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Hazardous Locations
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Introduction
Hazardous locations are those where there is ahazardous environment. The Codes forhazardou environments are specifically
concerned with the possibility of a fire orexplosion due to forseeable or avoidable humanerrors like:
Improper installation, selection, and design
Lack of proper maintenance Improper use
Carelessness or oversight
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Hazards
Fire is rapid oxidation/combustion which results inproducing heat, smoke, and light
An explosion is a violent and sudden expansion ofgases produced by rapid burning; it produces a very
strong force when shut in a small space and generallyhas a loud, sharp noise and may have a supersonicshock wave
The fundamental nature of electricity is to create aspark or generate energy which provides ignition in theright mix of fuel and air which leads to a fire orexplosion Both sparks AND heat have to be considered in electrical
equipment design
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Category System
Class I: Gases and flammable liquids such as gasoline
Class II: Dusts and particles such as coal, flour, grain, and paint pigments
Class III: Fibers such as carpet fiber
Within these categories, the type of material also defines severalsubclasses depending on material properties
Division 1: A situation where a hazardous atmosphere is present eithercontinuously or for long periods of time under normal operation, or inwhich they exist frequently because of repair and maintenance or leakage
Division 2: A situation where a hazardous atmosphere is not normallypresent except in the case of an accidental rupture or breakdown ofequipment, or there is an adjacent Division 1 area that can potentially spill
over. Unclassified: Not normally containing a hazardous atmosphere under
normal or abnormal conditions. Example: Piping without valves, checks,meters, or similar devices would be unclassified. Pneumatic coal linesdesigned and maintained according to NFPA 85 is unclassified
See NFPA 30A, 36, 51, 52, 54, 55, 58, 61, 85, 88A, 99, 407, 409, 495, 496,
654, 655 for specific classification guidance. See NFPA 30, 497, and 499 forgeneral guidance. All can be viewed on NFPA web site (www.nfpa.org)
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European Classifications
European system uses zones instead of
class/division
Does not have a direct equivalent to division 2
NEC allows either system (choose one)
European equipment ratings
Rated by the terms ATEX or Ex
European Manufacturers can self-certify without
any third party verification (CE)
Not acceptable to MSHA or OSHA
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Solutions
There are MANY approaches, and each has particularadvantages and disadvantages
As a PHA team, it is better to ensure that electricalequipment meets hazardous location requirements
and avoid a specific implementation Some methods: dust tight, dust-ignition proof,
nonincendive, explosion proof, hermetically sealed, oilimmersion, purged, pressurized, intrinsically safe,quartz/sand filled, encapsulated, ventilated
Each method has developed to serve specificequipment and atmospheres. Each has advantagesdepending on the classification and the process
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Method Overview
The following notes outline details on the major designmethods for performing safe electrical installations inhazardous locations as background material
Some (but not all) equipment will be specifically certified
for use with a particular method by a certifying agency (FMGlobal, TUV)
Some equipment is automatically approved Example: TEFC (Aurora standard) motors are approved for use in
Class II, Division 2 areas
Some methods have greater flexibility or higher powerlimitations at the expense of cost and weight. Others (oilfilled) are equipment specific.
Againa PHA team will want to ensure that the hazardouslocations are documented and that the electrical team is
aware of them, not specify solutions
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Coal Mine History
The use of electricity in the explosive environment wasinitially done in the mining industry where theefficiency and power of electricity was quicklyrecognized
The early approach was to burn any fire damp(methane) gases to reduce the risk of an explosion.Fire men were young miners who were covered withwet sacking and go ahead of the others with long wicksand hold them up in the highest parts of the mine to
burn off methane. Methane flames are also a differentcolor which was one of the first combustible gassensors
The method was effective but considered primitive
Flare towers are still in common use in refineries today
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Alternative Coal Solution
The first improved technique wasventing (mine ventilation) which isstill a major concern forunderground mines today
The second was the Davy safetylamp. This lamp had a wire meshthat cooled the hot gases whichprevented the methane around a
lantern from igniting 1800s, first example of explosion
proof method
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Pressuring/Purging Methods
Most flexibleany equipment can be used in any sizeenclosure
Can be high cost to operate and maintain Often less expensive due to flexibility and enclosure size
Methods are to pressurize an enclosure to preventhazardous atmospheres from entering, or to purgewith enough fresh air to achieve the same effect
Usually requires some sort of monitoring and control
to shut down if the purge/pressurizing system fails Must consider maintenance access especially with
pressurized systems, and repressurizing/repurging
Can require process calculations to verify effectiveness
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Explosion Proof Method
Dr. Ing Carl Beylingdeveloped flame proof in1908.
An explosion proofenclosure is designed towithstand the pressure ofan internal explosion
Also called dust ignitionproof in dusty areas
Very effective butenclosures are cast, heavy,expensive, and requireexpensive wiring withspecial seals
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Flameproof Gap
Misconception: Not gas tight
Allows gas/dust to exist inside equipment
Hot gas jets through the opening which causes it to cool
Openings are designed so that an escaping flame will becooled to the point where it will not ignite any gas or dustoutside the enclosure
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Nonincendive Method
Every make/break component (relay, switch,etc.) must: Be nonarcing
Use an electrical circuit that limits the power sothat the arc cannot cause anything to ignite
Be sealed so that a spark cannot cause ignition The only remaining concern is heat, which is
addressed by enclosure design
Enclosures must be weather proof and impact-resistant
Wiring must also be rated for the hazardouslocation
Note: Many intrinsically safe ventilation fansand other parts are actually mislabeled and arenonincendive
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Intrinsically Safe Method
Dates back to Senghenydd Coal Mine Disaster onOctober 13, 1913. Just prior to the explosion,older wet cell batteries were upgraded to drycell batteries (note: no MOC!)
It turns out that the higher current output of thedry cells created a spark large enough to ignitemethane in a coal mine. Older wet cells wereincapable of causing ignition.
Intrinsically safe was born! To date, there are no recorded cases of IS
equipment causing a fire or explosion
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IS Design
Barriers Barriers electrically limit both current and voltage to within
the safe limits required by the intrinsically safe design
Barriers must be located within the non-hazardous area
Generally this is less than 1.3 Watts (low power)
Some portable devices on the market have a specialbattery (back to the old wet cell concept) which isdesigned to allow the IS power supply inside thehazardous area
Some PLC hardware has barriers integrated into it as aconvenience. However manufacturers (Honeywell, AB)charge far more for convenience and it is usually lessexpensive to use a separate barrier
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IS Devices
Field devices must be designed to not accumulate energy to a levelwhere a spark or heat could form
Devices must be either rated as IS or else be a simple apparatus: All standard wiring (no special wiring required for IS)
Passive components: switches, junction boxes, resistors, and some
simple semi-conductor devices Sources of generated energy that do not exceed 1.5 volts, 100 mA, and
35 mW (thermocouples and photocells)
LVDT type position sensors
Magnetic pickup coils if the inductance does not exceed the limit on ISequipment
Vibration sensors were previously excluded but are now allowed
Most 120 VAC equipment cannot be approved but most 24 VDCinstrumentation and controls are approved
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IS Wiring
IS wirng is simply standard wiring methods
No special wiring is required
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Hazardous Location ComparisonMethod Power Wiring Best Use Cost
Intrinsically
safe
Very limited Standard LED pilot lights, push
buttons, most field
instruments, radios
Low
Nonincendive Unlimited Special Medium size motors,
lights, power relays
Medium
Explosion
proof/dust-
ignition proof
Limited Special Junction boxes, smaller
motors
Medium
to high
Dust tight Unlimited Tray/Conduit Standard process
equipment
Low
Sealed/oil
filled/sand
filled
Limited Special Transformers, some
motors, some medium
voltage equipment
Low
Purged/Pressur
ized
Unlimited Unliimted Entire MCCs, control
rooms, custom equipment
Low to
high