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An electricity meter is generally taken to be a device which measures the amount of electrical energy supplied to a customer of an electricity company. The most common type is more properly known as a (kilo)watt-hour meter or a joule meter.
Means of operation
Modern electricity meters operate by continuously measuring the instantaneous voltage (volts) and current (amperes) and finding the product of these to give instantaneous electrical power (watts) which is then integrated against time to give energy used (joules, kilowatt-hours etc).
The most common type of electricity meter is the electromechanical induction meter. This consists of an aluminium disc which is acted upon by two coils. One coil is connected in such a way that it produces a magnetic flux in proportion to the voltage and the other produces a magnetic flux in proportion to the current. This produces eddy currents in the disc and the effect is such that a force is exerted on the disc in proportion to the product of the instantaneous current and voltage. A permanent magnet exerts an opposing force proportional to the speed of rotation of the disc - this acts as a brake which causes the disc to stop spinning when power stops being drawn rather than allowing it to spin faster and faster. This causes the disc to rotate at a speed proportional to the power being used.
The aluminium disc is supported by a spindle which has a worm gear which drives the register. The register is a series of dials which record the amount of power used. The dials may be of the cyclometer type where for each dial a single digit is shown through a window in the face of the meter, or of the pointer type where a pointer indicates each digit. It should be noted that with the dial pointer type, adjacent pointers generally rotate in opposite directions due to the gearing mechanism.
Some newer meters are solid state and display the power used on an LCD. Most solid-state meters use a current transformer to measure the current. This means that the main current-carrying conductors need not pass through the meter itself and so the meter can be located remotely from the main current-carrying conductors, which is a particular advantage in large-power installations. It is also possible to use remote current transformers with electromechanical meters though this is less common.
Unit of measurement
The most common unit of measurement on the electricity meter is the kilowatt-hour which is equal to the amount of energy used by a load of one kilowatt over a period of one hour, or 3,600,000 joules. Some electricity companies use the SI megajoule instead.
Electricity retailers may wish to charge customers different tariffs at different times of the day. This is because there is generally a surplus of electricity at times of low demand, such as during the night (see supply and demand). Such tariffs are facilitated by meters which incorporate or are connected to a time switch and which have multiple registers. In the UK such tariffs are branded Economy 7 or White Meter and are commonly used in conjunction with electrical storage heaters. The popularity of such tariffs has declined in recent years, at least in the domestic market, due to the (perceived or real) deficiencies of storage heaters and the low cost of natural gas.
Domestic variable-rate meters normally only permit two tariffs ("peak" and "off-peak") and in such installations a simple electromechanical time switch may be used. Large commercial and industrial premises may use electronic meters which record power usage in blocks of half an hour or less. This is because on most electricity grids there are demand surges throughout the day, and the power company may wish to give incentives to large customers to reduce demand at these times. These demand surges often corresponding to meal times or, famously, to intervals in popular television programmes.
Means of reading
Most domestic electricity meters must be read manually, whether by a representative of the power company or by the customer. Where the customer reads the meter, the reading may be supplied to the power company by telephone, post or over the internet. The electricity company will normally require a visit by a company representative at least annually in order to verify customer-supplied readings and to make a basic safety check of the meter.
An electricity meter will be designed to operate across a specified range of voltage, current and frequency. Mechanical meters normally have an accuracy of better than 2%. Solid-state meters may have an accuracy better than 0.8%. The accuracy of the meter will be poorest at the extremes of its specified operating conditions.
Most domestic electricity meters do not account for reactive power but more sophisticated meters which do measure reactive power may be used in commercial and industrial environments where low power factor loads may be present.
In most countries, the required accuracy of the meter is specified by law. The accuracy of a meter may decline with age and mechanical meters may be affected by events such as voltage spikes caused by lightning strikes on the supply. The manufacturer will generally guarantee the accuracy of the meter for a certain number of years and after that period the electricity company must replace or recalibrate the meter.
Due to the deregulation of electricity supply markets in many countries, the company responsible for an electricity meter may not be obvious. Depending on the arrangements in place, the meter may be the property of the electricity distributor, the retailer or for some large users of electricity the meter may belong to the customer.
The company responsible for reading the meter may not always be the company which owns it. Meter reading is now sometimes subcontracted and in some areas the same person may read gas, water and electricity meters at the same time.
The location of an electricity meter varies with each installation. Possible locations include on a power pylon serving the property, in a street-side cabinet or inside the premises adjacent to the consumer unit / distribution board. Electricity companies may prefer external locations as the meter can be read without gaining access to the premises but external meters may be more prone to vandalism.
As stated above, the use of current transformers permits the meter to be located remotely from the current-carrying conductors. This arrangement is commonly used in larger installations, for example an outdoor substation serving a single large customer may have metering equipment installed in a nearby cabinet without the need to bring the very heavy cables leading out of the substation into the cabinet.
In North America, it is common for smaller electricity meters to clip into a standardised base unit. This arrangement allows the meter itself to be replaced without disturbing the supply and load cables which terminate in this base unit. Some base units may have a facility to bypass the meter whilst it is removed for service. The amount of electricity used without being recorded during this small time is considered insignificant when compared to the inconvenience which might be caused to the customer by cutting off the electricity supply.
In the UK, the supply and load terminals are normally provided in the meter housing itself, at least for smaller meters (up to around 100 amps).
Some customers may attempt to manipulate the meter such that it under-registers or even runs backwards, effectively using power without paying for it. Some may justify this by reference to the increasing costs of energy, profits of the electricity company, etc. Most would consider this to be fraud and such behaviour is probably illegal in most countries.
Techniques vary from unsubtle means such as physically breaking the meter housing and jamming the mechanism to more sophisticated methods involving applying magnets to the outside of the meter or altering the characteristics of the load with the intention of temporarily or permanently altering the characteristics of the meter.
The owner of the meter may employ a number of means to secure the meter against such actions. Most consist of physical means such as sealing the meter in such a way that the connections and mechanism cannot be tampered with without breaking the seal. Meter readers are also trained to spot signs of tampering.
The standard business model of electricity retailing involves the electricity company billing the customer for the amount of energy used in the previous month or quarter. If the retailer believes that the customer may for whatever reason not pay the bill then a prepayment meter may be fitted. This requires the customer to make advance payment before electricity can be used. If the available credit is exhausted then the supply of electricity is cut off by a relay.
In the UK, mechanical prepayment meters used to be common in rented accommodation. Disadvantages of these included the need for regular visits to remove cash, risk of theft of the cash in the meter and the lack of a means of applying a standing charge.
Many electricity customers are installing their own electricity generating equipment, whether for reasons of economy, redundancy or environmental reasons. Gas turbines, wind turbines and photovoltaic cells are all in common use. When a customer is generating more electricity than required for his own use, the surplus may be exported back to the power grid.
This exported energy may be accounted for in the simplest case by the meter running backwards during periods of net export, thus reducing the customer's recorded energy usage by the amount exported. More sophisticated meters permit such exported energy to be recorded and accounted for separately.
See also Net metering.
Other types of electricity meter
In addition to the types of meter described above which directly measure the amount of energy used, other types of meter are available.
Meters which measure the amount of current (coulombs) used, known as amp-hour meters, were used in the early days of electrification. These were dependant upon the supply voltage remaining constant for accurate measurement of energy usage which is not a likely circumstance with most supplies.
Some meters measured only the length of time for which current flowed, with no measurement of the magnitude of voltage or current being made. These were only suited for constant load applications.
Neither type is likely to be found in electricity retail use today.
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