INTRINSIC SAFETY
In Tech • www.isa.org • June 2001
51
A universal approach for
hazardous-area classifications
By Jim Peterson
Intrinsic safety
is cost effective
everywhere.
The adoption by the National Electric
Code of the International Electrotechnical
Commission standard allowing use of zones,
an alternative to the traditional division
method of area classification, was added to the
1996 code and greatly expanded in the 1999
code. The availability of two area classification
methods increases flexibility when choosing
the electrical equipment used in hazardous
areas. Though the traditional, North American
method of area classification in North America
is still useful, engineers can now use the zone
method for new installations or reclassifying
existing facilities.
The zone method allows designers to use a
wider variety of equipment and techniques
than the traditional method in all but Zone 0
applications, where intrinsic safety (IS) is the
only method that may be used. The only technique allowed in all area classifications worldwide is IS.
The designer’s choice of a technique
depends on the type of equipment required.
Low-power signal and process control equipment is readily available and easily applied in
all area classifications using IS, which is by far
the safest standard, regardless of the area classification. IS systems are not only safest but
also very cost effective. Further, IS is a truly
universal approach because devices certified
for a higher classification can be used in lower
classifications of a similar gas group and temperature rating.
Intrinsic safety’s advantages
When choosing a method of protection,
evaluate the following:
Are adequate field and interface devices available for the application? The number of IS field
devices and interfaces is large and steadily
growing. The devices now available include
transmitters, valve positioners, electropneumatic valve actuators, displays, currentto-pneumatic converter and pneumatic-tocurrent converter devices, proximity sensors,
and many more.
Can designers use general-purpose devices in a
hazardous area? The use of “simple apparatus”
devices that will neither store nor generate more
than 1.2 volts, 25 milliwatts, or 20 microjoules of
energy, as defined in NEC Article 504-2, is an
advantage of the IS method. Some examples are
light-emitting diodes, remote terminal displays,
thermocouples, and switches. These devices
require no certification when used with suitable
“associated apparatus” (intrinsically safe barriers).
Are live maintenance (powered loops) and
troubleshooting permitted in the hazardous area?
INTRINSIC SAFETY
52
June 2001 • www.isa.org • In Tech
Zone-based standard approaching adoption
By Nick Ludlam
The standard expected to form the basis for IS zones in U.S. practice is
undergoing review and balloting, and a final version may be available to
practitioners as early as next year.
ISA’s SP12.02 committee is responsible for developing a number of
standards related to the hazardous (classified) location type of IS protection.
The committee has prepared three standards: ISA-12.02.01–1999,
Electrical Apparatus for Use in Class I, Zones 0, 1 & 2 Hazardous (Classified)
Locations – Intrinsic Safety Requirements; ISA-RP12.2.02–1996, Recommendations for the Preparation, Content, and Organization of Intrinsic Safety
Control Drawings; and ISA-TR12.2–1995, Intrinsically Safe System
Assessment Using the Entity Concept. ISA-12.02.01–1999 resembles IEC
60079-11 (1988) but introduces differences to comply with American practice and to agree with the requirements of ANSI/NFPA 70, National Electrical
Code (NEC).
The ISA standard differs from the International Electrotechnical
Commission (IEC) standard in its requirement for an input voltage of
250 volts (V) for stand-alone associated apparatus, 120 V or 250 V for
non-stand-alone associated apparatus, and the addition of a section on
shunt diode zener barriers. Work on the standard started after publication of the 1996 NEC, which introduced the zone concept for hazardous
(classified) locations in a new Article 505.
ISA is presently planning to adopt a revised version of IEC 60079-11
(1999) as a standard. This work is nearing completion, and the draft is currently being balloted by the subcommittee. Once finalized, it is the subcommittee’s aim that this document will form the basis for IS for zones in the U.S.
One apparent problem is the existence of ANSI/UL 913–1997,
Intrinsically Safe Apparatus and Associated Apparatus for Use in Class
I, II and III, Division 1 Hazardous (Classified) Locations, and ANSI/UL
2279–1997, Electrical Apparatus for Use in Class I, Zone 0, 1 and 2
Behind the byline
Jim Peterson is a senior applications engineer at MTL, Inc. in
Hampton, N.H. His address is
peterson@mtlnh.com.
Hazardous (Classified) Locations. ANSI/UL 913 is the IS standard referenced in the 1999 edition of the NEC. ANSI/UL 2279 is one of the
documents listed in the NEC in Article 505 for zone concept. The differences among the standards appear to be considerable.
The ISA document emulates IEC 60079-11 and lists major technical
deviations in an annex. UL 2279 provides only the deviations from the IEC
document. This is because ISA has the exclusive U.S. rights to publish the
IEC text. ISA and Underwriters Laboratories (UL) are working together, however, to harmonize the technical requirements of the two U.S. versions of the
IS zone standards, and a correlating committee has been set up by ISA and
UL for this specific purpose. A co-published ISA/UL document will be available sometime in the future.
Although UL 913 covers a concept similar to both ISA-12.02.01 and
UL 2279 Part II, UL 913 is for Class I, II, and III, Division 1 hazardous (classified) locations. At this time, the major differences between the draft ISA12.02.01 standard and IEC 60079–11 (1997) standard include the addition
of a requirement for live maintenance and a section on control drawings.
The committee is now rewriting ISA-RP12.2.02 and ISA-TR12.02.03.
Work on these two documents is at an early stage, but the subcommittee is hoping to have the documents ready for publication in early 2002.
The ISA SP12.02 committee is working closely with ISA SP12.06 to
develop installation guidelines for intrinsically safe circuits in both Class
I, II and III, Division 1 hazardous (classified) locations and Class I, Zone
0 and 1 hazardous (classified) locations.
If everything goes according to plan, the three documents will be available early next year.
Behind the byline
Nick Ludlam is a senior engineer with Factory Mutual Global in Norwood,
Mass. His address is nicholas.ludlam@fmglobal.com.
The design of IS loops allows field devices and
cables to be safely serviced without shutting
down power. The low-power and fault-tolerant
circuitry associated with IS provides added
safety for personnel.
Is there an acceptable level of safety for both
personnel and property? By choosing IS, you are
afforded the safest proven method available.
Is mounting space a problem? IS devices, by
nature of their low-power design, are smaller
than other hazardous-area devices. The IS
interfaces can be the high packing density type,
which saves control-room real estate. Designers
can mount devices in Class 1, Division 2/
Zone 2 areas in general-purpose enclosures,
thus limiting expensive cable runs. Further,
incorporating IS eliminates requirements for
rigid sealed conduit because IS cable installation can use any method approved for ordinary
areas. Owners realize cost savings with installation, access to cables for maintenance and
repair, and overall cost of ownership.
Is adequate, clear, and concise technical support
of the chosen method readily available? IS is a
well-understood, universally accepted method
of protection with well-defined design, certification testing, and installation practices.
Are the field devices and interfaces compatible
with major control systems? A wide variety of
devices is available for interfacing IS field devices
with controllers, ranging from applicationspecific devices geared to the large distributed
control system and programmable logic controller manufacturers to custom solutions for
any conceivable controller configuration.
Traditionally, engineers use intrinsically
safe systems in only the most hazardous environments. IS need not be restricted to these
areas, however. The extremely high degree of
safety, low-cost maintenance, installation
ease, increasingly broad availability of components, and worldwide acceptance make IS
the most sensible choice for any hazardousIT
area application.