Final Project Proposal

Final Project Proposal
Class II Div II Hazardous Environment
Occupancy Sensor
Final Design Report
Joint ME (Team 26) and ECE (Team 166) Senior Design
ME 4972/ECE 4901
Fall 2012
Faculty Advisors
George Lykotrafitis (ME), Sung Yeul Park (EE)
Sponsored By:
Sponsor Advisor: David Behnke ([email protected])
Mechanical Engineering
Michael Gazda
James Fisher
Computer Engineering
Christopher Zannoni
Electrical Engineering
Christopher Zannoni
Joanne Hitchcock
Gledi Progonati
Engineering Physics
Russell Gee
This design report is a summary of the University of Connecticut Senior Design Project
sponsored by Sensor Switch. Sensor Switch is a manufacturing company which is an industry
leader in developing occupancy sensors for lighting control. The overall goal of the project is to
modify one of the company’s occupancy sensors for a Class II Division 2 environment. A group
of six engineering students from the University of Connecticut that consists of mechanical,
electrical, computer engineering, and engineering physics majors have been chosen to complete
this project. This design report will take an in-depth look at the process of producing a quality,
cost effective product for Sensor Switch. By the end of this report, the reader will fully
understand what the final design will look like and what this project will require to move
Table of Contents
*****Each section of this design report was written as stated above, however, it was put
together and edited by James Fisher, Michael Gazda, and Chris Zannoni. The abstract
was written by James Fisher
Definitions- All
Introduction – Russell Gee
a. Passive Infrared Sensor Technology – James Fisher
b. Intrinsic Safety – James Fisher
c. Enclosure – James Fisher/Michael Gazda
a. Hazardous Dust Limitations – JoAnne Hitchcock/James Fisher
b. Cost Limitations – JoAnne Hitchcock
Preliminary Design Solutions – Chris Zannoni
a. Design Solution 1 – Relay and Sensor Enclosed
b. Design Solution 2 – Relay Enclosed and Sensor Connected by a Cable
c. Design Solution 3 – Relay Enclosed and Sensor Connected (Wirelessly)
Design Process and Overcoming Challenges – Chris Zannoni
a. Vague Project Requirements
b. Initial Design Path and Difficulties
c. Overcoming the Challenges and Finding a Final Design Approach
Final Design Specifications
a. Material Properties – James Fisher
b. Enclosure Design Measurements – James Fisher/Michael Gazda
c. Zener Diode Barrier – James Fisher
Testing – Russell Gee
a. Sensor Testing - Chris Zannoni
b. Dust Circulation Testing
c. Pressure Testing
d. Impact Testing
e. Torque Testing
f. Thermal Endurance
g. Additional Testing
Summary – Gledi Progonati
Appendices – All
References – All
I. Definitions
a. Capacitor- A device used to store an electric charge, consisting of one or more
pairs of conductors separated by an insulator.
b. Class II Division 2 Environment- Dust not normally suspended in an ignitable
concentration (but may accidentally exist). Dust layers are present.
c. Class II Division 2 Group E- Metal dusts (conductive,*and explosive)
d. Class II Division 2 Group F- Carbon dusts (some are conductive,* and all are
e. Class II Division 2 Group G- Flour, starch, grain, combustible plastic or chemical
dust (explosive)
f. Conduit- A tube or trough for protecting electric wiring.
g. Conduit Fitting- apparatuses designed to connect conduits to each other or to an
entrance of an electrical enclosure.
h. Cut-Frequency- In physics and electrical engineering, a cutoff frequency, corner
frequency, or break frequency is a boundary in a system's frequency response at
which energy flowing through the system begins to be reduced (attenuated or
reflected) rather than passing through.
i. FM Approvals- A division of FM Global that offers worldwide certification and
testing services of industrial and commercial loss prevention products.
Recognized and respected across the globe, FM Approvals certification assures
customers that a product or service has been objectively tested and conforms to
the highest national and international standards.
j. FM Global- A Johnston, Rhode Island-based mutual insurance company, with
offices worldwide, that specializes in loss prevention services primarily to large
corporations throughout the world in the Highly Protected Risk (HPR) property
insurance market sector.
k. Fresnel lens- The design of this lens allows the construction of lenses of large
aperture and short focal length without the mass and volume of material that
would be required by a lens of conventional design. Figure A:1 shows a Fresnel
lens and Figure A:2 shows the equivalent conventional lens.
Figure A: Fresnel vs. Conventional lens
l. Inductor- A component in an electric or electronic circuit that possesses
m. Infrared Light - Infrared (IR) light is electromagnetic radiation with longer wavelengths
than those of visible light, extending from the nominal red edge of the visible spectrum at
0.74 µm (micrometers) to 300 µm.
n. Intrinsic Safety (IS) – The need to protect a piece of electrical equipment from
leading to the ignition of anything in its’ environment.
o. Low-pass filter- A filter that passes frequencies below a certain value and
attenuates frequencies above that value.
p. Micron- A unit of length equal to one millionth of a meter.
q. National Pipe Threading (NPT) - The U.S. standard for tapered threads used
on threaded pipes and fittings. The conical nature of this thread ensures a tight seal.
r. Occupational Safety and Health Administration (OSHA)? (already defined right
in beginning)
s. Passive Infrared (PIR)- A lighting control system that uses infrared beams to
sense motion. When beams of infrared light are interrupted by movement, the
sensor turns on the lighting system. If no movement is sensed after a
predetermined period, the system turns the lights off.
t. Polybutylene Terephthalate(PBT)- A thermoplastic polymer that is commonly
used for injection molding
u. Relay- Electrical device such that current flowing through it in one circuit can
switch on and off a current in a second circuit, essentially it is a switch.
v. Resistivity-A measure of the resisting power of a specified material to the flow of
an electric current.
w. Resistor- A device having resistance to the passage of an electric current.
x. Zener Diode - A Zener diode is a type of diode that permits current not only in
the forward direction like a normal diode, but also in the reverse direction if the
voltage is larger than the breakdown voltage known as “Zener knee” voltage.
II. Introduction
In an era of increasing energy costs, many companies are seeking ways to reduce
their power consumption. A simple way to lower energy costs, occupancy sensors are
an attractive method to limit power consumption of lighting and other electrical
equipment. Occupancy sensors are relatively simple: if a person is in a room, the
lights turn on; when they leave, the lights turn off. Most occupancy sensors, including
the specific sensor involved in this project, use passive infrared sensing technology to
detect a change in infrared radiation corresponding to that of a person and send a
signal to a relay that switches the lights on. This technology reduces costs by not only
saving power but also increasing the lifetime of lights and equipment, by allowing
them to turn off automatically when no one is present. In addition, occupancy sensors
eliminate the need for the user to physically turn on the lights when one enters the
room, which is a useful tool in the event one is unable to access a light switch.
While occupancy sensors are an incredibly useful and beneficial tool, they may
pose a problem in hazardous environments. If a combustible dust or gas is present in
the atmosphere, energy from the sensor (whether it is increased temperature from
electrical components or arcing from the switching of the relay) may be enough to
ignite the gas or dust. This may result in an explosion that can not only cause massive
amounts of damage to equipment, but may even result in injury or death to those
caught in the explosion. Therefore, like any electrical equipment in a hazardous
environment, an occupancy sensor must guarantee that it will not cause a dangerous
explosion. The goal of this project is to modify an occupancy sensor so that it can
safely be installed in a hazardous environment, specifically a Class II Division 2
environment—one where occasionally combustible dust clouds may arise.
This project is sponsored by Sensor Switch, based out of Wallingford, CT.
Founded in 1987, Sensor Switch designs and manufactures occupancy sensor and
photocell technology for a wide variety of applications, and is considered an industry
leader in innovation and production of occupancy sensor technology. Provided with
Sensor Switch occupancy sensors, it is this project’s goal to design and build a
product that can be installed in a Class II Division 2 environment.
III. Theory
This section will cover the basic theory of this project and the basis of the designs,
which will be further explained in the following subsections. As the final product will be
placed in an environment that could be surrounded by combustible dust, this modified
occupancy sensor has to protect all of its components from igniting this dust and creating
a possible explosion. There were different ways to tackle this project and create a product
that was effective as well as marketable.
a. Passive Infrared Sensor Technology
The occupancy sensor that will be used in this project employs Passive
Infrared Technology (PIR). This technology uses a set of sensors to pick up the
infrared waves given off from any object that has a temperature above absolute
zero. Human beings give off an infrared wave from their body temperature
between 8 and 12 microns. The energy that is detected is then used to send a
control signal back to the sensor, where it comes into communication with the
power supply and relay and consequently powers the light on.
However, infrared waves are only able to pass through certain materials. It is
unable to pass through simple materials such as glass, which hinders this project
from using cheaper materials. Therefore, alternatives need to be taken to get
around this concern. Figure 1 displays a block diagram of how this occupancy
sensor works.
Figure 1
There are two portions of this product that need to be considered when placing
it in a hazardous environment. The circuitry of the sensor, as well as the relay
which creates a spark, can lead to an explosion from the combustible dust.
Therefore, considerations have to be taken on how to protect these components.
Everything may be enclosed to protect it from the environment, or the circuitry
can be modified to be intrinsically safe. Either way, both of these components
have to be protected from interacting negatively with the environment.
b. Intrinsic Safety
Intrinsic safety is a way to protect the circuitry in the sensor from leading to
the ignition of the combustible dust by limiting the energy throughout it. This can
be done by modifying the circuitry, so that in the event of a short circuit, the
energy discharged by any component is below the Minimum Ignition Energy
(MIE) of the dust, and thus will not ignite a dust cloud. These constraints are
given by three different graphs below in Figure 2.
Figure 2
Starting from the top left, the first curve shows the ignition curve for a
resistive component, then the curve for a capacitive component, and the bottom
graph shows the curve for an inductive component. The curves on the three
graphs show where ignition will occur. In other words, if the point is above the
lowest curve, there is a possibility of ignition of the combustible dust. If this is the
case, the sensor is not intrinsically safe for the environment.
The most general method of solving an issue of intrinsic safety is simply
redesigning the circuit. Often, either simplifying or expanding subcircuits allows
eliminating the need for high-capacitance capacitors and high-inductance
inductors. For example, in a simple low-pass filter (Fig. 3) that filters out high
frequency signals coming from the microphonics sensor, the cut-off frequency of
this circuit is represented by Equation 1:
where R is the resistance and C is the capacitance. It is evident that increasing R
allows us to reduce C and still maintain the same cut-off frequency. While this is
a simple example, it shows the main process of redesigning the circuit for intrinsic
safety. First, the circuit must be understood, then the problems with intrinsic
safety identified, and finally it can be redesigned to meet the standards.
Figure 3: Low Pass Filter
c. Enclosure
Both the relay and the circuitry in the sensor must be protected. Since relays
are electrically operated switches that create a spark, such a spark could
potentially ignite the combustible dust in the product’s environment. The
occupational safety and health administration (OSHA) defines an enclosure in a
Class 2 Division II environment as follows:
“The apparatus is enclosed in a manner that will exclude ignitable amounts of
dusts and will not permit arcs, sparks or heat generated or liberated inside the
enclosures to cause ignition of exterior dust accumulations on the enclosure or of
atmospheric dust suspensions in the vicinity of the enclosure.” (OSHA)
The way this will be constructed will be covered in section ___ where all of
the specifications of this design will be covered.
IV. Limitations
This section will explore the limitations associated with the design of this device.
When operating an electronic device in a hazardous location, safety is a significant
concern. In the Class II Division 2 environment, there are many risks associated with the
potential presence of hazardous dusts and/or dust clouds. One of the primary concerns is
the potential to ignite these hazardous dusts. Storing too much energy in the system,
arcing sparks, and even operating at a certain threshold of voltage or current all pose a
threat to ignite dust.
Another design consideration for this device is cost. As almost everything is cost
driven in today’s market, cost is a large consideration. For a device to be cost-effective, it
must result in savings that are significantly greater than the cost of the device. These
factors must both be weighed carefully in the design of the occupancy sensor system.
a. Hazardous Dust Limitations
When dealing with a hazardous environment, either combustible gases or
combustible dust can be present. In this project, the environment present is a Class 2
Division II location, which is defined by OSHA as an environment that has
combustible dust abnormally present. The types of this dust are classified into groups
E, F, and G. Group E is a group of metal dusts, all of which are conductive and
explosive. Group F contains carbon dusts where some are conductive and all of the
dust is explosive. Lastly, group G consist of flour, starch, grain, combustible plastic
or chemical dust in which all are explosive. Because of these dust groups, this project
has many limitations. As stated earlier in section III. (Theory), these combustible
dusts require the relay and sensor to be enclosed unless they are modified to be
intrinsically safe.
b. Cost Limitations
Since the Class II Division 2 occupancy sensor is to be sold as a commercial
device, the cost of the product is to be taken into consideration. In practice, the cost of
the device should not be more than what the user would save by using it.
Furthermore, not only must it cost less, the savings should be significant. Specific to
this project, the user must not pay more for the device than what would be saved over
the course of three years’ use. In order to reduce the cost of electricity for the user,
this device must effectively reduce the usage of lights in the designated area as well
as run efficiently. Ideally, the device will run on a minimal amount of power and the
lights will only be turned on when a person is occupying the designated area.
Therefore, the device must be created to be as sensitive as possible to changes in a
room’s occupancy in order to minimize power consumption.
The amount of money that can be saved by switching to the occupancy sensor
lighting system is dependent upon how frequently the designated area is occupied and
the size of the area. In theory, more lights would be needed to adequately light a
larger area. By limiting the run time of all of these lights, the user would see more
overall savings than by limiting the run time of just one light. Furthermore, if the
designated area is frequently occupied, the lights would remain on nearly as often
with the occupancy sensor as without, so the savings would be minimal. Otherwise, a
minimally occupied area (such as a class II division 2 environment) would yield
much higher savings by only turning on the lights as needed. Table 1 shows that a
storage area, a typical area to be designated class II division 2, would be unoccupied
around 45-85% of the time. This percentage of time unoccupied allows for the
possibility of significant energy savings with an occupancy sensor lighting system.
Equation 2 shows the typical formula used to calculate savings:
X, Y, and Z represent size of area, percent time unoccupied, and savings.
Using 45% to calculate minimum occupancy, eq. 2 will yield $0.129 savings per
square foot per year. At a maximum unoccupied rate of 85%, the savings would be
$0.23 per square foot per year. Estimating a 100 square foot area, the savings of the
device would be between $38.70 and $69 over the course of 3 years.
Private Offices
Conference Rooms
Storage Areas
Hotel Meeting Rooms
Table 1: “Estimated Time Unoccupied”
V. Preliminary Design Solutions
After some basic research into a Class II Division 2 environment and using the
initial requirements and theoretical design facts provided by the sponsor Sensor
Switch, the team came up with 3 designs that would solve the problem at hand. The
following section will describe those designs in detail. This section will also provide
some comparison between the different designs
a. Design Solution 1 - Relay and Sensor Enclosed
In the initial meeting with Sensor Switch, the initial requirements and
some reported facts about the sensor were provided. They said that the sensor was
“very low power and [was] assumed to be intrinsically safe”. Throughout the
meeting, Sensor Switch expressed confidence that the sensor was intrinsically
safe. However, many group members expressed doubt, based on the words
“assumed to be”, that the sensor was actually intrinsically safe. With this in mind,
a simple option was explored that would meet the design requirements if the
sensor was found later to not be intrinsically safe.
The first design solution to solve the problem of the sensor not being
intrinsically safe was to enclose both the relay and the sensor inside a dust-tight
box. The dust-tight box would have a window made of a material that would
allow infrared (IR) waves to pass through to the sensor. Figure 4 below describes
this design in more detail.
Figure 4: Design Solution 1 – Relay and Sensor Enclosed
Although this design did solve the problem of keeping the intrinsically
unsafe sensor out of the hazardous environment, it was quickly rejected based on
some brief research into the IR-transmissible material needed for the window of
the enclosure. The cheapest material (germanium) that would allow IR waves to
pass through it and was strong enough to be in the required environment would
cost $150 for a 10mm-diameter window. With this high cost for the window
alone, manufacturing the sensor would not be cost-effective.
b. Design Solution 2 - Relay Enclosed and Sensor Connected by a Cable
Using the assumption that the sensor was intrinsically safe, a solution
believed to meet the initial requirement was developed. This solution involved the
relay only being enclosed in a dust-tight box, with the sensor connected by a cable
to the box with a dust-tight connector. The sensor would not be protected from the
environment as it is assumed to be intrinsically safe. This option would be more
cost-effective as it would require fewer components, be cheap to design, and be
cheap to manufacture. Since this was an attractive approach, we came up with a
rough 3D model to present this design. Figure 5 below shows a picture of this 3D
model and displays this design in more detail.
Figure 5: Design Solution 2 - Relay Enclosed and Sensor Connected by a Cable
Despite the above solution’s simplicity, the initial research into the
hazardous environment for this project hinted at possible problems with cabling in
a hazardous environment. These possible problems included length of wire,
connection restrictions between the IS circuit and the box, and many industry
regulations which were initially hard for the team to learn and understand. These
possible problems led to the following third solution to avoid any cabling
c. Design Solution 3 - Relay Enclosed and Sensor Connected (Wirelessly)
The third solution involves the relay once again being enclosed in a dust
tight box; however, this solution has the sensor on the lid of the box. In this
design, the sensor would be powered by inductive coils on the sensor and in the
box, and would communicate with the box wirelessly; alternately, the cable could
just go straight through the box’s top via a small hole, later filled with sealing
putty. A rough 3D model was constructed of this design. Figure 6 below show a
picture of this 3D model and depicts this design in more detail.
Figure 6: Design Solution 3 - Relay Enclosed and Sensor Connected (Wirelessly)
Using wireless power and communications would allow the box not to be
punctured, thereby preventing dust ingress. This design simplifies the installation,
as no wires would need to be connected from the sensor to the box, and it would
also separate the IS circuit in the hazardous environment from the vulnerable nonIS circuit inside the box. Using the initial hazardous environment research, it was
determined that the components needed for the wireless power and
communications would be safe enough to work in this type of environment.
This design, along with the others detailed in this section, gave some
preliminary ideas to collaborate with Sensor Switch on this project. However, as
we moved forward in the design process, there were several obstacles that we
encountered which made it difficult for us to proceed.
VI. Design Process and Overcoming Challenges
With any project, an initial design path is necessary for timely project completion.
However, the initial information given by Sensor Switch was very limited and vague. All
we were given was a short description of our project in general terms. Due to this, our
initial design path had many challenges to overcome. The following section will describe
the design path, the challenges that were presented to us, and the ways we overcame
those challenges.
a. Vague Project Requirements
Sensor Switch had very limited information about this project in its initial
stages. All that was presented was the following:
“…design an occupancy sensor intended to control the lighting in a Class
II, Div 2 location. Class II locations are defined as areas where
combustible dust exists (grain elevators, flour mills, etc.) Div 2 locations
are those where the hazard is not normally present.”
It was also provided that the design was to include one of their current products,
which would need to be modified to work in this type of environment.
This vague information presented numerous questions and challenges
immediately to the team. One major question that arose, was what regulations
must be followed in order to design a circuit and an enclosure to work safely in
this type of environment? Another question, was how to get this design certified
and tested? With these questions in mind, an initial design path was made for the
semester which is presented in the next section.
b. Initial Design Path and Difficulties
The initial design path, which was developed soon after our initial meeting
with Sensor Switch, is detailed as follows:
1. Determine what regulations pertain to our project.
2. Read those regulations and create a specifications sheet that we could use
as a guideline for our design.
3. Brainstorm solutions to problems based on this specifications sheet.
4. Research and validate solutions, based on cost and feasibility.
5. Make initial designs.
6. Finalize designs.
However, step one in this path seemed to find more questions than it answered.
Upon researching, a virtual firehose worth of information was found. The
information that was found came from many different sources, including
Underwriters Laboratory, FM Approvals, National Electric Code, International
Electric Code, OSHA, Canadian Standards Association, and many others. Each of
these sources had many different specifications and regulations pertaining to this
project. To complicate things further, many of these specifications and regulations
were vague and hard to follow. Some of these sources seemed to say very similar
things but with slight differences. However, it was quickly realized that the only
way to move forward on this project was to start reading all these regulations and
determine what was relevant to this project. With this massive amount of reading
and research, some information was found that seemed to be very similar to most
of the specifications. This information, which is presented in section II of this
report, pertains to intrinsic safety and how that limits the components that can be
used in a circuit exposed to a Class II Division 2 environment. Once this useful
information was found, which related mainly to the circuit, the sensor that was
provided needed to be accessed to determine if it met the IS requirements.
However, this information was limited on the amount that could be found. Due to
Sensor Switch’s limited knowledge about Class II Division 2 environments, it was
decided between the team and Sensor Switch to reach out to a Nationally
Recognized Testing Laboratory. This is when the team got in contact with a
company called FM Approvals.
c. Overcoming the Challenges and Finding a Final Design Approach
FM Approvals was contacted on October 18th, 2012 and about a week later
they responded with very exciting news. Using the website to gain information on
this project, FM Approvals decided it was in their best interest to give as much
help as was needed on this project. Multiple phone conversations with them led to
an in person meeting.
The meeting with FM Approvals was on November 8th, 2012. This
meeting was one of the most significant developments in this project. In this
meeting, FM Approvals went over the protection methods and regulations that
pertained to the project as well as the testing that our designs would be subjected
to, along with the time and cost of such testing. They also informed the team that
the sensor that was provided by Sensor Switch was not intrinsically safe. Through
this information, they helped figure out what part of the designs might or might
not pass the tests.
In addition, they directed the team to the exact regulations needed to
follow to design and build a circuit and enclosure to pass the industry tests so that
it could work in Class II Division 2 environments. The regulation to follow to
design an IS occupancy sensor is FM3610, which refers to ANSI/ISA60079-0 and
ANSI/ISA60079-11 standards. The regulation to follow to design a “dust ignition
proof” box to enclose the relay is FM3616. This information was brought to
Sensor Switch to help decide on a final design approach.
A meeting was arranged with Sensor Switch on November 20th, 2012 to
discuss the FM Approvals meeting and to present the final design approach. In
this productive meeting, the possible solutions were discussed along with all of
the information that was collected over the semester. Sensor Switch finally
decided that the team should proceed with either of two possible solutions; one,
see if a Fresnel lens could be found that would meet the requirements of “dust
ignition proof”, or two, design a IS occupancy sensor with the relay enclosed in a
dust ignition proof box. Finally this was brought this to the UConn senior design
advisors attention, who concluded that the second option was the best. This design
is detailed in the next section.
Final Design Specifications
This section of the design report will include all the specifics of the final
design for the enclosure and how to approach the rest of this project. During the
course of our research, we have concluded that the relay will be enclosed in a casing
while the sensor circuitry will be designed to be intrinsically safe. This means that the
sensor will be attached to the lid and out in the open in the Class 2 Division II
environment. The wiring will be directly connected through a hole in the lid and
sealed shut with putty. There will be a Zener Diode barrier present in the wiring
between the outside environment and the enclosure. The following subsections will
take an in-depth look at our final design.
a. Material Properties
The material used for the enclosure was a very important step in creating
the design because of the testing requirements for this environment. The first
decision that had to be made was choosing the material type (metal vs plastic).
Since plastics are easier to manufacture in high volume, this was the material
chosen, which we then narrowed down to certain types of thermoplastics. This
research led to a certain type of polymer called Polybutylene Terephthalate
(PBT). It is widely used in electronic applications, and its mechanical and thermal
properties are very appealing for a Class II Division 2 environment. It is a very
compatible plastic with plastic injection molding, which is vital to manufacturing
in high amounts. The properties of this polymer are shown in Table 2 below.
Young’s Modulus
Shear Modulus
Tensile Strength
Yield Strength
Thermal Conductivity
Melting Temperature
Table 2
These properties shown above provide us with a strong and durable
material that will have a long service life. PBT will be more than suitable to work
in this environment and will prove to make this product compatible with a Class II
Division 2 environment.
b. Enclosure Design Measurements
The enclosure will be of a cylindrical shape which will have a lid that will screw
into it. The base of the enclosure is shown in Figure 4.
Figure 4
The extruded square inside the base of the enclosure is where the relay will be placed.
This square will act as a holding place for it. There are two knockouts on the side of
the enclosure, used for conduit fittings that will be provided with our product. The
fittings that have been chosen are the Thomas and Betts’ H-100-Tb (Figure 5). The
dimensions of this fitting are shown in Table 3.
Figure 5
Table 3
Diameter A
Diameter B Diameter C Max. Panel
Throat Diameter E.
Thickness D
2 in.
1.8125 in.
1.0625 in.
.25 in.
1 in.
This conduit fitting provides for a dust-tight seal 360 degrees around the fitting. One
side of the fitting is placed on the inside of the knockout, while the other is placed on
the outside. They are screwed together and the teeth of the fitting bite into the
enclosure, which creates a dust-tight seal. The knockouts are molded onto the
enclosure using fillets. The reason for the use of fillets is that a fillet is able to spread
the stress applied over a larger area than a straight edge. This will reduce the risk of
failure in the mechanics of the design. The lid is screwed into the enclosure using
National Pipe Threading (NPT), a type of threading that tapers creating a conical
shape. By using this thread, the design will ensure that it is dust-tight. The dimensions
of the enclosure are shown in Table 4.
Base Height
3.5 in.
Base Outer Diameter
4.75 in.
Base Inner Diameter
4.50 in.
Base Thickness
.25 in.
Knockout Outer Diameter
2.00 in.
Knockout Inner Diameter
1.00 in.
Knockout Thickness
.25 in.
Knockout Fillet Radius
.125 in.
Bottom Fillet Radius
.200 in.
Table 4
Lastly, the sensor will be attached to the enclosure by a very simple method.
Two small prongs will be molded onto the sensor and then slipped into a track onto
the lid. Once the sensor is turned, the prongs will lock into the track, therefore
suspending it and attaching it to the lid. See Figure 6 to see this design and Appendix
A for drawing sheets of the final design.
Figure 6
c. Zener Diode Barrier
Enclosing the relay and leaving the sensor open to the environment as an
intrinsically safe circuit requires an energy limiting barrier between the two
components. The reason for this is that if there is a surge present, this barrier will
stop the surge from reaching the intrinsically safe circuit and hazardous area and
creating the possibility of an explosion. The energy limiting barrier that we will
use is a Zener Diode Barrier. The way this barrier works is when the voltage rises
above a certain point, the barrier immediately grounds the circuit. By doing this,
the intrinsically safe circuit will always remain protected from the hazardous
VIII. Testing
After design and production of a prototype, it is necessary to test the product to
ensure that it meets requirements and will remain safe in a Class II Division 2
environment. Through testing, we can prove that the product cannot and will not pose a
threat of dust ignition. In addition to testing the product in normal operating conditions,
other tests shall be conducted to ensure that even under adverse conditions, safety and
reliability can be guaranteed.
a. Sensor Testing
Physical testing for a simple circuit is usually not very different from
simulated results. However, since we are working with such stringent
requirements, mistakes or slight differences in values could produce catastrophic
results. This is why it is important to do exhaustive testing on our sensor circuit.
The testing that we plan to do is actual real world operational tests as well as
detailed component testing to determine exact voltage, current, capacitive, and
inductive values in the circuit. Also, if time permits and a safe area can be
provided, we plan on testing this circuit under short circuit conditions while in a
simulated class II division 1 environment (explosive dusts, always present) to see
if combustion occurs. We believe that the physical testing along with the
simulated testing we can perform will be sufficient to determine if our design will
pass the testing by FM Approvals.
b. Dust Circulation Testing
The first and most basic component of physical testing is the use of a dust
circulation chamber to show that no dust can penetrate the enclosure. This
requires either the purchase or construction of a chamber that will circulate dust,
while simultaneously drawing a vacuum within the enclosure. In this manner, dust
circulating in the chamber is encouraged to penetrate the enclosure. If, after being
placed in this dust circulation chamber for an extended period of time, no dust is
drawn into the enclosure, it is proven that the enclosure is in fact dust tight and
will sufficiently protect the relay from any dust.
Commercial dust circulation chambers cost thousands of dollars, and it does
not make fiscal sense to purchase such an apparatus for the testing of the
enclosure. Thus, a chamber will be custom-built by the team for dust penetration
testing. The chamber will consist of two subsystems; the chamber itself, which
will circulate dust, and a vacuum system that will draw an appropriate vacuum
within the enclosure.
The chamber itself will be constructed simply. The current design consists of
a 2x3x2ft box, constructed from aluminum. A sealed lid on the top of the chamber
will allow maintenance and testing setup. Supported on a platform, the enclosure
will sit in the middle of the chamber. A large clear window (made from clear
acrylic) will occupy the front of the chamber, allowing the user to monitor the
enclosure during testing. A small light will illuminate the enclosure on the inside.
At the bottom of the chamber, a funnel, in the form of a steep walled pyramid,
will channel falling dust into a commercial blower. This blower will draw the dust
in and blow it out of the bottom of the chamber through a duct that connects to a
wall of the chamber. Thus, dust will be chaotically circulated throughout the
chamber as the blower draws dust from the bottom and blows it back into the
chamber. Talcum powder will be used as dust for the purpose of testing.
Electrical vibrators will be placed on the outsides of the funnel to help prevent
dust from sticking to the funnel walls and assist in channeling dust into the
The vacuum system will create a vacuum in the enclosure. Of the two holes in
the enclosure, one will be blocked off with an airtight seal. An airtight pipe will
be connected to the second conduit entrance and will leave the chamber through a
hole. An air flow sensor, vacuum regulator, and dust filter will be connected in
series, and an electric commercial vacuum pump will draw the vacuum. The
vacuum regulator ensures the pressure within the enclosure does not fall below
the minimum pressure of 200mm of water, as specified in the FM Global 3611
document. It is important to filter out any dust from the system before reaching
the vacuum; in the event of dust leaking into the enclosure, it must be ensured that
dust cannot reach the vacuum pump, as it may cause damage. The air flow sensor
will be used to monitor the volume of air drawn out of the enclosure. If an
extraction rate of 40-60 times the volume of the enclosure is achieved, the test
will be conducted for 2 hours. In the event that the extraction rate is less than 40
volumes per hour, the test will continue for 8 hours, or until 80 volumes have
been extracted, whichever comes first (Section 14.3b, FM Global 3611).
c. Pressure Testing
Prior to any dust circulation testing, including testing after
impact/torque/thermal endurance, etc, the enclosure will be subjected to positive
internal pressure of at least 2kPa for at least 60 seconds, with all entrances sealed.
d. Impact Testing
The product will be subjected to an impact test. The enclosure will be
attached to a rigid installation. A 1 kg test mass, with a 25mm diameter steel
hemisphere impact point, will be dropped from 270mm, for a total impact energy
of 2.7 J. Impact testing will occur at room temperature, as well as the lower and
upper approved temperature according to enclosure specifications. After impact
testing, the enclosure will be tested in the dust circulation chamber to ensure it
remains dust-tight. (FM Approvals, 3600)
e. Torque Testing
Each conduit opening will be fitted with a test plug, The plug will be
tightened to the test value of 90 N-m. The enclosure will be tested in the dust
circulation chamber at room temperature after the torque testing, and it must
remain dust-tight. (FM Approvals, 3616)
f. Thermal Endurance
For 2 weeks, the enclosure will be heated to (95+/- 2)°C and (90+/-5)%
humidity, followed by 2 weeks at (20 +/- 2) °C above maximum service
temperature. The enclosure will be then tested in the dust circulation chamber.
The enclosure will also be placed for 24 hours in an ambient temperature (5-10)
°C below minimum service temperature. The enclosure will be then tested in the
dust circulation chamber. (FM Approvals, 3616)
g. Additional Testing
Additional testing, such as crush testing and moment testing, may also be
required, based on necessity, materials, and specifications.
IX. Summary
In this design report, we have shown that an occupancy sensor system would
be an appropriate and cost-effective 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, where combustible dusts are present
under abnormal circumstances. 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. Therefore, in order to implement the occupancy sensor
system in such an environment, this sensor must meet all safety regulations. If the
sensor was intrinsically safe and the relay enclosed, it would meet all the safety
regulations for Class II Division 2 environments. To modify the sensor circuit and
make it intrinsically safe, we will include limits to the amount of inductance,
capacitance, power, and especially heat output of the device. Also, to show that the
final design can work properly in this type of environment, tests will be performed
similar to FM Approvals (the industry standard). This product will meet the highest
standards of occupancy sensors in the market today while producing a cost effective
and appealing option for companies.
X. Appendix A: Drawing Sheets of Enclosure
a. Side View of Enclosure Base
b. Top View of Enclosure Base
c. Side View of Enclosure
XI. References
2012, “Company Overview.” from
2012, “Light Guide: Occupant Sensors.” from
2012, “Sensor Switch.” from
David Behnke, 2012, Director of Engineering Sensor Switch, Sponsor Advisor, private
David Behnke, 2012, Director of Engineering, Sponsor Advisor Sensor Switch, private
Hoffman, “Classification of Hazardous Locations.” from
John F. Crossen, 2012, Advanced Engineer FM Approvals, private communication.
Minimum Ignition Curves." GM International. GM International, n.d. from
“Occupancy Sensors." Infohouse. Utility Savings Initiative, Feb. 2004. from
OSHA Office and Training Education, 1996, “Hazardous (Clasified) Locations.” from
Paramount Industries, Inc., “NEC Hazardous Locations Definitions.” from
Principles of Explosion Protection,” Cooper Crouse-Hinds.
“Rhett, Herman. "An Introduction to Electrical Resistivity in Geophysics." Radford, 22 Mar.
2001. from
Siemens, 1997, S7-300, M7-300, ET 200M Automation Systems Principles of Intrinsically-Safe
Design, Siemens AG, 6 Chap.
Underwriter’s Laboratory, “Class II: Combustible Dusts.” from
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