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What You should know about electric power
Electric power is the product of two quantities: current and voltage. These two quantities can vary with respect to time (AC power) or can be kept at constant levels (DC power).
Most refrigerators, air conditioners, pumps and industrial machinery use AC power whereas most computers and digital equipment use DC power (the digital devices you plug into the mains typically have an internal or external power adapter to convert from AC to DC power). AC power has the advantage of being easy to transform between voltages and is able to be generated and utilised by brushless machinery. DC power remains the only practical choice in digital systems and can be more economical to transmit over long distances at very high voltages (see HVDC).
The ability to easily transform the voltage of AC power is important for two reasons: Firstly, power can be transmitted over long distances with less loss at higher voltages. So in power systems where generation is distant from the load, it is desirable to step-up (increase) the voltage of power at the generation point and then step-down (decrease) the voltage near the load. Secondly, it is often more economical to install turbines that produce higher voltages than would be used by most appliances, so the ability to easily transform voltages means this mismatch between voltages can be easily managed.
Solid state devices, which are products of the semiconductor revolution, make it possible to transform DC power to different voltages, build brushless DC machines and convert between AC and DC power. Nevertheless devices utilising solid state technology are often more expensive than their traditional counterparts, so AC power remains in widespread use.
Wires - about colors
To enable wires to be easily and safely identified, all common wiring safety codes mandate a colour scheme for the insulation on power conductors. In a typical electrical code, some colour-coding is mandatory, while some may be optional. Many local rules and exceptions exist per country, state or region.1 Older installations vary in colour codes, and colours may fade with insulation exposure to heat, light and ageing.
As of March 2011, the European Committee for Electrotechnical Standardization (CENELEC) requires the use of green/yellow colour cables as protective conductors, blue as neutral conductors and brown as single-phase conductors.2 The United States National Electrical Code requires a green or green/yellow protective conductor, a white or grey neutral, and a black single phase.3
The United Kingdom requires the use of wire covered with green insulation, to be marked with a prominent yellow stripe, for safe earthing (grounding) connections.4 This growing international standard was adopted for its distinctive appearance, to reduce the likelihood of dangerous confusion of safety earthing (grounding) wires with other electrical functions, especially by persons affected by red-green colour blindness.
In the UK, phases could be identified as being live by using coloured indicator lights: red, yellow and blue. The new cable colours of brown, black and grey do not lend themselves to coloured indicators. For this reason, three-phase control panels will often use indicator lights of the old colours.
The kilowatt hour (symbol kWh, kW?h, or kW h) is a derived unit of energy equal to 3.6 megajoules. If the energy is being transmitted or used at a constant rate (power) over a period of time, the total energy in kilowatt-hours is the product of the power in kilowatts and the time in hours. The kilowatt-hour is commonly used as a billing unit for energy delivered to consumers by electric utilities.
The symbol "kWh" is commonly used in commercial, educational, scientific and media publications, and is the usual practice in electrical power engineering.
Other abbreviations and symbols may be encountered:
"kW h" is less commonly used. It is consistent with SI standards (but note that the kilowatt-hour is a non-SI unit). The international standard for SI states that in forming a compound unit symbol, "Multiplication must be indicated by a space or a half-high (centered) dot (?), since otherwise some prefixes could be misinterpreted as a unit symbol" (i.e., kW h or kW?h). This is supported by a voluntary standard6 issued jointly by an international (IEEE) and national (ASTM) organization. However, at least one major usage guide and the IEEE/ASTM standard allow "kWh" (but do not mention other multiples of the watt hour). One guide published by NIST specifically recommends avoiding "kWh" "to avoid possible confusion".
The US official fuel-economy window sticker for electric vehicles uses the abbreviation "kW-hrs".
Variations in capitalization are sometimes seen: KWh, KWH, kwh etc.
"kW?h" is, like "kW h", preferred with SI standards, but it is very rarely used in practice.
The notation "kW/h", as a symbol for kilowatt-hour, is not correct.