| Contents for this page | Related topics |  |
|---|---|---|
|
The domestic power supply Correct wiring of appliances Earthing |
Heating effects of electric currents Inductive circuits Electric machines Alternating currents Resistive, capacitive and inductive AC circuits |
 Data Glossary |
| Learning Outcomes | ||
| After studying this section, you will (a) be familiar with the basics of electric supply to a dwelling, (b) know how to wire a power plug safely, and (c) understand the importance of earthing appliances. | ||
As we saw earlier in another topic, electric power is supplied to houses in South Africa at 220 V and 50 Hz. This can give rise to large currents, which are easily capable to give deadly electric shocks. It is important therefore that one should treat all electrical appliances with caution, and know something about the correct way to connect such appliances.
Electricity is normally supplied to dwellings from the transformer at the electrical sub-station by an underground armoured cable carrying two wires (
). One, called the LIVE wire has a red insulation, while the other one, with a black insulation, is called the NEUTRAL (depending on local regulations, a third "earth" wire may or may not be part of the cable) (). The outside armoured sheathing of the cable is frequently used to earth the supply. The live wire carries a voltage that alternates between +310 V and -310 V (giving a root mean square voltage of 220V). The neutral wire is a constant 0 V.
The supply first passes through a meter, and then to a distribution board that splits the current into various circuits, for example, a circuit for the stove, another for the geyser, one for lights, one for power sockets, and so on. This ensures that if a circuit is faulty (as the result of a short circuit, for example), the other circuits are unaffected. Switches on the distribution board allow each circuit to be turned off (there is also a mains switch that turns off power to all circuits). Power sockets and lights are connected in parallel to these wires.
 | |
 |
 |
Most modern appliances come nowadays with tamper-proof plugs. However, one must now and again connect a power cable to the older types of plugs. It is very important that this be done correctly! Power cables normally carry three insulated wires, as shown in the photograph below:
By international convention, the live wire has a brown insulation, the neutral a blue insulation, and the earth wire (to be discussed later) has a yellow or green insulation, or an insulation with bands of yellow and green.
The correct wiring of a three-prong power plug is shown below:
 |
 |
It is easy to remember!
Bottom Right = BRown.
Bottom Left = BLue.
Electricity is dangerous. Before working on any appliance, make sure it is unplugged from the power socket. If you are working on fixed wiring, turn of the electricity supply at the mains switch. Alterations to the electrical circuits in a dwelling should only be carried out by a qualified electrician. In South Africa, many people are killed annually by rigging makeshift wiring or by trying to steal power from the power supply.
Appliances that have a metal casing should be "earthed". That means that the metal case should be connected to the earth wire, which, when the plug is inserted into the power socket, will be connected to an earth wire. In the event of the live wire within the appliance accidentally touching the metal case (which would make the case "live", and very dangerous to touch!), a large current would then flow to earth. A fuse in the circuit would then "blow", cutting off the supply of electricity to that appliance.
Appliances that are encased in a plastic moulding with no metal parts showing are said to be "double insulated", and do not need to be earthed. They are normally connected to the power supply with a two-prong plug, carrying a live and a neutral wire.
The earth wire should be connected to the ground - ideally, by means of a stout copper peg driven into the ground. The earthing safeguard is provided at several points in the distribution system.
