Wednesday, January 9, 2013
Difference Between ia & ib Intrinsic Safety Protection
Difference Between ia & ib Intrinsic Safety Protection
ProtectionType
ia
Apparatus is designed so that it is
suitable for Zone 0 & it will not cause ignition when the maximum permitted
values are applied to its terminals:
a)
in
normal operation with application of those non-countable faults which give the
most
onerous
condition;
b)
in
normal operation with application of one countable fault and those
non-countable faults
which
give the most onerous condition; and
c)
in
normal operation with application of two countable faults and those
non-countable faults which give the most onerous condition.
Normal operation means that the
apparatus conforms to the design specification supplied by the manufacturer, and
is used within electrical, mechanical, and environmental limits specified by
the manufacturer.
Normal operation also includes open
circuiting, shorting, and grounding of external wiring at
connection facilities. When
assessing or testing for spark ignition, the safety factors to be applied to
voltage or current are 1.5 in conditions a and b, and 1.0 in condition c.
(These factors should properly be called “test factors.”
The real safety of intrinsic safety
is inherent in the use of the sensitive IEC apparatus to attempt to ignite the
most easily ignitable mixture of the test gas with hundreds of sparks. This
combination of conditions is many times more onerous than any likely to occur
in practice.)
North American Intrinsic Safety
design standards are equivalent to ia intrinsic safety.
ProtectionType
ib
Apparatus is designed so that it is
suitable for Zone 1, & it is assessed or tested under the conditions of a
and b above, with a safety factor on voltage or current of 1.5 in the condition
of a and b.
It is likely that Level of
protection ic apparatus, equivalent to and replacing Type of Protection nL,
suitable for use in Zone 2, will be standardized.
Figure below shows typical grounded
and ungrounded two wire intrinsically safe circuits.
Figure below illustrates the
principle that every ungrounded conductor entering the Division 1/Zone 0, or 1
location in this case where the transmitter or transducer is located, must be
protected against unsafe voltage and current by appropriate associated apparatus.
The boxes with three terminals represent barriers,
independently certified protective assemblies, certified and rated according to
the national standard. Nonintrinsically safe devices connected to the barrier
need only be suitable for their location, and must not contain voltages higher
than the Um rating of the barrier.
Many barriers are passive,
consisting of current limiting resistors and voltage limiting diodes in
appropriate configuration and redundancy to meet the requirements of the
standard. Others have active current or voltage limiting. Both types may be
combined with other circuitry for regulating voltages, processing signals, etc.
The user should follow the recommendation of the intrinsically safe apparatus manufacturer
in the control drawing, or discuss the selection of appropriate barriers with
the barrier vendor, all of whom have proven configurations for many field
mounted devices.
-Ref: Body of Knowledge
Labels: difference between ia and ib protection, hazardous area, ia, ia category, ib, intrinsic safe circuit, Intrnsic safety, level of protection ia apparatus, level of protection ib apparatus
Subscribe to Posts [Atom]
Post a Comment
Note: Only a member of this blog may post a comment.