practical electricity 2:uses and safety

a battery…a current can only flow when a circuit is

complete…

12V

0 V

the mains…similarly, a current can only flow when a circuit is

complete…

12V

0 V

a battery…consider this…

a battery…consider this…

12V

0 V

tell me why...… a bird won’t get electrocuted...

tell me why...… but a man would?

just imagine that...The current that flows in mains is similar to...

12V

0 V

just imagine that...The current that flows in mains is similar to...

240 V

Earth

electric shockDifferent amounts of current will have different

physiological effects on a person.

Current Effect

1 mA Threshold - No pain beyond this point

5 mA A frightening, but harmless shock

10 mA Uncontrollable muscular contractions

50 mA Pain and exhaustion. Breathing affected

100 mA Uncoordinated contraction of the heart

what happens...If an electrician accidentally touches a live wire of 240 V, what possible effect would he

experience if his skin

(a) were dry? (R dry skin = 100 000)

R = V/I I = V/ R I = 240/ 100 000 = 2.4 mA (mild shock)

(b) were wet? (R wet skin = 1000)

R = V/I I = V/ R

I = 240/ 1000

= 240 mA (DEATH)

just imagine...If I had to connect...

main dangers(a) electric shock• occurs when a current passes through your body• different amounts of current have different effects

(b) fire• due to overheating of cables

causes of dangersHazards are caused by...1. Damp conditions

– water can increase the conductivity of the skin– could cause shock when operating an appliance

with wet hands

causes of dangersHazards are caused by...2. Damaged Insulation

– insulation protects us from touching live wire– can cause shock when bare hands touch exposed

wire

causes of dangersHazards are caused by...2. Damaged Insulation

– insulation prevents short circuit between live and neutral wire

– can cause short circuit, increase current flow and causing overheating

LNE

causes of dangersHazards are caused by...3. overloaded cables

– having too many plugs at one power point can increase current at that power point

– can cause a fire when the cables overheat

causes of dangersHazards are caused by...4. poor or loose connections

– Poor or loose connections cause resistances to increase and cause overheating and electrical fires

what do you think...What are some ways we can use electricity

safely?

let’s try...How do I protect appliances?

I = 1.9 A

let’s try…How do I protect people?

let’s try…How do I protect people?

safe use of electricity1. Fuse

– safety device that breaks a circuit when the current exceeds a certain value

– usually used to protect electrical appliances– made of a piece of tin-coated wire

let’s try...Fuse

– rated with certain values

– fuse selected is slightly larger than the required current needed

– e.g. if an appliance needs a current of 3 A, then a fuse of rating 5 A is chosen

I = 1.9 A

I Fuse = ? AI Fuse = 2 A1.9 A

2.0 A

2.1 A

exampleA television set draws 1.2 kW from a 220 V supply. What fuse rating should you select for the fuse? Fuses available: 1A, 2A, 5A, 10A, 13A

safe use of electricity2. Earth Wire• safety device that breaks the fuse if there is a short

circuit to the metal case

safe use of electricity2. Earth Wire• safety device that breaks the fuse if there is a short

circuit to the metal case

safe use of electricity2. Earth Wire• safety device that breaks the fuse if there is a short

circuit to the metal case

safe use of electricity2. Earth Wire

appliances that have double insulation do not have an earth wire.

– the electrical cable is insulated from the internal metal parts of the appliances

– all internal metal parts are enclosed in a plastic

three types of wires1. Live wire

• connected to the mains

• is set at 220-240 V (in Singapore)

• is brown in colour

three types of wires2. Neutral wire

• is set at 0V

• Is blue in colour

three types of wires3. Earth wire

• the safety wire

• is connected to the ground

• is green and yellow in colour

precautions when wiring1. Ensure coloured wires are connected to the

correct pins

2. Check, with test pen, the ‘live’ wire.

Neutral(blue)

Earth(green & yellow)

Live(brown)

Fuse

how to wireHow would you wire a bulb?

how to wireIf you place the switch and the fuse in the neutral wire,

the current will flow as circuit is now closed!!!

how to wireIf you place the switch and the fuse in the live wire, the

current will NOT flow as circuit is still open!

how to wire

Better circuit Not so good circuit!

how to wire

how to wire

series

how to wire

parallelA parallel circuit is better because if any appliance has a broken circuit, the other appliances can still operate

how to wire

ring

Don’t draw!

how to wire

ring

how to wire

ring A ring circuit is better... Fewer cables are needed because there are two possible paths through which a circuit can flow, to and fro, from the mains supply to any particular circuit. This means that we can use, at any one time, twice as much current as would be carried within a single cable!