Electricity in the home

Chapter 52

Fuses

• A fuse is a safety device which cuts off the current in a circuit if the current goes above a certain level. The thin fuse wire overheats and melts and so breaks the circuit. Fuses are now replaced by circuit breakers.

Plugs

• Brown wire = Live wire connected to the fuse.

• Blue wire = neutral connected to the left hand side of the plug.

• Yellow green=Earth wire. Connected to the metal casing of the electrical device and runs to a metal plate buried in the ground

Measuring Electricity

• The watt is the unit of electrical power.

• It is a measure of how quickly an electrical appliance converts electrical energy to other forms of energy.

E.g. A 100 watt light bulb converts 100 Joules

of energy into light every second.

• The ESB use a unit called the kilowatt-hour(kWh).

• A kilowatt-hour is the electric energy used by a 1 kw appliance for 1 hour.

No. of kilowatt-hrs = No. of kilowatts x No of hours

Examples

• A 6 kw cooker is on for half an hour.

a) How many units does it use?

b) If each unit costs 11.5 cents how much does the electricity cost?

Answera) No of units = 6 x 0.5hrs = 3kWh(units)b) Cost = 3 x 11.5 = 34.5 cent

Example 2If each unit of electricity costs 12 cents, calculate the total

cost of using a 1500 W electric heater for 2 hours, a 150 W freezer for 4 hours, a 1000W hoover for 12 minutes, a 1200W iron for 10 minutes and an 8kW electric shower for 15 minutes.

Answer:

Total No of units = (1.5 x 2) + (0.15 x 4) + (1 x 1/5) + (1.2 x 1/6) + (8 x 1/4)

= 3 + 0.6 + 0.2 + 0.2 + 2 = 6kWh or 6 units.

Cost of electricity = 6 units x 12 cents = 72 cents

Example 3Question

The reading on an electricity bill 2 months ago was

21,185.Now the reading is 21,935. The price per unit is 11.75

cent. The standing charge is 6.52 euro and VAT @ 13.5% is

then added. What is the total bill for electricity over these 2

months?

AnswerNo of units = present - previous = 21935 - 21185 = 750Cost of electricity = 750 x 11.75 = 88.13Standing charge = 6.52Total (before VAT) = 94.65 VAT @ 13.5% = 12.78Total including VAT = 107.43

Electrical Power

Electrical power can be calculated using the formula

Power = Voltage x current

(watts) (volts) (amps)

The power rating of any electrical appliance is always marked on it. The ESB provides a voltage of 220 V.

Knowing these two values we can calculate the current an appliance uses.

Current(amps) = Power(watts)

Voltage(volts)

Example

Given the mains voltage is 220 V calculate the current flowing and the fuse s needed in a) 2.4 kW kettle b) a 720 W toaster.

Answer

a) Current(I) = P / V b) Current = P/V

= 2400W / 220 = 720 W/ 220V

= 10.9 A = 3.3 A

Fuse needed :13 A Fuse needed:5 A

Alternating current and Direct current

• Direct Current (d.c.) travels in one direction only.

• Alternating current (a.c.), supplied by the ESB changes direction many times per second.

• a.c. can be converted to d.c. using a rectifier.

HomeworkTextbook Chapter51-ElectricityintheHome

• A1.Fuse,melts,circuitbreakers.

• A2.Earth,live.

• A3.Watt,changes.

• A4.Kilowatthour,kWh.

• A5.Direct,d.c.,

• B1.

• (a)230V.

• (b)Whenthecurrentinthecircuitgetstoohigh,theheatingeffectcausesthethinfusewiretomelt,breakingthecircuit.

• (c)Itdoesnothavetobereplacedwhenittrips.(d)Onthelivewire,itcutsoffallcurrenttotheappliance.

• (e)(i)30A;(ii)10A.

• B2.

• (a)A=neutral,B=earth,C=live.

• (b)A=blue,B=yellow/green,C=brown.

• (c)Replacethe13Afusewitha3Afuse.

• (d)Themetalcasetoacopperrod,buriedintheground.

• (e)Ifthemetalcaseofanappliancebecamelive,theearthwirewouldcarrythecurrentsafelytoearth.

• B3.

• (a)2kW.

• (b)12units.

• (c)€1.80.

• (d)€1.20.

• (e)€0.90.

• (f)Kilowatthour.

• (g)806.

• B4.• (a)Fuse.• (b)X=neutral, Y=live.• (c)Thefuseisplacedonthelivewireforsafetyreasons.I

fafaultdevelopsandthefuseblows,itwillcutoffallcurrenttotheappliance.

• (d)Parallel.• (e)Thebulbsshinebrighter;ifonefails,theotherskeep

lighting.• (f)Anearthwire.• (g)3x100=300W=0.3kW;5x24=120hours;0.3x120x

0.15=€5.40.(h)Atwo-wayswitchisfoundinthehallandlanding.