I have been asked lately a few
questions that I would like to share with all.
1. What is considered Intrinsically Safe, Non Incendive and Explosion Proof? How these methods are often employed in classified
locations.
First thing I have always done
is look at the certifications the manufactures have for the devices to be considered
and used. Do they have drawings that must be followed? Do they have noted that
impact the usage and area to be used? What does the NEC say and what rules govern
its use or installation?
Note: No single field device or
wiring is intrinsically safe by itself, but is intrinsically safe only when
employed in a properly designed IS system. Such systems are usually provided
with detailed instructions from the manufacture to ensure safe use and
maintenance
The National Electrical Code
defines Intrinsically Safe as follows.
Article 504 Intrinsically Safe Systems (Always
refer to this article)
504.2 Definitions
Associated Apparatus.
Apparatus in which the circuits are not necessarily
intrinsically safe themselves but that affects the energy in the intrinsically
safe circuits and is relied on to maintain intrinsic safety. Such apparatus is
one^ of the following:
1. Electrical apparatus that
has an alternative type of protection for use in the appropriate hazardous
(classified) location
2. Electrical apparatus not so
protected that shall not be used within a hazardous (classified) location
Informational Note No. 1: Associated apparatus has
identified intrinsically safe connections for intrinsically safe apparatus and
also may have connections for nonintrinsically safe apparatus.
Informational Note No. 2: An example of associated
apparatus is an intrinsic safety barrier, which is a network designed to limit
the energy (voltage and current) available to the protected circuit in the
hazardous (classified) location, under specified fault conditions.
When considering the use of this
type of Device in a Hazardous location it becomes an intrinsically safe system
Per NEC. Make sure all parts of the system are listed and labeled for this use.
“Intrinsically Safe” is the
practice where one is restricting the energy available to electrical equipment
in this potentially hazardous area so that a spark or hot surface cannot occur
due to any type of electrical fault. The IEC (International Electrical Code) States
that “Equipment must not store or generate more than 1.2V, 0.1A, 20 micro
joules and 25mW.” A certified IS interface (Barrier) limits the voltages and
current that can reach the equipment in the hazardous area under fault
conditions.
2. Can Explosion-Proof devices be used in intrinsically
safe systems? The answer is Yes and No! (There is one exception!)
An explosion-proof device will
generally be self-contained and may be certified as explosion-proof (or
flameproof Ex d in the IEC world). It cannot be used as part of an
Intrinsically Safe system as the energy storage parameters will not be
certified. This also applies to Ex e or Ex n apparatus - but connecting non-IS
certified components into an IS loop is still a major area of non-compliance.
There is one exception. An
explosion-proof switch with no other electrical connections can be treated as
"simple apparatus" and connected into an IS circuit on the basis that
it will not store or generate sufficient energy to cause ignition. Where both
types of protection are in use, this simplifies spares holdings etc. In this
case, always use items installed on an IS loop that are clearly marked to
distinguish them from those where protection relies on the explosion-proof
features of the switch.
When using cabling for IS loops
in raceways and cable trays remember they must be separated from all other
cables not Intrinsically safe by no less than 2 inches and the tray must be
labeled every 25 foot with the wording “Intrinsic Safety Wiring” and it must
remain visible after installation.
There are several tested pieces
of equipment that can be found where duel certifications have been granted due
the fact it has been devolved to meet the requirements of both methods of
protection.
Explosionproof is covered in Article 500, 501,502,503, 505 and 506
of the NEC
The NEC® includes definitions
for several types of protection techniques acceptable when designing products
for use in hazardous (classified) locations: Explosion proof, dust ignition
proof, dust tight, purged/pressurized, intrinsically safe, and hermetically
sealed. These definitions set the criteria that must be met by all components
installed in hazardous (classified) locations.
To meet the criteria for the
explosion proof rating, an enclosure must be able to contain any explosion
originating within its housing and prevent sparks from within its housing from
igniting vapors, gases, dust, or fibers in the air surrounding it.
Additionally, the NEC states
that equipment must meet the temperature requirements of the specific
application in which it is to be installed. This means that the operating
temperature of the motor (and its enclosure) or other component cannot be
greater than the lowest ignition/combustion temperature of the gases or dusts
in the atmosphere where the component is to be installed.
All components are labeled on their nameplate with the distinct
classification in which they have been tested and approved for installation.
This protection technique shall
be permitted for equipment in Class I, Division 1 or 2 locations.
Nationally recognized testing
laboratories such as Underwriters Laboratories and Intertek use marks to denote
that the products they have tested conform to the standards set by the (NFPA)
and by other international standards organizations. These marks, which include
UL, CSA, ETL, and others, can be looked for to determine compliance with the
standards. Products that do not bear these marks may not meet the requirements
of the NEC.
Explosionproof Equipment
An explosion proof component is a tested piece of equipment
that will prevent an explosion within the component from causing an explosion
outside of the equipment.
An explosion-proof or flameproof device has no limitations
on energy, and this protection method can be used for high-voltage or
high-current applications, as well as for devices with high levels of inductance
or capacitance.
Nonincendive Circuit, Equipment and Component.
Nonincendive circuits and field
wiring is defined in the US National Electrical Code (NEC) in Article 100 and
its use in Article 500 and 505. ISA Standard S12.12 "Nonincendive
Electrical Equipment for Use in Class I and II, Division 2 and Class III,
Division 1 and 2 Hazardous (Classified) Locations" further defines Nonincendive
systems. An IEC standard for this type of protection (called Type Ex
"n") is IEC 79-15 "Electrical Apparatus with Type of Protection
"n" ". Another reference is "Electrical Instruments in
Hazardous Locations, 4th. Ed", by Ernest Magison, published by ISA (ISBN 1-55617-638-4).
NONINCENDIVE, OR SIMPLIFIED PROTECTION METHOD
The concept of non-incentive
circuitry is defined by the National Electrical Code, NFPA 70, as "a
circuit in which any arc or thermal effect produced, under intended operating
conditions of the equipment, is not capable, under specified test conditions,
of igniting the flammable gas, vapor or dust-air mixture."
To better understand the two key
phrases—"under intended operating conditions" and "under
specified test conditions"—refer to ANSI/ISA-S12.12. Electrical Equipment
for Use in Class I, Division 2 Hazardous (Classified) Locations.
This method, when applied to
electrical apparatus, makes the apparatus incapable of igniting a surrounding
dangerous mixture during normal functioning.
Nonincendive and intrinsic
safety protection methods are both based on the prevention concept. However,
for the Nonincendive approach, the device or circuit is not evaluated for
safety under fault conditions. As a result, energy surges, equipment faults and
static electricity are not addressed. For this reason, Nonincendive devices are
not approved for Division 1.
The determination of whether a
circuit or system is Nonincendive is left to the user. Most end-users are
reluctant to install equipment classified as Non Incendive in Division 2
locations without further protection.
The ambiguity of the specification leaves enough doubt that the system
will be safe under both normal and fault conditions that the decision is often
made to employ intrinsic safety as the protection method.
The prescribed methods of
construction are similar to the ones required by the increased safety
protection method—specifically, relating to components, enclosures, connection
elements, surface temperatures, distances, etc.
This technique, due to its
nature, is allowed only in Division 2 where the probability of danger is very
low. This appears to be a limiting factor, but it is important to keep in mind
that approximately 80% of the hazardous locations in a plant are classified as
Division 2.
So in a nutshell we can use:
Intrinsic Safety.
Shall be permitted for equipment
in Class I, Division 1 or 2; or Class II, Division 1 or 2; or Class III,
Division 1 or 2 locations.
Explosion Proof Equipment.
Shall be permitted for equipment
in Class I, Division 1 or 2 locations.
Nonincendive Circuits.
Shall be permitted for equipment
in Class I, Division 2; Class II, Division 2; or Class III, Division 1 or 2
locations.
I hope this helps clear some
confusion up and you have learned something new or more. Please contact me if
you find errors in this or would like to add to it as a whole.
