SAFETY FEATURES USED IN THE SAFETY FEATURES USED IN THE CIRCUITS OF THE MAINS CIRCUITS OF THE MAINS

SUPPLYSUPPLY

Physics 2Physics 2

Ad

dit

ion

al S

cien

ceC

hap

ter

14

Electrical PowerElectrical PowerRemember:

Electrical Power (W) = Current (A) × Voltage (V)

or

P = IV

Current = 0.25A

Voltage = 240V

Power = Current × Voltage

= 0.25 × 240

= 60W

Current = 12.5A

Voltage = 240V

Power = Current × Voltage

= 12.5 × 240

= 3000W = 3kW

Ad

dit

ion

al S

cien

ceC

hap

ter

14

Electrical PowerElectrical PowerRemember:

Electrical Power (W) = Current (A) × Voltage (V)

or

P = IV

Current = 0.4A

Voltage = 240V

Power =?

Current = 0.5A

Voltage = 240V

Power =?

Current = 2A

Voltage = 240V

Power =?

Current = 3A

Voltage = 240V

Power =?

Answer: 120W Answer: 480W

Answer: 96W Answer: 720W

Ad

dit

ion

al S

cien

ceC

hap

ter

14

Electrical PowerElectrical PowerYou must remember:

Electrical Power (W) = Current (A) × Voltage (V)

or P = IV

Appliance Power(W) Current(A) Voltage(V)

Hair dryer 3 240

heater 1200

iron 960

personal stereo

2.4

microwave 800

Ad

dit

ion

al S

cien

ceC

hap

ter

14

Electrical PowerElectrical PowerYou must remember:

Electrical Power (W) = Current (A) × Voltage (V)

or P = IV

Appliance Power(W) Current(A) Voltage(V)

Hair dryer 720 3 240

heater 1200 5 240

iron 960 4 240

personal stereo

2.4 0.01 240

microwave 800 3.33 240

Ad

dit

ion

al S

cien

ceC

hap

ter

14

Electrical plugElectrical plugRemember:

Te live wire carries the electricity

The fuse protects the appliance from damage

The earth wire protects us from danger from the circuit The plastic and the rubber is the material for the plug and wire cover

Earth

Neutral

Live

Fuse

Cord gripCable

Ad

dit

ion

al S

cien

ceC

hap

ter

14

Electrical plugElectrical plug

Which plug has been correctly wired?

A B C

D E F

Ad

dit

ion

al S

cien

ceC

hap

ter

14

Electrical Power and the FuseElectrical Power and the FuseYou must remember:

Electrical Power (W) = Current (A) × Voltage (V)

or P = IV

Current = ?

Voltage = 240V

Power = 3000W

Current = ?

Voltage = 240V

Power = 480W

Current = ?

Voltage = 240V

Power = 720W

Current = ?

Voltage = 240V

Power = 120W

3A 5A 13A

What fuse must be used?

Ad

dit

ion

al S

cien

ceC

hap

ter

14

The work of the fuseThe work of the fuseCurrent travels along the live wire, through the appliance (bulb in this case) and back along the neutral wire.

`

Ad

dit

ion

al S

cien

ceC

hap

ter

14

The work of the fuseThe work of the fuseThis shows what happens when there is a fault in the circuit.

Part of the live wire touches the side of the appliance.

A lot of current now flows along the earth wire.

The current is too strong for the fuse. It blows causing a break in the circuit.

Ad

dit

ion

al S

cien

ceC

hap

ter

14

Circuit BreakerCircuit BreakerMiniature circuit beakers (MCB) are used in modern consumer units. They replace fuses.

When the current exceeds the rated value, the strength of the electromagnet is enough to separate the contacts. The circuit is switched off.

Circuit breakers, like fuses, do not provide protection should you touch a live component.

Advantages of the fuse:-

1) It works a lot faster

2) It is possible to reset it.

Click here for further Click here for further information from the information from the webwebA

ddit

ion

al S

cien

ceC

hap

ter

14

(Residual Current Devices RCDs)(Residual Current Devices RCDs)These devices are designed to protect against electrical shock. This type of circuit breaker measures the current in the live and neutral wires. If everything is working correctly, then these two are equal. If there’s a fault, you get a small electrical shock but not strong enough to kill you. You should always use a RCD every time you work with outdoor equipment.

Power supply

neutral live

pivot

Current out Current in

Wire coils (electromagnet)

B

N

Switch on the drill

Break in the circuit

If there’s a difference between the live wire and the neutral, the electromagnet in the live side will pull the iron bar downwards, pushing the button up to create a break in the circuit. The effect of this is to prevent electricity from reaching the appliance. Switch on the drill to see the RCD at work.

Iron bar

You must reset the switch before using the RCD again.

Ad

dit

ion

al S

cien

ceC

hap

ter

14

Oscilloscope

Oscilloscope

Vol

tag

e (

V)

Time (T)

Vol

tag

e (

V)

Time (T)

DC and ACDC and AC

AC means “Alternating Current” – the current changes 50 times per second (frequency = 50Hz). This is the frequency that comes from the mains supply.

230V

DC means “Direct Current” – the current travels in one direction only e.g. in a battery

1 th/ s

50

1.5

V

1.5V

Ad

dit

ion

al S

cien

ceC

hap

ter

14

Why 230V?Why 230V?

The UK and European countries have used 230V since the start.

It would be very costly to change this to a lower voltage – appliances use 230V.

Many transformers would be required for our appliances to work at a lower voltage.

Thicker and more flexible cables would be required (lower voltage higher current to ensure the same power.

Many builders use tools that use 110V. This voltage reduces the risk of fatal electrical shocks.

The USA has chosen a voltage of 120V for its mains The USA has chosen a voltage of 120V for its mains supply. Because of this, some electrical appliance supply. Because of this, some electrical appliance manufacturers include a switch so that the correct manufacturers include a switch so that the correct voltage can be chosen for the appliance – using the voltage can be chosen for the appliance – using the appliance with a voltage of 120V would be damaging appliance with a voltage of 120V would be damaging in this country.in this country.

Ad

dit

ion

al S

cien

ceC

hap

ter

14