Evan's Space

Wonders of Physics


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Double-Insulated Appliance – Air purifier in the classroom

Double-insulated appliances are common around us. The air purifiers in our classroom are one good example.

In these appliances, only two wires are needed, namely Live and Neutral wires. Earth wire is not required as the casing is made of insulator like plastic for this case.

The classic air purifier in our classroom
The “double squares” symbol means that this appliance is double-insulated.
As you see, the earth wire is not required.


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Why the fuse and switch must be on the live wire?

A common mistake is to state that the reason for switch to be on the live wire is “so that the switch can turn on/off the appliance”. This explanation is wrong.

Refer to the view for the reason why switch and fuse must be on the live wire.

IMG-0582.JPG

In general, in the event where the fuse is blown or the switch is open, it disconnect the appliance from the high potential of the live wire. So the appliance will no become ‘live’ and it is save to touch even though there is a fault.

 


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Why do we need earth wire and fuse?

Simply put, electric appliance can be very simple with just a live wire bring in the current and a neutral wire to bring the current out. The appliance can just work like this forever (without the hassle of having earth wire, fuse etc), provided there is no electric fault developed.

Both the earth wire and the fuse are safety features to protect the user and the appliance respectively. Take a look at the video below to understand the rationale.

Recall these notes given:

Function of earth wire.JPG

 

Function of fuse.JPG

Click here for other related posts

double-insulation

3 pin-plug

 


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Why lighting circuit should be connected in parallel?

Lighting circuit, in fact all circuits at home, should be connected in parallel.

Reasons being:
When one bulb is spoilt or switched off, the rest of the bulbs can still function normally at normal brightness. This is because the potential difference across each bulb in the branches remains the same.

Refer to series and parallel circuits summary.

If the lighting circuit is connected in series, when one bulb is spoilt or switched off, it will be an open circuit and no current can flow through the circuit. Hence all the bulbs cannot function.

Why lighting circuit should be parallel

Refer this post for the concept of earth wire and the fuse


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How a fuse works?

A fuse is a safety device that is added to an electrical circuit to prevent excessive current flow. It has the same function as a circuit breaker. However , a fuse must be replaced once it melts (blows). A circuit breaker can be reset after it trips.

Electric symbol for fuse
Capture

Fuse is connected to the live wire. It consists of a short piece of thin wire. In the event of an electrical fault, when a current that exceeds its fuse rating flows through, it heats up and melts (fuse blows). When the fuse blows, the electrical appliance is disconnected from the high potential of the live wire (usually 240 V). Hence it protects the appliance and the user.

The choice of fuse is always slightly higher than the actual current flowing through the appliance, and it has to be a whole number. Typical household fuses off the shelves are rated at 1 A, 2 A, 3 A, 5 A, 10 A and 13 A. But in theory, we just state a fuse which is slightly higher and is a whole number.

 


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SP N2007 P1 Q16 – Which events will cause the fuse to blow?

An electric cable contains three wires live, neutral and earth. The cable is correctly wired to a plug which contains a 3A fuse. The insulation becomes damaged and bare metal wires show.

Five possible events can occur.

  • A person touches the earth wire.
  • A person touches the neutral wire.
  • A person touches the live wire.
  • The live wire touches the neutral wire.
  • The live wire touches the earth wire.

How many of these five events cause the fuse in the plug to blow?

A   1          B   2        C   3         D   4

Solutions: Option B

Consider the five events:

  • A person touches the earth wire – As the person is at 0V, same as the earth wire, there will be no current flowing through the person. So current through the circuit will not be affected, which is lower than the 3A fuse rating. Fuse will not blow.
  • A person touches the neutral wire.-  As the person is at 0V, same as the neutral wire, there will be no current flowing through the person. So current through the circuit will not be affected, which is lower than the 3A fuse rating. Fuse will not blow.
  • A person touches the live wire. – The live wire is at high potential of 240 V. The person will get an electric shock. But a common misconception is that if a person gets an electric shock, the current flowing through him is very large, which is wrong. In fact, the current is very small, much smaller than the fuse rating. Assuming the average body resistance of the person is 100 000 ohms, and the potential difference in Singapore is 240 V, since I = V/R = 240/100 000 = 0.0024 A, which is lower than 3A fuse rating. Hence the fuse will not blow.
  • The live wire touches the neutral wire. – This will create a short circuit as a large current which exceeds the fuse rating will from the live (240 V) to the neutral wire (0V) as that path has very low resistance. The fuse will blow.
  • The live wire touches the earth wire. – This will create a short circuit as a large current which exceeds the fuse rating will from the live (240 V) to the earth wire (0V) as that path has very low resistance. The fuse will blow.


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How a fuse works

A fuse is a safety device which is connected on the Live wire.

In the event of electrical fault (e.g. live wire touches the metal casing), the large short circuit current which exceeds the fuse rating will cause the fuse to melt and break the circuit. This stops the current from entering into the electrical appliance and thus protects the electrical appliance.

Fuse has to work together with Earth wire.

<p><a href=”http://vimeo.com/24927865″>Fuse</a&gt; from <a href=”http://vimeo.com/user7367248″>evantoh</a&gt; on <a href=”https://vimeo.com”>Vimeo</a&gt;.</p>


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Double Insulation symbol on a hair dryer

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The double square symbol represents double insulation. This means that the electrical appliance (a hair dryer in this case) has double layers of insulation. Besides the rubber tubing which cover the wires, the other casing is made of electrical insulator (plastic in this case). This protects the user in the event if the wires become loose and touch the casing. The casing will not become ‘live’.

20130326-190110.jpg


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Thick Cables for High Power Electrical Appliances

Thick_cable_1Thick_cable_2Thick_cable_3Thick_cable_4Thick_cable_5

High power electrical appliances are those which draws high current.

The thinner the wire (smaller cross-sectional area), the higher the resistance of the cable which brings the current into the electrical appliances.

 

More thermal energy will be generated due to this higher resistance in the cable. The cable might be too hot or catch fire.

So it is advisable to have thicker cables for high power electrical appliance as thicker cables have lower resistance.


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Double Insulation

Why some electrical appliances do not need earth wire or they only have 2 pins (live and neutral)? For some 3 pins plug, the earth pin is actually plastic (that means no earth wire). Why?

Double_insulation_01Double_insulation_02Double_insulation_03

Why make the earth pin with plastic since there is no earth wire?
Having the plastic earth pin is to provide convenience for user to plug in the 3-pins. Without the earth pin the plug wouldn’t go into the socket. The earth pin is needed to open a cover inside the socket that stops people from touching the live parts.

(Thanks Bob Barrett for the suggestion to improve the explanation)


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kWh and J are units of electrical energy

When we pay the electricity bill, we pay for the
A) energy used.
B) power used.
C) voltage used.
D) current used.

Solutions: Option A

When calculating cost of electricity, we have to use E = Pt to find the electrical energy (kWh) used. Then we multiple by the cost of 1 unit of kWh (currently around $0.29).

In normal times, when finding electricity energy in joules (J), all physical quantities in E = Pt must be in SI units. (P in W and t in s).

Hence it is important to know that alternative units for electricity energy is kWh besides J.