HAZARDOUS LOCATION CLASSIFICATION
INTRINSIC SAFETY
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 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):
In all cases the intrinsically safe barriers and equipment MUST be wired per an approved drawing.
Capacitance and inductance of the wiring and cables must be included in the loop evaluation. Fig. 1
Positive Single-Channel Zener Barrier With Negative Ground.Fig. 2 Positive Dual-Channel Zener
Barrier With Floating Leads. Note: Terminals 3, 4, 5 and 6 are Common and are Bonded to the Mounting
Tabs for Positive Redundant Grounding.
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