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News>Issue Listing>September/October 2005>Installation of Intrinsically Safe and Nonincendive Systems
| Installation of Intrinsically Safe and Nonincendive Systems |
With technology rapidly advancing, new developments in sensing and process control have increased the number of products using intrinsically safe or nonincendive circuits as their protection technique for hazardous locations. |
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| by Benjamin P. Schaefer and David P. Malohn |
With technology rapidly advancing, new developments
in sensing and process control have increased the number of products using
intrinsically safe or nonincendive circuits as their
protection technique for hazardous (classified)
locations as defined in Articles 500 through 505 of the
2002 National Electrical Code ANSI/NFPA 70 (NEC)1.
With lower installation and maintenance costs, these
protection techniques have found increased favor in
applications traditionally requiring explosionproof
devices. Though these concepts have been in use in
industry for many years, some confusion still exists in
regards to the installation of these devices. Following
the proper installation requirements is important to the
overall safety of intrinsically safe and nonincendive
circuits.
Intrinsically Safe Circuits and
Nonincendive Circuits
Intrinsically safe circuits and
nonincendive circuits use the same basic concept to
minimize the risk of ignition of hazardous atmospheres.
Both rely on energy limitation with some important
differences.
Nonincendive circuits are permitted in Division 2
and Zone 2 classified locations only. Intrinsically safe
circuits, however, are suitable for use in Division 1
and 2, and Zone 0, 1 and 2 classified locations. The
major difference between the two types of equipment is
that nonincendive circuits are evaluated for ignition
capability under normal operating conditions, while
intrinsically safe circuits are evaluated under fault
conditions. During an evaluation of an intrinsically
safe circuit, it is allowed to introduce up to two
independent faults per the requirements in the
applicable product standard in an attempt to increase
the energy available in the circuit. Under these
conditions, the circuit must still be incapable of
causing ignition of the specified gas either by
introduction of a spark (shorting of two points in the
circuit) or by thermal effects of individual components
within the circuit.
When determining the inability of a circuit to
cause ignition of a flammable gas or vapor, it is
necessary to consider not only the voltage and current
available, but also the capacitance and inductance in
the circuit. Since most circuits contain energy storage
components such as capacitors and inductors, the energy
stored by these components must be considered when
determining the energy available in the circuit. The
addition of capacitance or inductance to a circuit that
is otherwise incapable of causing ignition may cause the
circuit to become ignition capable. This is especially
critical when considering fixed equipment interconnected
by cables. These cables typically have known values of
capacitance and inductance which must be considered in
the final system installation. Before considering how
the capacitance and inductance can be accounted for in a
system, it is helpful to have a brief understanding of
the equipment. The following definitions, taken from NEC 500.2 and 504.2, are provided for types of equipment
related to fixed installations of intrinsically safe and
nonincendive circuits.
Associated Apparatus. "Apparatus in which
the circuits are not necessarily intrinsically safe
themselves, but that affect the energy in the
intrinsically safe circuits and are relied on to
maintain intrinsic safety." This type of equipment
contains components that are designed to limit the
voltage and current available to intrinsically safe
equipment to which it is connected. Associated apparatus
is generally intended to be installed in unclassified
locations, though it can be installed in classified
locations if another protection technique, such as an
explosionproof enclosure, is used.
Intrinsically Safe Apparatus. "Apparatus in
which all the circuits are intrinsically safe."
Associated Nonincendive Field Wiring Apparatus. "Apparatus
in which the circuits are not necessarily nonincendive
themselves but that affect the energy in nonincendive
field wiring circuits and are relied upon to maintain
nonincendive energy levels."
Nonincendive Field Wiring Apparatus. "Apparatus
intended to be connected to nonincendive field
wiring."
Control Drawing. "A drawing or other
document provided by the manufacturer of the
intrinsically safe or associated apparatus, or of the
nonincendive field wiring apparatus or associated
nonincendive field wiring apparatus, that details the
allowed interconnections between the intrinsically safe
and associated apparatus or between the nonincendive
field wiring apparatus or associated nonincendive field
wiring apparatus." Nonincendive field wiring
systems and intrinsically safe systems shall be wired in
accordance with the control drawing, as required by
501.4(B)(3), 502.4(B)(3), 504.10 and 505.15(C)(1)(g). The control drawing identification is required to be
marked on the product.
When referring to intrinsically safe apparatus and
nonincendive field wiring apparatus in the body of this
text, the term field device is used to indicate
both, unless specifically indicated otherwise.
Similarly, associated apparatus is used for both
associated apparatus providing intrinsically safe
outputs and for associated nonincendive field wiring
apparatus.
Combination of Apparatus
Associated apparatus and intrinsically safe apparatus
may be combined to form intrinsically safe systems. Much
in the same way, associated nonincendive field wiring
apparatus and nonincendive field wiring apparatus may
also be combined to form a system. These combinations of
apparatus are required to be listed together as a
complete system or the associated apparatus and the
field device are listed separately and then must satisfy
additional requirements.
Intrinsically Safe and
Nonincendive Systems
Apparatus listed as a
system are easier to install since all aspects of the
installation are defined on the control drawing provided
with the equipment. Figure
1 is a simplified example of a system control
drawing:
On the system control drawing, each piece of equipment
that may be connected in the system is defined
specifically by manufacturer and part number. Only the
specific equipment included on the control drawing may
be connected into the system. Other equipment, including
portable communication devices, may not be used since
any additional equipment could add either energy storage
components or additional power sources which have not
been evaluated in combination with the circuits.
Additional equipment could therefore render the circuit
ignition capable. Additional guidance for intrinsically
safe systems can be found in Wiring Practices for
Hazardous (Classified) Locations Instrumentation Part 1:
Intrinsic Safety, ANSI/ISA RP12.62 as indicated in NEC 504.1.
It should be noted that the control drawing
provides information regarding the interconnection
cable. In some cases, the drawing will indicate a
maximum length of cable to be used between the pieces of
equipment. In other cases, a maximum allowable value of
capacitance and inductance is given and it is necessary
to calculate the allowable length of cable based on its
specified capacitance and inductance per unit length.
The capacitance and inductance of the cable shall either
be taken from the cable manufacturer’s datasheet for
the cable, or when not available, then standard values
of 60 pF/ft for capacitance and 0.2 µH/ft for inductance
are to be used. These values will be found on the
control drawing, and are taken from the Standard for
Safety, Intrinsically Safe Apparatus and Associated
Apparatus for Use in Class I, II, and III, Division 1,
Hazardous (Classified) Locations, ANSI/UL 9133.
Entity Concept
Another method of
interconnecting equipment is commonly known as the
entity concept. This has some unique advantages over
systems. Using this concept, a listed associated
apparatus and a listed field device, not necessarily
manufactured by the same company, may be connected
together without obtaining a listing for the specific
combination. This allows for much greater flexibility in
the field when purchasing equipment. However, this
concept requires special attention be paid to the
control drawings, not only for the associated apparatus,
but also for the field device. The control drawings for
each piece of equipment are provided with electrical
ratings known as entity parameters, which represent the
limits of the equipment, either with respect to the
maximum output characteristics, or the maximum allowable
input characteristics. These parameters follow.
Associated Apparatus
Entity Parameters
Associated apparatus which
has been assigned entity parameters, whether providing
intrinsically safe or nonincendive field wiring
connections, are assigned the parameters shown below.
Where two designators are shown, they are
interchangeable.
Voc or Uo,
this represents the maximum open circuit voltage that
may be present at the specified terminals under the most
adverse conditions.
Isc or Io,
this represents the maximum short circuit current that
may be present at the specified terminals under the most
adverse conditions.
Ca or Co,
this represents the maximum capacitance that may be
connected to the specified terminals without
invalidating safety.
La or Lo,
this represents the maximum inductance that may be
connected to the specified terminals without
invalidating safety.
In some cases associated apparatus may also be provided
with a Po and/or a Lo/Ro parameter. The Po parameter
represents the maximum output power from the specified
terminals. The Lo/Ro parameter is an inductance to resistance ratio and
indicates the maximum inductance per ohm of resistance
that may be safely connected to the specified terminals
of the associated apparatus.
Intrinsically Safe and
Nonincendive Field Wiring Apparatus Parameters
Intrinsically safe and
nonincendive field wiring apparatus are provided with
their own unique set of parameters. It should be noted
that these parameters are based on maintaining a circuit
that is not ignition capable and are not necessarily
related to the proper functioning of the equipment. This
equipment is assigned the parameters shown below.
V max or Ui,
this represents the maximum voltage that may be
connected to the specified terminals of the device to
maintain safety. Operational voltages are required to be
lower than this parameter.
Imax or Ii, this represents
the maximum current that may be connected to the
specified terminals of the device to maintain safety.
Again, the operational current will be lower than this
parameter.
Ci, this represents the
apparent capacitance at the specified terminals of the
device.
Li, this represents the apparent
inductance at the specified terminals of the device.
Pi, this represents the maximum
power that may be supplied to the specified terminals to
maintain safety. Operational limits will be lower than
this power value.
Li/Ri, this represents the maximum
internal inductance to resistance ratio at the specified
terminals.
Combining Associated Apparatus
and Field Devices
When combining associated
apparatus with field devices, it is necessary to use the
above noted entity parameters. The entity parameters of
the associated apparatus must be compared with the
entity parameters of all field devices connected into
the system. When compared, the entity parameters must
satisfy the conditions shown in Table 1.
To illustrate the use of these parameters, the following
two drawings are provided. Figure
2 is a simplified control drawing for an associated
apparatus. Figure
3 is a simplified control drawing for a field
device.
On the control drawing for the associated apparatus, the
Voc parameter is 28 V. Since this value is less than or
equal to the Vmax of the field
device, 30 V, the parameters satisfy the required
relationship. The same holds true for the Isc and Imax values, since 93 mA is less than 100 mA. Next,
a determination of the maximum allowable cable length is
required. The total amount of capacitance at the
terminals of the field device, Ci,
plus the total capacitance in the cable, Ccable,
cannot exceed the allowable capacitance of the
associated apparatus, Ca.
Therefore, the difference between the Ca of the associated apparatus and the Ci of the field device will provide the maximum value of
capacitance allowed in the cable. In this case, the
cable could have 30 nF of capacitance (80 nF minus 50 nF).
Assuming the cable used in the system had a maximum
capacitance of 60 pF/ft., a cable length of 500 ft.
could not be exceeded. A similar calculation would need
to be made for inductance, and the shortest cable length
calculated between inductance and capacitance would be
the maximum allowed.
When multiple field devices are connected to the
same terminals of an associated apparatus, the Ci value of each field device must be added; the same
applies to the Li values.
On the control drawings of certain types of associated
apparatus with multiple circuits, an additional set of
entity parameters can be found. These parameters are
designated Vt and It, and are
provided in place of or in addition to Voc and Isc. The parameter Vt is defined as the maximum open circuit voltage that can
appear across any combination of terminals of a multiple
circuit associated apparatus. Similarly, It is defined
as the maximum current that can be extracted from any
combination of terminals of an associated apparatus. In
all situations, it is important that the associated
apparatus be used only as described on the provided
control drawing.
Other Notes on Control Drawings
In addition to details
about the interconnection of the circuits in an
intrinsically safe or nonincendive circuit combination,
additional information is provided on control drawings.
This information may include, but is not limited to, the
following:
• The maximum nonhazardous location voltage, often
designated Um, is the maximum
voltage that can be applied to each of the
non-intrinsically safe terminals of an associated
apparatus without affecting intrinsic safety. Generally,
this voltage is 250 V, though other values are possible.
Since associated apparatus is evaluated to determine
that it can maintain intrinsic safety even under fault
conditions at its non-intrinsically safe terminals up to
and including the Um value, the installer of the
equipment must ensure that the equipment powering the
associated apparatus does not use, store or generate in
excess of this voltage.
• Grounding considerations (see below for additional
details)
• Restrictions on routing, insulation or types of
cables
• Restrictions on the types of associated apparatus to
be used
Simple Apparatus
While the entity concept described
above considers that both the associated apparatus and
field device have been listed with the appropriate
parameters, there are field devices that can be
installed and considered intrinsically safe or
nonincendive even though they do not carry entity
parameters. Such a device is known as simple apparatus,
which is defined in 504.2 as an electrical component or
combination of components of simple construction with
well defined electrical parameters that does not
generate more than 1.5 V, 100 mA and 25 mW, or a passive
component that does not dissipate more than 1.3 W and is
compatible with the circuit in which it is used. Some
examples of simple apparatus include passive components
such as switches, junction boxes with terminal blocks,
resistance temperature devices (RTDs) and LEDs, and
sources of generated energy such as thermocouples and
photocells. These types of devices are not required to
be listed per 504.4, and can be installed using the
entity parameters for the associated apparatus as
described above for determining the allowable cable
length for an acceptable connection.
Grounding
Correct grounding is
critical to maintaining the safety of many systems. For
example, a common type of associated apparatus is a
shunt zener diode barrier, which uses zener diodes to
clamp the voltage at its intrinsically safe circuit
terminals to the value specified for its Voc or
Uo entity parameter. In order for these diodes to
perform this function, they must have a fixed reference
to earth ground. These types of barriers are provided
with ground terminal, and their control drawings provide
information on the requirements for connection to this
terminal. When the associated apparatus is connected to
an intrinsically safe device, it is normally required
that the intrinsically safe device be isolated from
earth ground. This is to prevent the possibility of a
ground loop occurring between the intrinsically safe
device and the ground terminal of the associated
apparatus. Since the voltage drop in the ground path
between the devices is unknown, this could have an
adverse affect on the intrinsic safety of the
connection. This is not a concern with some other types
of associated apparatus, e.g., galvanically isolated
barriers, which generally use a transformer and are not
required to be grounded to maintain intrinsically safe
circuits. This type of associated apparatus is sometimes
required for certain types of intrinsically safe
equipment, e.g., devices such as pressure transducers
that cannot be isolated from earth ground due to their
construction. The requirements for grounding of an
intrinsically safe circuit are provided in 504.50.
Always refer to the control drawing for the specific
devices to ensure that the correct grounding and
grounding electrode connections are made.
Wiring
Due to the limited energy
levels available in intrinsically safe and nonincendive
field wiring circuits, wiring methods normally not
permitted in hazardous (classified) locations can be
used in their installation. For example, in a Division 1
classified location, installation normally requires
threaded rigid metal conduit. However, per 504.20,
intrinsically safe circuits may be installed using any
wiring method allowed by the NECâ for unclassified locations. Similarly, per 501.4(B)(3),
502.4(B)(3) and 505.15(C)(1)(g), nonincendive field
wiring circuits can also be installed using unclassified
location wiring methods. It is important to note,
however, that these circuits may require wires or cables
having additional insulation to guarantee suitable
separation. NEC 501.4(B)(3), 502.4(B)(3), 504.20
and 505.15(C)(1)(g) requires that different circuits be
installed in separate cables or in cables where each
conductor is within a grounded metal shield or each
conductor has a minimum insulation thickness of 0.25 mm
(0.01 in.).
Separation of Intrinsically
Safe and Non-Intrinsically Safe Wiring
To reduce the possibility
of their interconnection, additional requirements exist
for the separation of intrinsically safe and
non-intrinsically safe circuits. NEC 504.30(A)
requires a minimum separation distance of 50 mm (2 in.)
be provided between intrinsically safe and
non-intrinsically safe conductors. This separation may
also be accomplished by using a grounded metal or
insulating partition, or by locating either of the
circuits in a grounded metal shield capable of carrying
the maximum prospective fault current should either
circuit be connected to earth.
Identification of Intrinsically
Safe Field Wiring
Raceways, cable trays, and
other wiring methods for intrinsically safe system
wiring shall be identified with permanently affixed
labels with the wording "Intrinsically Safe
Wiring" or the equivalent. NEC 504.80
states that the distance between labels shall not be
more than 7.5 m (25 ft.). Alternatively, color coding
may be used as a method of identification provided that
the color light blue is used, and that no other
conductors or wiring are colored light blue.
Summary
The incorrect installation
of intrinsically safe and nonincendive circuits can
result in a system that could become ignition capable.
Therefore, care must be exercised when determining the
suitability of the combinations of associated apparatus
and field devices. Furthermore, it is necessary to
follow the applicable requirements in the Code concerning grounding, wiring, separation and
identification. The control drawing provided with each
piece of equipment provides the necessary information to
determine the acceptability of the installation.
1 National Electrical Code (NEC), ANSI/NFPA 70,
National Fire Protection Association, Inc. (NFPA),
Quincy, MA USA, 2002.
2
ANSI/ISA RP 12.6-1995, Wiring Practices for Hazardous
(Classified) Locations Instrumentation Part 1: Intrinsic
Safety, ISA – The Instrumentation, Systems, and
Automation Society, Research Triangle Park, NC USA
3 ANSI/UL 913, Intrinsically Safe Apparatus and
Associated Apparatus for Use in Class I, II, and III,
Division 1, Hazardous (Classified) Locations,
Underwriters Laboratories Inc., Northbrook, IL USA.
Edward M. Briesch,
P.E., is senior staff engineer, hazardous locations, for
the Conformity Assessment Services Department of UL.
Benjamin Schaefer, a 17-year veteran of UL, is a primary
designated engineer for intrinsically safe equipment and
systems. In addition to intrinsically safe equipment, he
has worked with explosion-proof and nonincendive
equipment. He is a delegate to the IEC SC 31G
subcommittee on intrinsic safety and is a member of the
IEC SC 31G MT4 maintenance team for intrinsic safety,
and is also a member of the ISA S12.06 subcommittee for
Wiring Practices for Hazardous (Classified) Locations.
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