Unit 7- Practical Electricity Ii [compatibility Mode]

Unit 7 Practical Electricity II

2009 Mr Chung T W David, Coral Secondary School

Lesson Objectives y state some electrical hazards and precautionary

measures to ensure the safe use of electricity in the home.

2009 Mr Chung T W David, Coral Secondary School

Practical Electricity II y Attention! This lesson may save your life!

y Electricity can be very dangerous! Do not take it

for granted! 2009 Mr Chung T W David, Coral Secondary School

Living and Physics

Living and Physics y Demonstration: Deadly Spark y Observation: y See what happens to the wire

URL: http://www.youtube.com/watch?v=pNsUDmlG6js

2009 Mr Chung T W David, Coral Secondary School

Living and Physics

Living and Physics y Demonstration: Deadly Spark y Questions: y What was that spark? y What Wh t caused d th the sparkk to t occur? ?

2009 Mr Chung T W David, Coral Secondary School

Living and Physics

Living and Physics Common Household Wiring y Live or ‘Hot’ wire (Brown or Red) y This is a wire at high

voltage. y This voltage may vary between countries y In Singapore, this voltage is 240V. y Neutral (Blue or Black) y This is a wire at 0V. 2009 Mr Chung T W David, Coral Secondary Schooly

It is kept at zero by earthing.

Living and Physics

Living and Physics Common Household Wiring y Earth wire (Yellow & Green) y This is a wire at 0V. y No current flows through

under normal operation y It is connected to the ground

2009 Mr Chung T W David, Coral Secondary School

What is a short circuit? y A short circuit is an accidental low-resistance

connection between two nodes of an electrical circuit that are meant to be at different voltages y An A example l iis th the accidental id t l connection ti off th the Live and Neutral wires for Household Wiring y Causes an overflow of current in the circuit

2009 Mr Chung T W David, Coral Secondary School

Living and Physics y Grouping: In pairs y Duration: 5min y Questions: y What was that spark? y What caused the spark to occur?

2009 Mr Chung T W David, Coral Secondary School

Electrical Hazards Ha ards y

Electric Shocks and Electrocution Electrical Fires

y

P Possible ibl C Causes:

y

1. 2. 3.

Damaged Insulation Overheating of Cables Damp Conditions

2009 Mr Chung T W David, Coral Secondary School

Electrical Hazards y Electric Shocks and Electrocution y Occurs: When a person is injured or killed by

large a ge cu current e t flowing o g through oug the e body body. y Example: touching exposed live wire. y Electric Shocks and Electrocution y Occurs: A short circuit occurs causing the wire to

be overheated and a fire starts. y Example: E l C Connecting ti ttoo many electrical l ti l appliances to same output socket, causing a fire to break. 2009 Mr Chung T W David, Coral Secondary School

Electrical Hazards 1. Damaged Insulation y The wires of electrical appliances are insulated with

insulating materials such as rubber y Due to wear and tear tear, the rubber can become damaged and the wires exposed y If a person comes into contact with the exposed wire…

2009 Mr Chung T W David, Coral Secondary School

Electrical Hazards 2. Overheating of Cables y Refers to the unusually

large amount of heat generated by over oversized current flowing through the conducting wires y Overloading of a power point can g p generate a huge amount of heat too

2009 Mr Chung T W David, Coral Secondary School

Electrical Hazards 3. Damp Conditions y Wet W t conditions diti off the th

skin greatly reduce the electrical resistance of the skin. skin y Dry skin has a typical resistance of 100 kΩ or more However more. However, it may drop below 1kΩ when wet y With a voltage of 240V 240V, a deadly current of 0.23A can flow through the body 2009 Mr Chung T W David, Coral Secondary School

Electrical Safety Safet y Never overload a socket y Never use instruments to poke into an electrical y y y y y

socket Replace old wiring Do not wet electrical appliances, plugs or sockets Never use electrical gadgets in wet places Disconnect the appliance when cleaning it Call the electrician

2009 Mr Chung T W David, Coral Secondary School

Safety Precautions in Using Electricity (I) y Earthing y Why is there an earth wire? y When a fault occurs, the earth wire will carry the

current to the ground directly. directly y This prevents the person who is in contact with the ground and touching the ‘live’ appliance to be electrocuted. y Appliances with metal surface are usually earthed. earthed

2009 Mr Chung T W David, Coral Secondary School

Safety Precautions in Using Electricity (II) y Switches y Used to cut off electric current when we are not operating the electrical appliance y Must be fitted to the live wire

lamp neutral

electric contact

240 V supply live

springy metal

fuse

two-way switches

2009 Mr Chung T W David, Coral Secondary School

Safety Precautions in Using Electricity (III) y Fuses y Consist of a thin piece of copper wire, the fuse is given a

current rating (i.e the maximum current it can carry without ith t melting) lti ) y When a fault occurs and the size of the current flow exceeds the rated value of the fuse, the fuse will ‘blow’ (the fuse wire will melt, cutting off current flow) y Must be fitted to the live wire

Normal O ti Operation

L N 2009 Mr Chung T W David, Coral Secondary School

Safety Precautions in Using Electricity (III) y Fuses y Consist of a thin piece of copper wire, the fuse is given a

current rating (i.e the maximum current it can carry without itho t melting) y When a fault occurs and the size of the current flow exceeds the rated value of the fuse, the fuse will ‘blow’ (the fuse wire will melt, cutting off current flow) y Must be fitted to the live wire

Abnormal O ti Operation

L

Fuse

No current flow 2009 Mr Chung T W David, Coral Secondary School

N

Safety Precautions in Using Electricity z

Switch and Fuse

•

must always be wired into the ‘live’ wire!

neutral live (off)

neutral live (on)

correct 2009 Mr Chung T W David, Coral Secondary School

incorrect

Safety Precautions in Using Electricity (IV) z

Circuit Breakers

• •

A special type of switch that automatically cuts off electricity supply in a circuit if the flow of current becomes too large The circuit breaker can only be reset after the fault has been corrected Miniature Circuit Breakers (MCB) work like fuses but they ‘trip’ trip instead of ‘blowing’ Earth Leakage Circuit Breakers (ELCB) detect small leakages in the earth wire and cut off the current in the live wire

• •

2009 Mr Chung T W David, Coral Secondary School

Lesson S Summary mmar y Household Wiring and Short Circuits y Hazards of Electricity y Electrical Safety Rules y Safety Precautions in Using Electricity

2009 Mr Chung T W David, Coral Secondary School