Mains electricity: Difference between revisions

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Type F mains power plug & socket

The term mains usually refers to the general purpose alternating current (AC) electrical power supply (as in "I've connected the appliance to the mains"). The term is not usually used in the United States and Canada. In the US, mains power has a variety of names. These include household power, household electricity, domestic power, wall power, line power, AC power, city power, and grid power. In Canada, it is sometimes called hydro, because much of the Canadian electrical generating capacity is hydroelectric.

Worldwide, many different mains power systems are found for the operation of household and light commercial electrical appliances and lighting. The different systems are primarily characterized by their

• Voltage
• Frequency
• Wall sockets (receptacles)

but also by their

• Earthing system (grounding)
• Protection against overcurrent damage (e.g., due to short circuit), electric shock, and fire hazards
• Parameter tolerances

All these parameters vary among regions. The voltages are generally in the range 100–240 V, the two commonly used frequencies are 50 Hz and 60 Hz, and domestic wall sockets are typically limited to 10–20 A rated current.

Some territories use standards different from those of the countries they belong to (such as Hong Kong in China). Foreign enclaves, such as large industrial plants or overseas military bases, may have a different standard voltage and frequency from the surrounding areas. Some city areas may use standards different from that of the surrounding countryside. Regions in an effective state of anarchy may have no central electrical authority, with electric power provided by incompatible private sources.

Many other combinations of voltage and utility frequency, including direct current, were formerly used, with frequencies between 25 Hz and 133 Hz and voltages from 100 to 250 V. The modern standard combinations of 230 V/50 Hz and 120 V/60 Hz did not apply in the first few decades of the 20th century and are still not universal.

Industrial plants with polyphase power systems will have different, higher, voltages installed for large equipment (and different sockets and plugs), but the common voltages listed here would still be found for lighting and portable equipment.

Voltage levels

All European and most African and Asian countries use a supply that is within 10% of 230 V, whereas Japan, North America and some parts of South America use a supply between 100 and 127 V.

A distinction should be made between the voltage at the point of supply (nominal system voltage) and the voltage rating of the equipment (utilization voltage). Typically the utilization voltage is 3% to 5% lower than the nominal system voltage; for example, a nominal 208 V supply system will be connected to motors with "200 V" on their nameplates. This allows for the voltage drop between equipment and supply. Voltages in this article are the nominal supply voltages and equipment used on these systems will carry slightly lower nameplate voltages.

The choice of utilization voltage is governed more by tradition than by optimization of the distribution system. In theory a 230 V distribution system will use less conductor material to deliver a given quantity of power. Incandescent lamps for 120 V systems are more efficient and rugged than 230 V lamps, while large heating appliances can use smaller conductors at 230 V for the same output rating. Practically speaking, few household appliances use anything like the full capacity of the outlet to which they are connected. Minimum wire sizes for hand-held or portable equipment is usually restricted by the mechanical strength of the conductors. One may observe that both 230 V system countries and 120 V system countries have extensive penetration of electrical appliances in homes. National electrical codes prescribe wiring methods intended to minimize the risk of electric shock or fire.

Many areas using (nominally) 120 V make use of three-wire, single-phase 240 V systems to supply large appliances. Three-phase systems can be connected to give various combinations of voltage, suitable for use by different classes of equipment.

Following voltage harmonization all electricity supply within the European Union is now nominally 230 V ± 10% at 50 Hz . For a transition period (1995–2008), countries who previously used 220 V will use a narrower asymmetric tolerance range of 230 V +6% −10% and those (like the UK) who previously used 240 V will use 230 V +10% −6%. Note that no change in voltage is required by either system as both 220V and 240V fall within the lower 230 V tolerance bands (230 V ±6%). In practice this means that countries such as the UK that previously supplied 240 V continue to do so, and those that previously supplied 220 V continue to do so. However equipment should be designed to accept any voltages within the specified range, and in practice most do so.

In the United States and Canada, national standards specify that the nominal voltage at the source should be 120 V and allow a range of 114 to 126 V (-5% to +5%). Historically 110, 115 and 117 volts have been used at different times and places in North America. Main power is often spoken of as “one-ten”; however, 120 is the nominal voltage.

Voltage tolerances are for steady-state operation; momentary heavy loads, or switching operations in the power distribution network, may cause short-term deviations out of the tolerance band. In general, power supplies derived from large networks with many sources will be more stable than those supplied to an isolated community with perhaps only a single generator.

As of the year 2000, Australia has converted to 230 V as the nominal standard with a tolerance of +10% -6%., this superseding the old 240 V standard, AS2926-1987. As in the UK, 240 V is within the allowable limits and “240 volt” spoken as “two forty volt” remains a synonym for mains in Australian and British English.

In Japan, the electrical power supply to households is at 100 V. Eastern and northern parts of Honshū (including Tokyo) and Hokkaidō have a frequency of 50 Hz, whereas western Honshu (including Nagoya, Osaka, and Hiroshima), Shikoku, Kyūshū and Okinawa operate at 60 Hz. To accommodate the difference, appliances marketed in Japan can often be switched between the two frequencies.

History of voltage and frequency

Voltage & frequency around the world

A 50 Hz ±5 Hz vibrating-reed mains frequency meter for 220 V (this device was made in Czechoslovakia in 1967)

The system of three-phase alternating current electrical generation and distribution was invented by several persons in the 19th Century including Nikola Tesla, George Westinghouse and others. Thomas Edison developed direct current (DC) systems at 110 V and this was claimed to be safer. For more information about the early battles between proponents of AC and DC supply systems see War of Currents. The 110 volt level was chosen to make high-resistance carbon filament lamps practical and economically competitive with gas lighting. While higher voltages would reduce the current required for a given quantity of lamps, the filaments would become increasingly fragile and short-lived; Edison selected 100 volts as a compromise between distribution costs and lamp costs. generation was at 110 volts to allow for a voltage drop between generator and lamp.

In the 1880's only carbon-filament incandescent lamps were available, designed for a voltage of around 100 volts. Later metal filament lamps became feasible. In 1899, the Berliner Elektrizitäts-Werk (BEW), a Berlin electrical utility, decided to greatly increase its distribution capacity by switching to 220 volt nominal distribution, taking advantage of the higher voltage capability of metal filament lamps. The company was able to offset the cost of converting the customer's equipment by the resulting saving in distribution conductors cost. This became the model for electrical distribution in Germany and the rest of Europe and the 220-volt (later 230-volt) system became common. North American practice remained with voltages near 110 volts for lamps.

In 1883 Edison patented a three wire distribution system to allow DC generation plants to serve a wider radius of customers. This saved on copper costs since lamps were connected in series on a 220 volt system, with a neutral conductor connected between to carry any unbalance between the two sub-circuits. This was later adapted to AC circuits. Most lighting and small appliances ran on 120 V, while big appliances could be connected to 240 V. This system saved copper and was backward-compatible with existing appliances. Also, the original plugs could be used with the revised system.

Many different power frequencies were used in the 19th century. As the 20th century continued, more power was produced at 60 Hz (North America) or 50 Hz (Europe and most of Asia). Standardization allowed international trade in electrical equipment; much later, the use of standard frequencies allowed international connections of power grids. The first units at the Niagara Falls generating station produced 25 Hz power and some early systems used 25 Hz. A few industrial customers still use 25 Hz power in the Niagara Region of Ontario and Western New York, from the hydro-electric plants on the Niagara River or from frequency changers operated off the 60 Hz network. Residential and commercial customers in Canada were converted to 60 Hz equipment starting in 1949.

The German company AEG (descended from a company founded by Edison in Germany) built the first European generating facility to run at 50 Hz, allegedly because 60 was not a preferred number. At that time, AEG had a virtual monopoly and their standard spread to the rest of the continent. In Britain, differing frequencies (including 25 Hz, 40 Hz, and DC) proliferated. Implementation of the National Grid in the United Kingdom starting in 1926 compelled the standardization of frequencies among the many interconnected electrical service providers. Notably, the large NESCO network in the north-east part of England was converted at great expense from 40 Hz to 50 Hz to match the national grid. The 50 Hz standard was completely established only after World War II. Similarly, parts of California used 50 Hz power and did not convert to 60 Hz until the late 1940's.

Frequency stability

In AC distribution networks, frequency variations control the transfer of energy between different parts of the network, and help the operators of generators to match demand and supply. If additional loads are connected somewhere in a network, nearby generators will supply larger currents, which slows down their rotational speed due to Lenz's law. This reduces their output frequency and their network vicinity will fall behind in phase compared to other parts of the network. This phase difference in turn increases currents flowing in from generators further away. After a short time, all generators in the network will have settled at a new, lower frequency. Control systems in power plants then detect this drop in the network-wide frequency and open steam valves in turbines to accelerate the generators back to their target frequency. This counteracting usually takes a few tens of seconds due to the large rotating masses involved. Temporary frequency changes are an unavoidable consequence of changing demand. Exceptional or rapidly changing mains frequency is often a sign that an electricity distribution network is operating near its capacity limits, dramatic examples of which can sometimes be observed shortly before major outages.

Long-term stability and clock synchronization

Frequency stabilization of large interconnected power systems allow line-operated clocks to keep accurate time. Network operators will regulate the daily average frequency so that clocks stay within a few seconds of correct time. In practice the nominal frequency is raised or lowered by a specific percentage to maintain synchronization. In the continental European UCTE grid, the deviation between network phase time and UTC is calculated at 08:00 each day in a control centre in Switzerland, and the target frequency is then adjusted by up to ±0.02% from 50 Hz as needed, to ensure maintain long-term frequency average of exactly 3600×24×50 cycles per day. In North America, whenever the error exceeds 2 seconds for the east, 3 seconds for Texas, or 10 seconds for the west, a correction of ±0.02 Hz (0.033%) is applied. Time error corrections start and end either on the hour or on the half hour. A real-time frequency meter for power generation in the United Kingdom is available online.[1] Smaller power systems may not maintain frequency with the same degree of accuracy.

Voltage regulation

To maintain the voltage at the customer's service within the acceptable range, electrical distribution utilities use regulating equipment at electrical substations or along the distribution line. At a substation, the step-down transformer will have an automatic on-load tap changer, allowing the ratio between transmission voltage and distribution voltage to be adjusted in steps. For long (several kilometers) rurual distribution circuits, automatic voltage regulators may be mounted on poles of the distribution line. These are autotransformers again with on-load tapchangers to adjust the ratio depending on the observed voltage changes.

At each customer's service, the step-down transformer has up to five taps to allow some range of adjustment, usually + or - 5% of the nominal voltage. Since these taps are not automatically controlled, they are only used to adjust the long-term average voltage at the service and do not regulate the voltage seen by the utility customer.

See also

• Domestic AC power plugs and sockets
• Electricity
• Energy meter
• Potential difference
• Power connector
• Three-phase electric power

External links

• Grid monitor tools
• Real-Time Data from ERCOT