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Intrinsically
safe equipment is defined as "equipment and wiring which is incapable of
releasing sufficient electrical or thermal energy under normal or abnormal
conditions to cause ignition of a specific hazardous atmospheric mixture in
its most easily ignited concentration." (ISA-RP12.6) This is achieved by
limiting the amount of power available to the electrical equipment in the
hazardous area - to a level below that which will ignite the gases.
 Three
elements are needed for a fire/explosion: fuel, oxygen and a source of
ignition. An I.S. system assumes the fuel and oxygen is present in the
atmosphere. Because of this assumption, which may or may not be true, the
system is designed so the electrical energy or thermal energy of a
particular instrument loop can never become high enough to cause ignition.
Traditionally, protection from fire/explosion in hazardous environments has
been achieved by using explosion proof enclosures, or purging which isolates
the explosive gas from the electrical equipment. Intrinsically safe
equipment cannot replace these methods in all applications, but where
possible can provide significant cost savings.
APPLICATIONS
A hazardous area may contain flammable gasses or vapors, combustible dusts,
or ignitable fibers. There are different systems used in Europe or the
United States to classify the type of hazard and whether the hazard is
always present or only present in an emergency condition such as a spill or
failure of venting equipment. In most cases the equipment is designed for
the worst case, which would be to assume the explosive atmosphere is always
present and the electrical or thermal energy is the lowest required to cause
a fire or explosion.
Most
applications require a signal to be sent out of or into the hazardous area.
The equipment mounted in the hazardous area must first be approved for use
in an intrinsically safe system. The barriers designed to protect the system
must be mounted outside of the hazardous area in an area designated as
Non-hazardous or Safe in which the hazard is not and will not be present.
Equipment
which has been designed for and is available for use in hazardous areas with
intrinsically safe barriers includes:
• 4-20 mA
Two Wire Transmitters
• Thermocouples
• RTD's
• Strain Gages
• Pressure, Flow, & Level Switches
• I/P Converters
• Solenoid Valves
• Proximity Switches
• Infrared Temperature Sensors
• Potentiometers
• LED Indicating Lights
• Magnetic Pickup Flowmeters
Most of the
apparatus that is mounted in the Hazardous area will have to be approved and
certified for use in the Hazardous area with an approved barrier designed
for use with that apparatus. Some simple devices like thermocouples, RTDs,
LEDs and contacts can be used in the hazardous area without certification as
long as it is wired in conjunction with an approved barrier.
APPROVALS
Intrinsic safety equipment must have been tested and approved by an
independent agency to assure its safety. The customer should specify the
type of approval required for their particular application. The most common
Agencies involved are as follows:
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COUNTRY |
AGENCY |
|
USA |
FM, UL
|
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CANADA |
CSA
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GREAT
BRITIAN |
BASEEFA
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FRANCE |
LCIE
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GERMANY |
PTB
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ITALY |
CESI
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BELGIUM |
INEX
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Approval by
any of the above European Agencies constitutes a CENELEC approval allowing
the units to be considered approved in many of the European countries. |
