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  • INTRODUCTION

    It is very important to note that any electrical work or installation must be undertaken by a qualified and registered electrician. This section is intended to give the reader a general understanding of an electrical installation and its principles, thereby assisting parties planning an electrical installation to communicate better.

    While the electrical trade is strictly governed by regulations, the detail of what is required in an installation is sometimes sadly overlooked particularly in a residential building. All too often one sees chalk markings on walls showing electrical points just before plastering commences.

    A simple look at electricity:

     

      • Electricity must be allowed to flow for it to be useful.

     

      • The flow of electricity is called Current.

     

      • Current is measured in Amperes (amps).

     

      • Electricity flows because of different pressures in the live and neutral wires. This pressure is called Voltage (volts).

     

      • Flow (current) is also affected by resistance which is measured in Ohms.

     

      • Power is measured in Watts and this is a measurement of how much electrical energy is used by something over a certain period of time.

     

     

    TERMINOLOGY

     

    Conductors

    Conductors allow electricity to flow freely. Most metals are conductors. Copper and aluminium are good conductors. The most commonly used conductor is copper, but brass can also be used. Aluminium is used when copper is too heavy or too expensive.

    Insulators

    Insulators resist the free flow of electricity. Most non-metal materials can act as insulators. The most common insulators are rubber and plastic (some older buildings still have ceramic insulation – this is very effective but no longer used). For example, the flex connecting your kettle to the plug (the wire is covered in plastic, which is an insulator) keeps you from being shocked by the electricity being conducted through it.

    Volt

    The unit used as a measure of electrical pressure. Symbol – V. Similar to Kpa pressure in a water pipe. In South Africa, the three phase voltage available for domestic use is 220 Volts (between neutral and a phase). Normally, appliances sold on the market are manufactured to operate at this voltage. Sometimes appliances have settings for various different voltage operations for use in other parts of the world. Make sure that the setting is suitably adjusted for this country.

    Voltage Drop

    When all conductors of an AC installation are carrying their design load, the difference in voltage (the voltage drop) between the point of supply and any point of outlet or terminals of fixed appliances should not exceed 5% of the standard or declared voltage.

    Ampere

    The unit used as a measure of the electrical rate of flow or the measure of the amount of electric charge passing a point in an electric circuit per unit. Symbol – A. Similar to litres per minute in a water flow system. Each and every appliance connected to an electricity supply system will draw a certain amount of current in Amps when in operation. A toaster makes use of this current flow through a filament to generate heat to brown the toast. The light bulb is similar except that the heat generated is so high that light energy is given off.

    Watt

    The unit which shows the current drain of consumption of an appliance connected to a circuit. Symbol – W. Most appliances are rated in Watts. It is a very useful unit to determine the size and rating of a conductor for a particular appliance. A variation of the Watt is the Watt-hour and this unit shows the electrical consumption over a fixed period. The electrical supply utility (In South Africa, Eskom or City Power) makes use of this unit to charge for the use of electrical energy.

    Domestic Voltage

    Domestic Voltage means voltage not exceeding 230V +/- 10% alternating current phase to neutral.

    Low Voltage

    Should not be confused with the low voltage application usually used for lighting and other appliances that commonly run at 12 volts. Low voltage technically means a set of nominal voltage levels that are used for the distribution of electricity, the upper limit of which is accepted to be an AC voltage of 1 000 V (or a DC voltage of 1 500 V).

    Medium Voltage

    Means a set of nominal voltage levels in the range 1 kV to 44 kV that is above low voltage and below high voltage.

    Circuit/Conductor

    A circuit is a conductor (e.g. copper wire) connected in such a way that electrical energy is allowed to flow through it. In household wiring, there are normally two conductors that make up a circuit; one is the live conductor and the other the neutral. Very often they will be colour coded to make identification of the type of circuit and conductor easy. A third conductor (normally a non-insulated copper wire) is provided for an earth return; this is a safety feature should the insulation or proper connection of live conductors fail.

    Circuits

    Electricity enters a home via a 220v cable into the distribution board that contains circuit breakers (or fuses in older houses). These circuit breakers divert the electricity along wires running in conduits in the walls and roof, forming separate circuits around a house which deliver electricity to all the switches and power points. A building has multiple circuits to help the electrical current stay small, reducing the risk of overloading the circuits. Some appliances, such as a stove, dishwasher, air-conditioner/ heater or dryer, draw more power than a light point and therefore may require their own circuit.

    It is important that the correct size of wire or cable is used for the maximum potential load on a circuit.

    The following specifications may be used for guidance:

     

      • 1 or 1.5 mm² for lighting, depending how many fittings are connected;

     

      • 2.5 mm² for power circuits and ring mains, again depending on how many socket outlets are connected to the circuit and their application;

     

      • 4 or 6 or 10 mm² for cookers, water heaters and air-conditioning, depending on current demand and output temperatures.

    Note:
    To work out how many watts an appliance is drawing, check the voltage and the amps of the appliance.

     

      • Volts x Amps = Watts

    Therefore: 1 Amp at 12 volts = 12 watts.

     

      • 1 Amp at 120 volts =120 watts.

     

      • Watts/Volts = Amps

    Therefore: A 1200 watt hairdryer at 120 volts draws 10 amps (1200/120 = 10).

    AC & DC Current

    AC is the abbreviation for alternating current and DC is the abbreviation for direct current. AC is the type of electricity supplied by the electricity supply utility to our industry and homes. DC is the type of electricity that would be supplied by a battery or an appliance such as a car battery charger where the voltage and current are constant. All the appliances found in our homes that operate on the mains supply are designed to work with AC. Some appliances, however, need batteries, such as portable radios and these work on DC.

    Single & Three Phase Power Supply

    Single-phase is an electrical supply with only one live phase, plus the neutral. Three-phase is the most commonly supplied electricity, 230/400 volts, from which 230 volts can be obtained between a phase and neutral in a four-wire system, but can be higher when used in commercial and industrial applications. Looking at the cable three phase would have four wires excluding earth, and the single phase would have two wires excluding earth. Some industrial motors and machinery require three phases to operate.

    Information supplied by Buildaid Publishing.

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