Protection Level is specified for several kinds of protection. The required
Protection level is linked to the intended use in the zones described below:
|
Group |
Ex risk |
EPL |
Minimum type of protection |
|
|
I (mines) |
energized |
|
Ma |
|
|
I (mines) |
de-energized in
presence of Ex atmosphere |
|
Mb |
|
|
II (gas) |
explosive atmosphere > 1000 hrs/yr |
0 |
Ga |
ia, ma |
|
II (gas) |
explosive
atmosphere between 10 and 1000 hrs/yr |
1 |
Gb |
ib, mb, px, py, e, o, q, s |
|
II (gas) |
explosive
atmosphere between 1 and 10 hrs/yr |
2 |
Gc |
n, ic, pz |
|
III (dust) |
explosive surface > 1000 hrs/yr |
20 |
Da |
|
|
III (dust) |
explosive
surface between 10 and 1000 hrs/yr |
21 |
Db |
|
|
III (dust) |
explosive
surface between 10 and 10 hrs/yr |
22 |
Dc |
|
[edit]Temperature
classification
Another important consideration is the
temperature classification of the electrical equipment. The surface temperature
or any parts of the electrical equipment that may be exposed to the hazardous
atmosphere should be tested that it does not exceed 80% of the auto-ignition
temperature of the specific gas or vapor in the area where the equipment is
intended to be used.
The temperature classification on the
electrical equipment label will be one of the following (in degree Celsius):
|
USA°C |
UK °C |
|
|
|
T1 - 450 |
T3A - 180 |
T1 - 450 |
G1: 360 - 400 |
|
T2 - 300 |
T3B - 165 |
T2 - 300 |
G2: 240 - 270 |
|
T2A - 280 |
T3C - 160 |
T3 - 200 |
G3: 160 - 180 |
|
T2B - 260 |
T4 - 135 |
T4 - 135 |
G4: 110 - 125 |
|
T2C - 230 |
T4A - 120 |
T5 - 100 |
G5: 80 - 90 |
|
T2D - 215 |
T5 - 100 |
T6 - 85 |
|
|
T3 - 200 |
T6 - 85 |
|
|
The above table tells us that the
surface temperature of a piece of electrical equipment with a temperature
classification of T3 will not rise above 200 °C.
[edit]Auto-ignition temperatures (vapors & gases)
The auto-ignition temperature of a
liquid, gas or vapor is the temperature at which the substance will ignite
without any external heat source. The exact temperature value determined
depends on the laboratory test conditions and apparatus. Such temperatures for
common substances are:
|
Gas |
Temperature |
|
580 °C |
|
|
560 °C |
|
|
493 °C |
|
|
425 °C |
|
|
305 °C |
|
|
290 °C |
|
|
102 °C |
The surface of a high pressure steam
pipe may be above the autoignition temperature of some fuel/air mixtures.
[edit]Auto-ignition temperatures (dust)
The auto-ignition temperature of a dust
is usually higher than that of vapours & gases. Examples for common
materials are:
|
Substance |
Temperature |
|
460 °C |
|
|
340 °C |
|
|
340 °C |
|
|
Grain dust |
300 °C |
|
300 °C |
[edit]Type of protection
To ensure safety in a given situation,
equipment is placed into protection level categories according to manufacture
method and suitability for different situations. Category 1 is the highest
safety level and Category 3 the lowest. Although there are many types of
protection, a few are detailed
|
|
Ex Code |
Description |
Standard |
Location |
Use |
|
Flameproof |
d |
Equipment construction is such that it can withstand
an internal explosion and provide relief of the external pressure via flamegap(s) such as the labyrinth created by threaded
fittings or machined flanges. The escaping (hot) gases must sufficiently cool
down along the escape path that by the time they reach the outside of the
enclosure not to be a source of ignition of the outside, potentially
ignitable surroundings. Equipment has flameproof gaps (max 0.006" propane/ethylene, 0.004"acetylene/hydrogen) |
IEC/EN 60079-1 |
Zone 1 if gas group & temp. class correct |
Motors, lighting, junction boxes, electronics |
|
Increased Safety |
e |
Equipment is very robust and components are made to
a high quality |
IEC/EN 60079-7 |
Zone 2 or Zone
1 |
Motors,
lighting, junction boxes |
|
Oil Filled |
o |
Equipment components are completely submerged in oil |
IEC/EN 60079-6 |
Zone 2 or Zone
1 |
Heavy current
equipment |
|
Sand/Powder/Quartz Filled |
q |
Equipment components are completely covered with a
layer of Sand, powder or quartz |
IEC/EN 60079-5 |
Zone 2 or Zone
1 |
Electronics,
telephones, chokes |
|
Encapsulated |
m |
Equipment components of the equipment are usually
encased in a resin type material |
IEC/EN 60079-18 |
Zone 1 (Ex mb)
or Zone 0 (Ex ma) |
Electronics (no
heat) |
|
Pressurised/purged |
p |
Equipment is pressurised to a positive pressure
relative to the surrounding atmosphere with air or an inert gas, thus the
surrounding ignitable atmosphere can not come in contact with energized parts
of the apparatus. The
overpressure is monitored, maintained and controlled. |
IEC/EN 60079-2 |
Zone 1 (px or
py), or zone 2 (pz) |
Analysers, motors, control boxes, computers |
|
Intrinsically safe |
i |
Any arcs or sparks in this equipment has
insufficient energy (heat) to ignite a vapour Equipment can be installed in ANY housing provided
to IP54. |
IEC/EN 60079-25 |
'ia': Zone 0
& |
Instrumentation,
measurement, control |
|
Non Incendive |
n |
Equipment is non-incendive
or non-sparking. A special standard for instrumentation is IEC/EN
60079-27, describing requirements for Fieldbus Non-Incendive Concept (FNICO) (zone 2) |
IEC/EN 60079-15 |
Zone 2 |
Motors, lighting, junction boxes, electronic
equipment |
|
Special Protection |
s |
This method, being by definition special, has no
specific rules. In effect it is any method which can be shown to have the
required degree of safety in use. Much early equipment having Ex s protection
was designed with encapsulation and this has now been incorporated into IEC
60079-18 [Ex m]. Ex s is a coding referenced in IEC 60079-0.
The use of EPL and ATEX Category directly is an alternative for “s” marking. The
IEC standard EN 60079-33 is made public and is expected to become effective
soon, so that the normal Ex certification will also be possible for Ex-s |
IEC/EN 60079-33 |
Zone depending upon Manufacturers Certification. |
As its
certification states |
The types of protection are subdivided
into several sub classes, linked to EPL: ma and mb, px, py and pz, ia, ib and
ic. The a subdivisions have the most stringent safety requirements, taking into
account more the one independent component faults simultaneously.
[edit]Multiple
protection
Many items of EEx rated equipment will
employ more than one method of protection in different components of the
apparatus. These would be then labeled with each of the individual methods. For
example a socket outlet labeled EEx'de' might have a case made to EEx 'e' and
switches that are made to EEx 'd'.
[edit]ANSI/NFPA
areas description
Class I, Div. 1 -
Where ignitable concentrations of flammable gases, vapors or liquids are
present continuously or frequently within the atmosphere under normal operation
conditions.
Class I, Div. 2 -
Where ignitable concentrations of flammable gases, vapors, or liquids are
present within the atmosphere under abnormal operating conditions.
Class II, Div. 1 -
Where ignitable concentrations of combustible dusts are present within the
atmosphere under normal operation conditions.
Class II, Div. 2 -
Where ignitable concentrations of combustible dust are present within the
atmosphere under abnormal operating conditions.
Class III, Div. 1 -
Where easily ignitable fibers or materials producing combustible flyings are
present within the atmosphere under normal operation conditions.
Class III, Div. 2 -
Where easily ignitable fibers or materials producing combustible flyings are
present within the atmosphere under abnormal operating conditions.
Common Materials within
Associated Class & Group Ratings, such as "Class I, Division 1, Group
A":
Class I Areas: Group
A: Acetylene / Group B: Hydrogen / Group C: Propane and Ethylene / Group D:
Benzene, Butane, Methane & Propane
Class II Areas: Group
E: Metal Dust / Group F: Carbon & Charcoal / Group G: Flour, Starch, Wood
& Plastic
Class III Areas: NO
GROUP: Cotton & Sawdust
For more information see Article 500 of
NFPA 70 - The National Electric Code, as published by the National Fire
Protection Association.
[edit]Equipment category
The equipment category indicates the
level of protection offered by the equipment.
Category 1 equipment may be used in
zone 0, zone 1 or zone 2 areas.
Category 2 equipment may be used in
zone 1 or zone 2 areas.
Category 3 equipment may only be used
in zone 2 areas.
[edit]Labeling
All equipment certified for use in
hazardous areas must be labelled to show the type and level of protection
applied.
[edit]Europe
In Europe the label must show the CE mark and the code number of the certifying
body. The CE marking is complemented with the Ex mark, followed by the
indication of the Group, Category and, if group II equipment, the indication
relating to gases (G) or dust (D). For example: Ex II 1 G (Explosion protected,
Group 2, Category 1, Gas) Specific type or types of protection being used will
be marked.
· EEx ia IIC T4. (Type ia, Group 2C gases, Temperature category 4).
· EEx nA II T3 X (Type n, non-sparking, Group 2 gases, Temperature category
3, special conditions apply).
In the United Kingdom, industrial
electrical equipment for hazardous area has to conform to standard BS 60079 and
in some cases, certified as meeting that standard. Independent test houses
(known as Notified Bodies)are established in most European countries, and a
certificate from any of these will be accepted across the EU. The DTI appoint
and maintain a list
of Notified Bodies within the UK, of which Sira and Baseefa are the most well known.
[edit]North America
In North America the suitability of
equipment for the specific hazardous area must be tested by a Nationally Recognized Testing
Laboratory. Such institutes are UL, MET, FM, CSA or Intertek (ETL),
for example.
The label will always list the
Class(es), Division(s) and may list the Group(s) and temperature Code. Directly
adjacent on the label one will find the mark of the listing agency.
Some manufacturers claim
"suitability" or "built-to" hazardous areas in their
technical literature, but in effect lack the testing agency's certification and
thus unacceptable for the AHJ (Authority Having Jurisdiction) to permit operation
of the electrical installation/system.
All equipment in Division 1 areas must
have an approval label, but certain materials, such as rigid metallic conduit,
does not have a specific label indicating the Cl./Div.1 suitability and their
listing as approved method of installation in the NEC serves as the permission.
Some equipment in Division 2 areas do not require a specific label, such as
standard 3 phase induction motors that do not contain normally arcing
components.
Also included in the marking are the
manufacturers name or trademark and address, the apparatus type, name and
serial number, year of manufacture and any special conditions of use. The NEMA enclosure rating or IP code may also be indicated, but it is
usually independent of the Classified Area suitability.