What is the Meaning of terms Volts, Watts and Kilowatt Hours?
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The correct use and meaning of many electrical terms have, in many cases, become unclear through general use. These explanations attempt to show the correct usage of these terms. |
| Electrical Terminology - VOLTS, WATTS and KILOWATT HOURS |
| These are the electrical terms most frequently used, both at work and at home. For example you may (or may not) know: |
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That the electricity supplied to your home is at 240 volts; |
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That transmission lines (the ones using large steel towers) operate at "high voltage"; |
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That you use light bulbs of different wattage (e.g. 75 watt or 60 watt) in your home; |
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That when you purchase a new electrical heater, one of your considerations is its wattage (e.g. 1,000 watts); and |
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That when you pay your quarterly electricity bill, you are paying for the kilowatt-hours of electricity you have used in your home during that period. |
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| Electrical voltage can be thought of as a measure of the electrical "pressure" applied to the electrical system to force the electricity to flow through the wires. A commonly used analogy is the water supply to your home where the water pressure forces the water through the pipes. |
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| Voltage is measured in volts (V). For convenience, higher voltages are identified in kilovolts (kV) where 1 kV = 1,000 V. |
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| In the "high voltage" parts of the electrical supply network, voltages of 11 kV, 33 kV, 66 kV, 110 kV and 275 kV are common. |
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| In the "low voltage" part of the electricity supply network, your home is supplied with 240 V electricity and, if you have a large air conditioner, with 415 V electricity. (More correctly, these should be called a 240 V single phase supply and a 415 V 3-phase supply. We will talk about "phases" in a later discussion.) |
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| Electrical wattage is a harder concept to visualise because it is, essentially, a measure of how fast electricity is being used - more correctly the "rate of use of electrical energy". For example, a 2,000 W electrical heater would use electrical energy twice as fast as a 1,000 W heater. For convenience, the term kilowatt (kW) is often used instead of 1,000 watts. The wattage of the new 1,000 W electrical heater mentioned above therefore could be identified as 1 kW. |
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| At the other end of the electricity supply system, power stations are producing electricity to match the rate at which electricity is consumed by the end users (plus losses). The "rate at which electrical energy can be produced" determines the wattage of a power station. Usually, power stations are rated in terms of kilowatts (kW) or megawatts (MW) where 1 MW = 1,000 kW. |
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| Electrical energy is commonly measured in terms of kilowatt-hours (kWh) here 1 kWh = 1,000 watt-hours. As a simple example, the 1 kW electrical heater mentioned above would use 1 kWh of electrical energy during each hour it is switched on (i.e. electrical energy used in 1 hour = 1 kW x 1 hour = 1 kWh). |
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| Large amounts of electrical energy are measured in terms of megawatt-hours (MWh) where 1 MWh = 1,000 kWh, or gigawatt-hours (GWh) where 1 GWh = 1,000 MWh. |
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| Note: The "rate of use of electrical energy" (kW) in your home is continually varying. The meter in the switchboard of your home is designed to overcome this variability in its recording of your consumption of electrical energy (kWh). Your electricity account uses data from this meter to identify the amount of electrical energy (kWh) you have used (and for which you have to pay) during the period. |
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What is meant by the terms AC, DC and frequency? |
| Electricity is said to flow when electrons in a suitable material (a "conductor") are induced to move in a particular direction when a suitable force (an "electromotive force" or EMF) is applied to the material. This flow of electricity is called an electrical current and is measured in terms of amperes (usually shortened to amps). The EMF is measured in terms of volts. |
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| Direct Current (DC) electricity is the easiest to visualise because here the electrons (the electrical current) always move in the same direction. A battery is the EMF source most commonly used to produce small amounts of direct electrical current. |
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| For example, the common torch uses a battery as the EMF source. Electrical current flows from one side (e.g. the positive side) of the battery, through the element in the torch bulb (in the process heating the element to produce light) and completes the circuit back to the other side (e.g. the negative side) of the battery. |
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| Alternating Current (AC) electricity can be thought of as electricity that flows in one direction for a short period of time, then reverses its direction of flow for a short period of time, then reverses flow again, and again, and again.. Why does it do this? It's because the EMF source is not constant and changes its polarity (positive and negative sides) in a regular manner. The rate at which the electrical current changes direction through a full cycle (flows in one direction, changes direction and flows in the opposite direction then changes back to the original direction) is called its "frequency". |
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What is meant by the term 'Phases'? |
| Alternating Current (AC) electricity changes its direction of flow in a regular, cyclic manner. Because electrical current flows in response to an applied voltage, the voltage of the AC supply must also have been changing polarity from positive to negative and back again at the same frequency as the alternating current. |
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| The distribution line supplying your home may be single phase and have only two wires strung between the poles (we will use the overhead power lines as examples because they can be easily seen). However, the distribution line may be made up of 4 lines. What are the others? The other lines carry the currents from two other electrical circuits, making a total of three circuits. Because these circuits are electrically linked (see below), they are called phases. The reason why there are only 4 lines is because the 3 phases have a common neutral line (i.e. 3 active lines and 1 common neutral line). |
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| In this "low voltage" part of the distribution system, the voltage between the active and neutral wires is 240 volts. The neutral wire is kept at the same electrical potential as the earth, so that the voltage between the active and earth is also 240 volts. The voltage between the phases in the low voltage distribution system is 415 volts (in Australia). A 415 volt 3-phase supply is able to deliver more energy than a 240 volt single phase supply. A 3-phase supply to a home would normally be required only for large electrical loads. 3-phase supplies are common in industrial areas and shopping centers. |
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| What is ‘Earthing’, and how do safety switches work? |
The power points in your home have three sockets. Some of your appliances have three pin plugs while other appliances have only two pin plugs. Why? The answer lies in the concept of "Earthing".
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| The lower two sockets in a power point are connected to the active and neutral wires. The top socket is connected to a separate wire which is "earthed" (connected to the earth). The need for a separate earth wire can be explained by considering your toaster.
A toaster usually has a metal enclosure. Because metals are electrical conductors (i.e. allow electricity to pass through them), this metal enclosure is "earthed" so that, if the active wire came in contact with the metal enclosure, the electricity would pass to earth. An appliance that requires its enclosure to be earthed must therefore have a three pin plug (active, neutral and earth).
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| An increasing number of appliances are enclosed in materials that prevent the flow of electricity through them (i.e. they are electrical insulators). Because these insulated enclosures do not need to be earthed, the appliances may have plugs with only two pins (active and neutral). |
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| If, for any reason, the active wire comes in contact with the earth wire, the electrical current (flow of electricity) passing through the active wire to earth through the earth wire could be large enough to activate the overload protection device in the active wire's circuit. |
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| A circuit is the term used to describe an active wire that can be isolated from within your home's switchboard. For example, your electric hot water system and your electric stove usually have their own circuits. Your power points may all be connected to the one "power" circuit or they could be divided into two or more separate power circuits. Your lights could also be supplied from one or more "lighting" circuits. Each circuit in your home should be protected against overload by a device that senses the current passing through the active wire of the circuit and isolates the circuit when the current is too high. The overload protection device could be a fuse (a special type of wire that melts if the current passing through it is more that its rated current). More usually now, the overload protection device could be a type of switch that operates on current overload. |
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| We saw above that the overload protection device in a circuit could operate if the active wire contacted the earth wire. A current overload could also occur if the active wire came into direct contact with the neutral wire. |
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Let us now look at what happens when you become part of an electric circuit. If you contact an active wire and you are electrically connected to the earth, current will pass through you to earth. If your connection to earth is poor (e.g. you are standing on a carpet or a wood chair), you may be lucky enough to escape with only a mild shock. If not, the size and duration of the electric current passing through you to ground may have more drastic consequences! A current could also flow through you if you contact both the active and neutral wires. |
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| In both these cases, the current passing through you may be high enough to activate the circuit's overload protection device and turn off the supply to that circuit - but by the time that happened, it may have been too late so save you. What is needed is a device that would sense that something is wrong and switch off the supply of electricity before you are injured. |
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One such device is the "safety switch". This device is also called an "earth leakage circuit breaker" because of the way it operates. To understand how it operates, we need to realise that, in a normal circuit, the current flowing in the active wire of the circuit is exactly the same as the current in the neutral wire of the circuit. If a fault occurs in the circuit and some current flows to earth, the current in the neutral wire would be less than the current in the active wire. A safety switch senses this imbalance in currents and isolates the circuit if the imbalance becomes greater than a preset value. Because safety switches can sense and react to this type of situation much quicker (and at a smaller current) than a normal overload protection device, severe electrical shocks and electrocutions are prevented. |
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However, it must be realised that safety switches cannot protect you if you come in contact with both the active and neutral wires because in this case current does not flow to earth and there is no imbalance in the active and neutral currents. |
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The advantages of Earthing. |
The practice of Earthing is widespread, but not all countries in the world use it.
There is certainly a high cost involved, so there must be some advantages. In fact there are two. They are:
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The whole electrical system is tied to the potential of the general mass of earth and cannot 'float' at another potential. For example, we can be fairly certain that the neutral of our supply is at, or near, zero volts (earth potential) and that the phase conductors of our standard supply differ from earth by 240 volts. |
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By connecting earth to metalwork not intended to carry current (an extraneous conductive part or an exposed conductive part) by using a protective conductor, a path is provided for fault current which can be detected and, if necessary, broken. The path for this fault current is shown in |
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Danger in an unearthed system. |
a. Apparent safety: no obvious path for shock current.
b. Actual danger: shock current via stray resistance and capacitance.
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| Useful Electricity Tips |
Don’t keep electrical appliances switched ‘ON’ in idle condition.
Always use Earth Leakage Circuit Breaker (ELCB) to avoid leakage and unbalance in circuit.
Please ensure total isolation of Phase and Neutral wires.
Always use magnetic switches for appliances with heavy motors.
Inverters and Generators should be totally isolated from each other.
Avoid use of extension chords and don’t leave them dangling or trailing on the floor.
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