HOUSEHOLD ELECTRICITY
| Learning objectives: By the end of the subtopic learners should be able to: |
- Explain the source and transmission of electricity.
- Describe safe domestic electrical wiring.
- Describe the wiring of a two and a three pin plug.
- State household electricity billing.
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- Power lines supply electricity from where it is generated.
- In Zimbabwe, the supply and regulation of electricity is done by by the Zimbabwe Electricity Supply Authority (ZESA).
- Electricity is used for various purposes in industries and homes.
Transmission of electricity
- Generators produce electricity.
- In Zimbabwe, hydro electricity is produced at Kariba dam.
- Hydro means water.
- Thermal electricity is produced at Hwange.
- Steam is used to turn turbines which produce electricity.
- Electricity produced from these main power stations is transmitted throughout the country using pylons.
- At a power station electricity is generated as alternating current (a.c).
- It is generated at a very high voltage.
- A step up transformer is used to reduce the current and increase voltage to enable it to be transmitted over long distances with very little loss.
- Electricity is then stepped down by a transformer before distributed into individual households.
Alternating current (a.c)
- It is current that changes direction all the time.
- In a.c one point changes from positive to negative many times a second.
- There are no positive or negative terminals.
- There are only two conductors, the live wire and the neutral wire.
Direct current (d.c)
- A battery cell produces direct current.
- It moves in one direction from the positive terminal of the cell through a conductor and back to the cell through a negative terminal.
- It cannot be stepped up or down like a. c.
- It cannot be transmitted over long distances, since most power will be lost along the way.
Domestic wiring
- Households usually have fuse boxes or magnetic current circuit breakers (MCB's).
- There are two separate circuits in a house, one for mains which supply all power points for plugs and one for lights.
- The MCB receives power from the main supplies which is connected to the main grid.
- The following are important features of a circuit:
- MCB's are isolator switches which cut off the house circuit from outside. They are necessary when house repairs are being done.
- A meter measures the amount of electricity used by the household. Payment of electricity is based on meter readings.
- Live and neutral wires are connected to the outside transmission. The earth is connected into the ground using water pipes.
- Parallel connections are used in households as they allow appliances to be switched on and off without affecting other appliances.
Overloading an electric circuit
- When appliances connected to a circuit draw more current than it is supposed to carry, it is said to be overloaded.
- If an appliance drawing 7amps is connected to a lighting circuit of 5amps it becomes over loaded.
- This will cause conductors to become heated up and melt insulation of live and neutrals wires.
- If this occurs it will cause a short circuit.
- This will cause a spark which may result in a fire that could burn the house.
Short circuit
- A short circuit occurs when a wire breaks inside an appliance and touches the casing.
- The circuit is completed when you touch the casing and results in an electrical shock.
Fuses
- Each plug has a 13A fuse.
- It means the appliance uses 13 amps of current.
- When a fault occurs in an appliance the fuse will blow and protects the appliance from damage.
- When this happens, the fuse can be replaced by a new one and the appliance inspected by a qualified electrician for any fault.
- A fuse is placed on the live wire in a circuit.
Earthing
- Conduits are earthed by running thick copper from the pipe to the earth.
- The third connection on a plug socket is joined to the earthed conduit.
- The main switch and fuses are in a metal casing that is earthed in the same way.
- When there are no earth connections and you touch the live appliance your body would earth the appliance.
- You would receive an electric shock or will be electrocuted.
Switches
- A switch is always placed on the live wire of a circuit.
- A switch is a circuit breaker.
- In its off position, a switch makes a gap in the circuit so that current cannot flow.
The three pin plug
- It is wired so that the live wire in the fuse is in contact with the live wire in the circuit.
- The wires are colour coded as follows:
Brown/red…………………..live
Blue …………………………….neutral
Green/yellow……………..earth
- To connect the three pin plug properly, make sure the colours of each of the three wires are connected as below.
Two pin plug
- A two pin plug has connections of live and neutral wires only.
- It has no fuse or earth.
- Users are protected from electrical shocks because:
a) There is thick insulation that covers the plug.
b) Appliances with two pin plugs use very small amounts of current.
c) The appliances have built-in fuses.
Paying for electricity
- The electricity used in households is measured in kilowatts per hour (kWh).
- 1 kWh is the amount of energy consumed by a 1 kW appliance for an hour.
- Cost depends on the cost of supply at any given time.
- The formula used is:
Number of units used = number of watts × number of hours
SUMMARY
- Electricity is supplied as a.c current in homes.
- Switches turn appliances on and off.
- A fuse is used to protect the appliance in case of a short circuit.
- A 3 pin plug has 3 wires namely live, earth and neutral wires.
- Power = energy used time. It is measured in joules per second or watt.
- The amount of electricity used in the household is measured by a meter.
- The kilowatt/hour (kWh) is the unit of energy used.
