Introduction
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.
 In order to have a fire or
explosion, fuel, oxygen and a source of ignition must be present. An intrinsically safe system assumes the fuel and oxygen
is present in the atmosphere, but the system is designed so the electrical energy or thermal energy of a particular instrument
loop can never be great enough to cause ignition.
Traditionally, protection from explosion in hazardous environments has been accomplished
by either using EXPLOSION PROOF apparatus which can contain an explosion inside an enclosure, or PRESSURIZATION
or purging which isolates the explosive gas from the electrical equipment. Intrinsically safe apparatus cannot replace
these methods in all applications, but where possible can provide significant cost savings in installation and
maintenance of the equipment in a Hazardous area. The basic design of an intrinsic safety barrier uses
Zener Diodes to limit voltage, resistors to limit current and a fuse.
APPLICATIONS
A
Hazardous Area may contain flammable gasses or vapors, combustible dusts,
or ignitable fibers or flyings. 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. (Refer to Pages Z-93, 94 in The
OMEGA Complete Temperature Measurement and Control Handbook and
Encyclopedia® for U. S. Classifications). 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 mAdc Two Wire Transmitters
- Thermocouples
- Rtds
- 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:
Country AGENCY
USA - FM, UL
CANADA - CSA
GREAT BRITIAN - BASEEFA
FRANCE - LCIE
GERMANY - PTB
ITALY - CESI
BELGIUM - INEX
NOTE: 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.
Products to be mounted in the hazardous area can be approved either under the LOOP or
ENTITY approval concept.
The LOOP concept specifies the exact part number and products that can be used in
the loop. No deviation from the specified units is allowed.
The ENTITY concept specifies parameters which any approved intrinsic safety barrier
must meet. This allows the user to select barriers from different approved manufacturers. Under entity
approval two items may be interconnected if the following conditions are met (refer to chart below):
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