ELECTRICS & LIGHTING – WHAT’S INVOLVED
Electricity and Electronics are Related Fields. While both these related fields are specialised and take considerable training to be tackled in their entirety, there are projects involving both electrics and electronics that anyone with a reasonable aptitude and understanding can tackle. These range from wiring a plug and fitting a dimmer switch, to repairing a computer mother board.
But we do have one major word of warning: ELECTRICITY IS POTENTIALLY LETHAL, and only a qualified registered and/or licensed electrician may install electrical systems. It makes perfect sense, after all even something as simple as wiring a plug incorrectly could trip out the system, and faulty wiring could cause a fire. At worst, an electric shock can kill!
We use electricity for lighting and for cooking and very often also for heating or cooling the buildings we live and work in. We know that it is in short supply and that the costs of producing it keep increasing. But how many of us really understand what electricity is and where it comes from?
Most of us take electricity for granted because we use it every day of our lives. All it takes is the touch of a switch to make a light go on or to start a power tool. We can’t see it, and we can only smell it if something goes wrong and it causes an explosion or makes something burn.
Where electricity comes from
Electricity is generated at power stations, and in South Africa it is Eskom (the Electrical Supply Commission of South Africa) that is responsible for the power stations all over the country.
Coal is the major source of energy for our power stations. First it is pulverised to dust and then burnt in huge furnaces. Then the heat released by the burning coal boils water that is converted to steam at a very high temperature and pressure. This super-heated steam is used to turn a turbine that has a shaft that is coupled to the rotor of a generator. The rotor is a cylindrical electro-magnet that spins inside large coils of copper, generating the electricity.
But this is just the beginning. To get electricity from the power stations to our homes and buildings it has to be transported along power lines. Most of Eskom’s power stations generate electricity at about 22 000 volts (or 22 kV).
Voltage is the measure of electric pressure or force, and a volt is the unit that we use when we measure the force of electric current. To be able to move the electricity over long distances, the electricity must be carried at very high voltages, even higher than the electricity that has been generated. This is done via transformers that can step-up the voltage to as much as 765 kV, and then feed the electricity into Eskom’s national grid.
But we can’t use electricity that has such a high voltage. Factories normally use a voltage of 11 kV (11 000 volts), while the electricity we use in our homes, shops and offices is usually supplied in an electrical circuit with a driving force of either 380 volts or 220 volts. It may be supplied to the building in what we call a single phase, in which case there will be two wires, one that is live and one that is neutral. Or it may be supplied in a three-phase system and there will be three live wires and one neutral wire.
All the electricity that Eskom generates is fed into the national grid for distribution. But Eskom does not distribute all the electricity. Most municipalities buy electricity in bulk from Eskom and then resell it to their consumers. Whoever supplies the electricity will supply and install a meter box so that the amount of electricity used can be measured.
Electricity for homes and buildings
For safety reasons, our laws state that only qualified electricians are allowed to install electricity in our buildings. They must also be registered with the Electrical Contracting Board of South Africa and work according to the relevant South African National Standards (SANS). But electricians have to work very closely with contractors and supervisors to be sure that power points are positioned exactly where they are required.
When we build our homes, offices, schools, hospitals and other buildings, the owner of the property or the contractor applies to Eskom, or to the local authority (or municipality), for an electrical connection. An electrician, appointed by the owner or contractor, then wires the building and installs all the different elements including a distribution board to control the power through a series of circuits, isolators, plug points and light fittings. He or she also earths the current so that if there is too much power or a fault in the system, the live electricity will go to ground (or earth). When supply cables run overhead, the earth cable is usually connected to a copper rod that is pushed into the ground. Otherwise the main earth terminal, which is where the electrician connects wire in an electrical circuit, is connected to a device that is clamped to the mains service cable.
Now we’re going to look at some of the elements that relate to electricity:
Distribution boards are where you will find the mains switch and other switches that you can use to isolate the various circuits in the building. All modern distribution boards also have an earth-leakage unit and various circuit breakers (or trip switches) that will trip if there is an electrical fault.
Local authorities have strict requirements regarding the type and size of distribution board that must be used.
They are usually located in the kitchen or in the garage, but they must be easily accessible in case of a power failure or fault.
Earth leakage units tell us when the electricity supplied to our buildings goes to ground by simply shutting off the power supply. This protects us from the possibility of fire or from getting an electric shock which could be lethal. Earth- leakage units react much quicker than old-fashioned fuses or circuit breakers that do the same job.
Circuits are the complete circle that the electric current travels. The load or amount of electrical power that can be produced in any one circuit is limited, and the electrician will decide which power points will be linked to which circuit. He or she must be careful to organise the circuits so that the electricity doesn’t overload and trip unnecessarily.
The number of circuits used in any building will depend on what is required. In an ordinary house we normally have at least one circuit for the lights and then another for the plugs. There is also a separate circuit for the water heater (or geyser) and another for the stove, because these appliances both draw a lot of power.
Lighting circuits always run separately to other circuits and they use thinner cable with a lower amperage. This is because lights use a lot less current than appliances like fridges and washing machines, or machines that we use on building sites. Usually houses will have at least two light circuits so that if there is a problem with one circuit at night, there will still be light from the other circuit.
Isolators are units that we use to either isolate the system from the mains supply, or to isolate one or more circuit.
Wiring homes and buildings
To be able to wire a home, the builder lays conduits which are plastic or metal pipes through which the wiring will be threaded by the electrician. The conduiting can run either within the floor slab or within the roof space, from where it is chased, or cut, into the walls so it can be positioned to extend to various outlet points where light fittings and plug points are needed.
Even though a qualified electrician is responsible for the wiring itself, it is a good idea for all supervisors and contractors to be able to identify different types of cable and flex. The insulation material around the wires inside cables and flex are different colours to make it easy to identify them. Green or green and yellow is the earth, live wires are brown or red, and neutral is either light blue or black.
It is usually the builder’s responsibility to link the distribution box either to the meter box or to the connection box on an outside wall of the building.
Once the wiring in a building is complete the entire system must be checked by an accredited person (from Eskom or the municipality) who will issue a certificate of compliance to say that the system has been installed correctly. Only then will the supply authority link up the wiring to the mains supply so that we can switch it on.