Kilowatt Counter

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Kilowatt Counter


The conceptual design brief stated that the product must be (in order of importance)
Purpose: Reducing the total power used
Scope: University of Toronto residences
1. Cheap to manufacture
2. Effective in reducing phantom load (material does not affect this and thus will not
Conceptual Design Summary:
be discussed)
The Kilowatt Counter is an extension that plugs into the electrical outlet and the
3. Safe
appliance will be plugged into it. The device is capable of measuring the daily power usage
4. Small (the size of the device is outside the scope of this decision and is hence
and there is a set power usage limit on each device. When the user approaches a certain daily
limit, it will produce an alarm in the form of sound and blinking light. Once the limit is
5. Light
reached, it will temporarily cut the power through that socket for 10 minutes. If the limit is
In response to this, we have chosen to use CPVC in the main body of the device. The
reached again after the power cut, the power will be cut for double the previous time.
reasons for choosing CPVC are as follows:
It is a plastic
The following objectives are set by the conceptual engineering firm:
Below, a table has been compiled comparing the characteristics of a variety of possible

materials that could have been used to make the body of the device. From this, decisions
- Safety (metric: user safety)
were made based on the ranked objectives provided by the Conceptual Design Report.
- Effectiveness (metric: total reduced amount of energy)
- Small (metric: device's dimensions)

Cost US$/tonne
Highly Variable
Highly Variable
- Light (metric: device's weight)
1014 to 1016
- Manufacturing price (metric: price per device)
- Easy to implement (metric: time and steps required to install)
- Easy to use (metric: additional steps required to use)
Young's Modulus
Highly Variable
E (MPa)iv
860 - 2100
Detail Design Decisions:
Each design decision satisfies certain objectives mentioned in the parenthesis:
Temperature oC4

Density (g/cm3)
- Material (safety, small, light, manufacturing price)

- Reinforcement (effectiveness)
- Manufacturability (manufacturing price)
2 July 2011 price for Merchant bar according to
- Dimensions (small, light, easy to use, easy to implement, manufacturing price)
- Ease of installation ( easy to implement )
- The Algorithm (effectiveness, easy to use)

Of all the different materials, plastic is the cheapest to produce. This is because, although
We have chosen to use a thermoplastic for this device as it will ease the production of our
Steel may be cheaper on a ton by ton basis than plastic, plastic is 8 times less dense than steel.
gadget thus lowering production costs leading to it being cheaper. This is because we can use
Hence, for the same price, one will be able to buy a much larger volume of plastic and hence
several methods of production which take advantage of the material's thermoplastic
be able to make more products for the same price, resulting in it being cheaper to
properties9. For example, the main tube structure of the plug can be extruded through a mold
and then allowed to cool. Later, two disks (also made through the process of extrusion) will
be attached onto open sides using heat welding (this is using the thermoplastic properties to
Plastic was also chosen on account of being safe. In this case, safety was measured as a
facilitate production).
product of its insulative properties and its ignition temperature. From the table, we can see
that overall, plastic has the highest values for insulation, and second highest for ignition
temperatures. In this case, the insulative properties of a material were valued over its ignition
temperature as the danger of electric shocks to the user through the device were viewed as a
PolyVinylChloride (CPVC), High Density Polyethene (HDPE) and Polypropene. We chose
greater safety risk than the device melting due to overload. As plastic scored high on both
these as they are already widely used in the production of devices10 in today's world. We
accounts, it was chosen.
then found and compared the physical and electrical properties of these materials as shown in
the following table11
Of all the properties which were considered above, plastic comes out the worst in terms
of toughness with a Young's Modulus of about a sixth of the next lowest competitor.
However, we believe that this will not be a problem. This is because in today's world, plastic
Price ($
is almost omnipresent in household appliances. This lends strong evidence that plastic is
Volume Resistivity ()
capable of handling the rigours of residential life.
Surface Resistivity ()
1013 - 1014
Young's Modulus E
It is a Thermoplastic
There are three types of plastics in existence, Thermosets, Thermoplastics and
Density g/cm^3
Elastomers8. Thermoplastics are plastics which will repeatedly melt once hitting a certain
Temp. Soften/melt *C
temperature and be able to be reformed into another shape. This cycle can be carried on

From the following table, we have decided to choose to use CPVC. The reasons are
A Thermoset is different as it can only be melted once. Once it has cooled and "set", it
stated below:
will not melt anymore unless exposed to some very high temperatures.
An Elastomer is a type of plastic with a very high degree of flexibility, coupled with a
very low value of Young's Modulus. As such, Elastomers will be discounted as a potential

material as a rigid device shell is desired.
11 Data from

7 Phil Collins

"92.1 The temperature rise of a cord connector, when measured at the points
The cost of a unit mass of CPVC is substantially lower than that of Polypropene or HDPE.
described in 92.2 shall not be more than 30EC (54EF) when the device is carrying its
This was one of the factors leading to us choosing CPVC for the material as Cost of
maximum rated current."16
Manufacture is a very important objective stated by the Conceptual Design Brief. This will
Thus, a plastic that melts at 100-120 degrees will be well within the safe range for this
directly affect the cost of manufacture without having a negative effect on anything else
product provided the circuitry and other components of the device are within safe standards.
Volume resistivity
This means that our material will not melt, even when the device is overloaded.

Volume resistivity of the plastics did not come into consideration during the selection of
plastics as they are all within acceptable safe ranges and thus all fulfill the criteria of safety
The Conceptual Design Report explicitly stated that:

"The product will be designed mainly for cheapness of manufacture"
Surface Resistivity
Of all the three plastics, CPVC has the lowest surface resistivity. This is actually good as
If a product is simple to manufacture and does not require a large energy input during the
the surface resistivity of a material determines how difficult it is for static charge to build up
assembly process, the cost of manufacture will be much cheaper due to the reduction in
on it. Having a lower surface resistivity means that a product designed out of CPVC will
labor/energy costs. This is why Ease of Manufacture is one of the chief influences on the cost
have a smaller amount of static build-up within it. This is important as the static build-up
might cause harm to the internal circuitry.
of manufacture. As such, it will be discussed in detail.

Two main features of the item's manufacturability will be discussed:
Method of producing component parts
CPVC has the highest Young's Modulus. What this translates into is that it is very tough

and can withstand the greatest punishment of all the plastics. This is because it has a high
Method of assembly
Young's Modulus of 2957 MPa, which means that it is able to withstand large forces without

bending, thus protecting the interior circuitry.
In Figure 1 an exploded view of the shell of the product is displayed and the component

parts numbered. These parts will be produced in different ways. Parts 2, 3 and 4 will all be
produced by method of extrusion. This is because these particular pieces have a simple and
Melting Point
constant cross section which does not require a large amount of precision. Of all the methods

CPVC was also intentionally chosen for its low melting point because this would
of production (injection molding, extrusion blow molding etc17) the process of extrusion is
facilitate the production of the materials. This will hopefully result in a lower production cost
most suited for making these components.
(as less heat energy needs to be produced to melt the plastic so it can be worked into its
proper shape). According to the UL standard for safety,

16 UL #498 ISBN 0-7629-0028-8 Pg. 105
17 Information on production methods obtained from http://www.the-


As effectiveness is the prioritized objective mentioned by the conceptual engineering

design firm, reinforcement is one of the essential detail design decisions. The whole purpose

of our product is to regulate the amount of energy consumed by students who live in
residences. Thus, by reinforcing the device we can increase its effectiveness; otherwise,

students would simply remove the device from the outlet. By regulating the energy
consumption, we would be able to decrease the standby energy as well as raising student

awareness. As we strive to satisfy all the objectives, we need to keep in mind that user safety

is always our main goal.

In order to make sure that the students have no choice but to use this device, as desired by
Figure 1
Figure 2
the conceptual engineering firm, it needs to be fixed on the wall. Therefore, the kilowatt
Part 1 of Figure1 has a much more complex shape as can be seen in Figure2 as it has
counter will be attached to the wall with two screws and brackets on each side, in order to
apertures as well as screw mounts to which the electric meter will be anchored. Therefore,
prevent the user from taking it out. These brackets should be made of PVC as the body of the
extrusion is inappropriate as a method of producing this particular component. Instead,
device is.
injection molding will be used instead. This is more expensive to set up and run than
extrusion, however it benefits from being able to produce multiple relatively accurate copies
Regarding to screws, we have to choose between the steel screw and the plastic screw.
of the shape quickly.
The advantage of steel screw is its toughness and its longevity. However, for our device, the
bracket is plastic, so the steel screw does not make the connection between the device and the
The method of assembly will be achieve using a relatively simple "top-down" method
socket stronger. Even though plastic screw is not as strong as the steel one, it is good enough
shown in Figure 2 where separate components were installed on top of one another with only
for the purpose of our device.
gravity holding them in place until the screws were used to anchor them. This method was
used as it minimizes the amount of material needed to hold everything together (such as
In perspective of safety, plastic screw is non-conductive, while steel screw is conductive.
using glue to hold everything together), and some pieces (such as the button covers) are held
Therefore, plastic screw is much safer than steel one, since the screws are needed to be put in
together by being pinched in place.
the socket, and it may be connected to the power line.
These four component pieces can then be welded together at the edges using relatively
When we consider the cost of our product, the plastic screw is much cheaper than the
low temperatures of 65oC (from the low melting point of PVC). This low melting
steel screw. The price for a certain volume of steel is about 8 times as the price for plastic.
temperature means that less heat energy is needed in the production of this product and thus
The use of plastic screws also has its side effect. Since they are easy to break or take off,
its manufacturing costs will be lowered as well.
it's hard to make sure that students will not break the rule. However, one of the main

purposes of our product is to raise students' awareness, in terms of saving energy, so the
existence of our product will definitely meet the purpose.

The following diagram explains how we fix our product on the wall. The outlet and the
Another design decision is the interval in which the device will be shut off. Once the
screw are both plastic so that you can't just use the screwdriver to put the screw in to socket.
amount of power consumption approaches the daily limit, the kilowatt counter will
The solution is to use hand driller to drill two holes corresponding the screw position
temporarily cut the power for 10 minutes. If the power consumption increases again after the
power cut, the power will be cut for double the previous time, until it reaches the daily limit.
The goal of this product is to reduce standby power in order to save energy; however,
having a set limit for daily energy usage will make student life more difficult. In order to
reduce this negative impact, a melodic alarm is chosen for this device.
Since one of the objectives defined by the conceptual design engineering firm is that the
device needs to be small, dimension is another important engineering design decision.
The shape of the kilowatt counter is a cylinder with the diameter of 3.5 cm and the height
of 4 cm. The diameter of the device is chosen based on the standard outlet size, so that when
the device is attached to the wall socket it would not block the other outlets around it, which
results in user satisfaction. The height of the cylinder is chosen based on the fact that

according to UL standards (UL - 498) the height of any object plugged into an outlet not be
more than 5.1 cm, and it should be large enough so that the interior circuitry would fit inside
All things considered, for our product, using plastic screws is much better than using steel
of it.
screws in terms of functioning, safety and cost. Thus, we choose plastic screws to fix the
device on the outlet.
The shape of the device is a cylinder instead of a cube, in order to avoid any sharp edges.
This device will be sticking out of every outlet and there is a chance that people passing by

that outlet would hit this device. In order to satisfy another important objective, safety,
The algorithm:
cylinder is a better option for not having any sharp edges.

The algorithm that this device follows is very important detail design decision, since only a
reasonable algorithm will result in an effective and usable product.

While the appliance is operational, the device will count the total amount of energy that

has passed through it and display it on the screen. If it starts to approach a daily energy limit,
it will produce an alarm in a form of a sound and a blinking light. The first detail design

decision is that this limit of power consumption will be set by the manager of the residence,
since it will vary depending on the needs of the costumer.

Ease of Installation

One of the main objectives introduced by the conceptual engineering design firm is "Ease
of Installation". This device will be simply plugged into the existing outlet and attached with

screws. No modifications to the wiring circuits are required.

In order to make the installation easy, the reinforcement of the device is placed on either
side of the device. This will allow the installer to be able to attach the device to the existing

outlet without needing to go through any additional steps.

First the installer has to set the limit for the amount of electricity used per day then all
that needs to be done is to place the two screws on the brackets, plug in the device, and screw

This figure shows the front view of
it in place.

the kilowatt counter
We had three possible design options for reinforcing the device on the wall:

1. Using screws inside the device ( attaching the base of the cylinder to the wall) and
The screen and the two buttons are placed at the top of the cylinder with the length of
then attaching the main body of the device to its base:
3cm to make the power usage more readable for the user. Also the two brackets are placed at
This method would make it harder for the students to remove the device but it would
each side of the device for the purpose of reinforcement.
make the installation process more difficult as well.

2. Using the clipping method, where a piece of plastic would go through the grounding
hole in the outlet and then open up, so that the device would be fixed on the wall:

This method would make it impossible for the students to remove the device, since
the only way would be breaking that piece of plastic; however, it would make the

installation very easy. In addition, in case of repair it would be difficult to dissemble
the device from the outlet once it is installed.

3. Using brackets and screws on each side of the device seems to be the best option.
Since it is easy to install and it will not allow the students to easily remove the device

from the outlet. Therefore, this design decision satisfies the effectiveness as well as
ease of installation.
This figure shows the front view of the

kilowatt counter

Solid Modeling:
Top view:
Front view:

Side view:
Back view:

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