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Typical US domestic electricity meter

An electric meter or energy meter is a device that measures the amount of electrical energy supplied to a residence or business. These are customers of an electric company.

The most common type is more properly known as a (kilo)watt-hour meter or a joule meter. Utilities record the values measured by these meters to determine demand during a specific time period. The demand is used to generate an invoice for the electricity.

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.

The type of meter described above is used on a single-phase AC supply. Different phase configurations use additional voltage and current coils.

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.

Solid state meters can also record other parameters of the load and supply such as maximum demand, power factor and reactive power used etc.

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.

Demand meters

In its simplest form, a demand meter has a gauge whose pointer moves a marker. When the gauge falls back, friction keeps the marker in place. When a demand meter is read, its marker is reset, usually with a magnet from outside the sealed meter enclosure. Computerized demand meters usually find the fifteen-minute interval in the month with maximum demand. Often they also record a month worth of fifteen-minute averages. Some demand meters measure the temperature of a conductor, or simulate the heating of the conductor, in order to track "running" demand. Running demand meters usually log the times when a maximum demand is exceeded, or they log the times when the meter enters a different tariff rate (see below).

So, why would anyone care? Electricity cannot be stored, so Electricity retailers need to arrange the necessary generators to meet the maximum demand. New generators are long-term capital investments, so demand also directly affects the retailers' and power-providers' accounting, and need for long-term debt. In particular, when interest rates are high, generating companies are reluctant to install new capacity, and want customers to reduce demand, so the retailers use meters to find and punish high demand.

Since the need for new equipment is really set by how hot the existing equipment runs, demand meters are really an attempt to find and control the sources of overheating in the electrical grid. Heating is caused by current. Therefore, demand meters ideally measure Volt-amperes, which combine both reactive (reflected) and actual (consumed) AC power, because both consumed and reflected power cause currents and therefore heating in distribution equipment.

Multiple tariff (variable rate) meters

Electricity retailers may wish to charge customers different tariffs at different times of the day. This is because there is generally a surplus of electrical generation capacity at times of low demand, such as during the night (see supply and demand).

Multiple tariffs are made easier by time of use (TOU) 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 most electricity grids have 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 advertisements in popular television programmes.

Some multiple tariff meters use different tariffs for different amounts of demand. These are usually industrial meters.

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.

Newer electronic meters can be read automatically. Remote meter reading is an application of telemetry. Often, meters designed for semi-automated reading have a serial port on that communicates by infrared LED through the faceplate of the meter. In some apartment buildings, a similar protocol is used, but in a wired bus using EIA-485 to connect all the meters to a single plug. The plug is often near the mailboxes. In the European Union, the most common infrared and EIA-485 protocol is "FLAG", a simplified subset of mode C of IEC 1107. In the U.S. and Canada, the favoured infrared protocol is ANSI C12.18. Some industrial meters use a protocol for programmable logic controllers, MODBUS. The most modern protocol proposed for this purpose is DLM/COSEM which can operate over any medium, including serial ports. The data can be transmitted by Zigbee, WiFi, telephone lines or over the power lines themselves. Some meters can be read over the internet.

Accuracy

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.

Ownership

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.

Location

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.

Connection

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).

Security

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 (to saturate the magnetic circuits) or altering the characteristics of the load (adding capacitance, or instantaneous high currents) with the intention of temporarily or permanently altering the characteristics of the meter.

The owner of the meter normally secures the meter against such acts. Meters are usually sealed so that the connections and mechanism cannot be tampered-with without breaking the seal. Meters may also measure VAR-hours (the reflected load), neutral current (elevated by most electrical tampering), ambient magnetic fields, etc. The normal tactic, legal in most areas, is to switch the metering rate to the meter's maximum designed rate when tampering is detected. Meter readers are trained to spot signs of tampering, and in this case, the maximum rate may be charged each billing period until the tamper is removed. Substations and some transformers have a high-accuracy meter for the area served. Power companies normally investigate discrepancies between the total billed and the total generated, in order to find and fix power distribution problems. These investigations are also an effective method of discovering tampering.

Prepayment meters

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. In some countries, if the retailer believes that the customer may for whatever reason not pay the bill, a prepayment meter may be installed. 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.

Modern solid-state electricity meters in conjunction with smart card technology have removed these disadvantages and such meters are commonly used for customers considered to be a poor credit risk. In the UK, one system is the PayPoint network, where rechargable tokens (Quantum cards for natural gas, or plastic "keys" for electricity) can be loaded with whatever money the customer has available.

Power export

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.

High-End Energy Meters

Energy meters have come a long way from being just passive instruments determining consumption of energy during a predetermined period. The high-end meters today come packed with a plethora of features, with major breakthroughs in being non-tamperable and highly accurate at the same time.

Block Diagram

Basic Working Of a Modern Electronic Energy Meter

As in the block diagram, the meter has a Power Supply, A metering Engine, A processing and communication engine i.e a microcontroller, other add-on modules such as RTC, LCD display, Communication ports/ Modules etc.

Power Supply

The power supply has the responsibility of providing power to various electronic components in the meter. Meters usually use less than 50 milliwatts, in order to reduce the pwoer company's costs (it is fraud for it to charge customers for the pwoer meters' consumption). Normally the power supply uses a large capacitor, charged by the high voltage through a diode, and slowly drained through a resistor network and voltage regulator. This is far less expensive than a transformer, or the switching power supply used in a PC.

Metering Engine

The metering engine is given the voltage and current inputs and has a voltage reference, samplers and quantisers followed by an ADC section to yield the digitised equivalents of all the inputs. These inputs are then processed using a Digital Signal Processor to calculate the various metering parameters such as powers, energies etc.

The largest source of long-term errors in the meter is drift in the preamp, followed by the precision of the voltage reference. Both of these vary with temperature as well, and vary wildly because most meters are outdoors. Characterizing and compensating for these is a major part of meter design.

Processing and Communication Section

This section has the responsibility of calculating the various derived quantities from the digital values generated by the metering engine. This also has the responsibility of communication using various protocols and interface with other addon modules connected as slaves to it.

RTC and Other Add-on Modules

These are attached as slaves to the processing and communication section for various input/output functions.

Time of Use Metering

They facilitate load control and planning on the part of utilities. This is effectively achieved using a concept called Time of Usage (TOU) metering. This involves dividing the day, month and year into tariff slots and with higher rates at peak load periods and low tariff rates at off-peak load periods. While this can be used to automatically control usage on the part of the customer (resulting in automatic load control), it is often simply the customers responsibility to control his own usage, or pay accordingly (voluntary load control). This also allows the utilities to plan their transmission infrastructure appropriately.

Communication Technologies

Another development has been in the communication facilities that these high end meters boast of. They now come equipped with Low Power Radio, GSM, GPRS, Bluetooth, IrDA apart from the now conventional RS-232 and RS-485 wired link. They now store the entire usage profiles with time stamps and relay them at a click of a button. The demand readings stored with the profiles accurately indicate the load requirements of the customer. This load profile data is processed at the utilities and renders itself to a variety of representations, all sorts of graphs, reports et el.

Pre-Payment Electricity Meter

Newer developments have also been in the field of Prepayment Electricity Supply, analogous to the Prepaid phone connection most of us are used to. These deploy Smartcards, both of conventional and the contact-less variety, to recharge the meter with a prestipulated amount of electricity, and optionally, to warn the utilities about any alarming signs in the usage behaviour of the costumer by taking the data back to them at the next recharge.

AMR & RMR

Energy Theft is a major concern across the orld. All of us must be aware of the dire consequences that loom large on the power industry due to this. A very accurate description of this can be found in Overload by Arthur Hailey. The weakest cog in the wheel is the human element involved in the reading of the meters and also the meters being easily accessible to the custumer who may then try and tamper with it. A solution to this ever burgeoning problem would be to eliminate the human element involved. On the part of meter reading this can be done with a set of technologies called AMR (Automatic Meter Reading) and RMR (Remote Meter Reading), of which the above mentioned communication technologies are a good implementation. On the part of the customer, this can be effectively achieved using off-site metering, that is an electronic meter is placed at the junction point where all the connections originate, inaccessible to the end-user, and it relays the readings via the AMR technology to the utility.

When energy theft is suspected, the power utility will sometimes install a second, remote meter. This meter is often disguised as a utility transformer. Any substantial discrepancy between the energy usage reported by this meter and the meter on the customer's property can be assumed to be caused by energy theft.

The movement in the energy meter development is moving in the forward direction and has come a long way towards a loss free and a hassle free energy usage across the world.

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 charge (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.

References

• Handbook for Electricity Metering- the "Bible", covers theory and practice from the most basic "an electron is..." to the most esoteric. A meter's-eye view of the entire electric power distribution system. Expensive, but worth every penny.

External links

• [1] Paper describing in detail the mathematics and physics behind induction meters
• [2] Photos and descriptions of historic and modern North American electricity meters
• General Electric (U.S.)
• Landis + Gyr (Germany)
• Capital Power (India)
• Itron - Solid-State Electricity Meters

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