Class II Division 2 Occupancy Sensors
James Fisher, Michael Gazda, Russell Gee, JoAnne Hitchcock, Gledi Progonati, Chris Zannoni
Department of Electrical and Computer Engineering, University of Connecticut, Storrs, Connecticut 06269
Sensor Switch, 900 Northrop Road, Wallingford, CT 06492
Occupancy sensors detect human occupancy in an area using passive infrared technology. To use this
system in class II division 2 hazardous environments, it must meet specific safety regulations including
heat output, power, inductance, and capacitance for example. The focus for this device will be one that
meets dust ignition proof, intrinsically safe requirements in order to be compliant with the safety
regulations.
Background
Occupancy sensors are devices that allow for
automatic lighting of a room or area. Rather than using
the traditional system of turning on the lights via a light
switch, the sensor system will do it based on occupancy.
This takes out the human factor which could allow for
extended use of the lights even when no one is present
in the area. The current technology being utilized to
detect human occupancy is passive infrared (PIR) and
microphonics. PIR detects occupancy by absorbing the
infrared energy only in the range of which the human
body produces This energy is focused onto the sensor
with a lens such as a Fresnel lens (a flat lens that is
cheap to manufacture) [1]. This lens allows the sensor to
absorb the energy from many different locations and
when the temperature reading indicates a difference
between consecutive areas, occupancy is detected. A
signal is then sent from the device to a relay to
automatically turn on the lights. Microphonics is the use
of a microphone to use audible clues to indicate
continued occupancy in the area [2]. For example, if a
person enters the area, but remains still for an extended
period of time, the sensor may come to the conclusion
that the room is unoccupied. However, if the person is
talking or typing on a computer, for example, the
microphone in the device will pick up the sound and
keep the lights on. The devices are set to turn off the
lights after a predetermined length of time during which
the PIR is not detecting a presence and the microphone
is not detecting sound. Since the microphone only keeps
the lights on after a human presence has been detected
through PIR, there is no possibility of a false positive to
keep the lights on without human occupancy. In most
circumstances, occupancy sensor systems help to reduce
the cost of lighting by limiting power consumption only
to times of human occupancy. This could also extend
the life expectancy of the light bulbs thus reducing the
cost of both the electricity and frequent replacement of
bulbs.
There are so many benefits to the occupancy
sensor system that it would be a great investment for
any area where occupancy is infrequent or limited in
duration. Thus, an ideal location for these systems
would be a Class II Division 2 hazardous environment.
This environment is one in which combustible dusts are
present under abnormal circumstances. For example,
warehouses that store packaged flour or bags of
charcoal would be considered Class II Division 2
environments [3]. Although the presence of hazardous
dusts is infrequent, the area must be treated as if it were
constantly dangerous to reduce the risk of igniting any
dusts. In order to implement the occupancy sensor
system, it must first be guaranteed compliant with all
safety regulations for this environment. These
regulations include limits to the amount of inductance,
capacitance, power, and especially heat output of the
device among other regulations [4]. A device that is dust
ignition proof covers all of these limitations. By
definition, dust ignition proof means:
“A dust ignition proof component prevents
dust entering from outside. Arcs, sparks and
heat generated inside of the enclosure will
not be able to ignite the exterior surroundings
near the component.” [6]
Hazardous Dusts
Hazardous dusts fall into many different categories
depending on each one’s electrical resistivity. Within
class II division 2, there are three groups: E, F, and G
with group E being most hazardous and group G being
the least hazardous. Group E hazardous dusts are
typically metal dusts that are both conductive and
explosive. The resistivity of these dusts is less than 100
kΩ/cm. Some examples of group E dusts are aluminum,
calcium silicide, and titanium. Group F dusts have a
resistivity between 100 kΩ/cm and 100 MΩ/cm; only
some are conductive, but all are explosive. Carbon
black, activated Charcoal, and asphalt dusts are all
examples of group F. Finally, Group G dusts are not
conductive, but all are explosive with a resistivity
greater than 100 kΩ/cm. Flour, starch, grain,
combustible plastics and explosive, chemical dusts
would fall under this category.
abnormal circumstances. Figures 1, 2, and 3 below
show the ignition curves for inductive, resistive, and
capacitive circuits respectively. In order for the device
to be considered intrinsically safe, the parameters must
fall below the curves. Another possibility would be
separating the intrinsically safe portion of the device
from the parts that aren’t using an energy limiting
barrier such as a zener diode barrier.
Design Restrictions: Intrinsic Safety
If a device were intrinsically safe, it would
meet all the safety regulations for class II division 2
environments. Intrinsic safety by definition is:
"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)
By limiting the electrical and thermal energy of the
device, the possibility of ignition is removed making
this the safest standard available. An intrinsically safe
device is cheap, universally accepted, and easy to
install. The limitation on the energy of the device means
that the device would be very simple in design using as
few components as possible. This would make it
cheaper to manufacture and easier to install meaning
less installation costs for the consumer and an overall
less expensive product. Also, since the device is
incapable of igniting any hazardous dusts within the
class II division 2 dust groups (E, F, and G), it would be
universally accepted as a safe product. Thus, there
would be a larger consumer market available to sell the
product in.
An intrinsically safe device could be composed
of simple apparatuses [4]. A simple apparatus would be
one that comprised of components that store little or no
energy. There are energy curves related to each type of
system indicating the amount of energy allowed before
ignition would be caused. Similarly, there are
temperature restrictions depending on the dusts that may
be present in the environment. For intrinsic safety, the
device must not be capable of getting near the
temperature of lowest ignition under either normal or
Fig. 1: “Inductive Circuit Ignition Curves”
Fig. 2: “Resistive Circuit Ignition Curves”
enclosed within the box could ignite, or char, dust layers
or clouds near the device. Also, all raceways and
conduits made to the enclosure must be sealed and made
of either PVC or metal that meets the requirements
previously stated. Any wires going to the device need to
be supported to reduce the tension on connections that
could eventually expose the internal components to the
hazardous environment. The enclosure should have a
long life time to limit replacement costs and ensure the
longevity of the device, thus, it should be corrosion
resistant. Another significant consideration is the
transmissibility of the material used to make the
enclosure. For wireless communications to be possible,
the material must be chosen carefully.
Fig. 3: “Capacitive Circuit Ignition Curves”
Enclosure Considerations
In order to protect the device from the
environment as well as keep any internal sparks, arcs, or
explosions from normal or abnormal operating
conditions away from the environment, the enclosure
must be well designed. For this design, at minimum, a
dust ignition proof enclosure is required. OSHA
(Occupational Safety and Health Administration) has
many requirements for an enclosure to be considered
dust ignition proof. For example, the apparatus must
completely enclose arcs and sparks so that they do not
come in contact with the hazardous dusts in the
atmosphere [5]. Furthermore, it will not allow the
surface of the enclosure to become hot enough to ignite,
or char, the lowest combustion temperature dusts even
under surface dust accumulation conditions. Table 1,
shown to the right, demonstrates the different
temperature codes and the maximum allowable
temperature the device may reach to be coded as such.
It’s critical that the heat transfer of the enclosure remain
as low as possible with low thermal conductivity to
eliminate the risk of combustion. If the heat transfer
isn’t low enough, device temperature of components
North American
Temperature Code
T1
T2
T2A
T2B
T2C
T2D
T3
T3A
T3B
T3C
T4
T4A
T5
T6
IEC/CENELEC/NEC 505
Temperature Classes
T1
T2
T3
T4
T5
T6
Maximum Temp.
°C
°F
450
842
300
572
280
536
260
500
230
446
215
419
200
392
180
356
165
329
160
320
135
275
120
248
100
212
85
185
Table 1: “Temperature Codes with Max. Temperatures”
Cost Considerations
In order for the device to be useful to the
consumer, it must not cost more than the savings they
would expect from using it. Based on calculations, this
means the device should cost less than $430-$1200 this
range depends on the size and typical occupancy of the
area to use the sensor system. To meet this goal, the
material chosen for the enclosure should have a low cost
which meets all the previously stated requirements. It
should also have a low production cost. Injection mold
plastics, for example, would be ideal for this. The
enclosure design should also limit installation time. This
would reduce both the cost to install the system, which
in turn would reduce the overall cost of the product.
Another way to ensure a low-cost system would be to
use inexpensive components that meet the requirements.
References:
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Problems with Occupancy-Sensing Lighting
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2012.
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(Classified)
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Oct.
2012.
<http://www.osha.gov/doc/outreachtraining/ht
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<https://moodle.dce.fel.cvut.cz/file.php/17/Ma
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Protection." Automation Catalog 8000. R
Stahl, 2012. Web. 26 Sept. 2012.
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2012.
<http://www.engineeringtoolbox.com/hazardou
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