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An electricity meter, electric meter, electrical meter, energy meter, or kilowatt-hour meter is a device that measures the amount of electric energy consumed by a residence, a business, or an electrically powered device. Electric meter or energy meter measures the total power consumed over a time interval. Electric utilities use electric meters installed at customers' premises for billing and monitoring purposes. They are typically calibrated in billing units, the most common one being the kilowatt hour (kWh). They are usually read once each billing period. When energy savings during certain periods are desired, some meters may measure demand, the maximum use of power in some interval. "Time of day" metering allows electric rates to be changed during a day, to record usage during peak high-cost periods and off-peak, lower-cost, periods. Also, in some areas meters have relays for demand response load shedding during peak load periods.

(1) Electric energy meters can be divided into DC electric energy meters and AC electric energy meters according to the circuits they use. AC electric energy meters can be divided into single phase energy meters, three-phase three-wire electric energy meters and three-phase four-wire electric energy meters according to their phase lines; (2) Electric energy meters can be divided into electrical-mechanical electric energy meters and electronic electric energy meters (also known as static electric energy meters, solid-state electric energy meters) according to their working principles. Electromechanical electric energy meters are used in AC circuits as ordinary electric energy measuring instruments, and the most commonly used ones are inductive electric energy meters. Electronic energy meters can be divided into fully electronic energy meters and electromechanical energy meters; (3) Electric energy meters can be divided into integral electric energy meters and split electric energy meters according to their structure; (4) Electric energy meters can be divided into active electric energy meters, reactive electric energy meters, maximum demand meters, standard electric energy meters, multi-rate time-of-use electric energy meters, prepaid electric energy meters, loss electric energy meters and multi-functional electric energy meters according to their uses; (5) Electric energy meters can be divided into ordinary installed electric energy meters (0.2, 0.5, 1.0, 2.0, 3.0 grades) and portable precision electric energy meters (0.01, 0.02, 0.05, 0.1, 0.2 grades) according to their accuracy levels.

A relay is an electrically operated switch. It consists of a set of input terminals for a single or multiple control signals, and a set of operating contact terminals. The switch may have any number of contacts in multiple contact forms, such as make contacts, break contacts, or combinations thereof. Relays are used where it is necessary to control a circuit by an independent low-power signal, or where several circuits must be controlled by one signal. Relays were first used in long-distance telegraph circuits as signal repeaters: they refresh the signal coming in from one circuit by transmitting it on another circuit. Relays were used extensively in telephone exchanges and early computers to perform logical operations. The traditional form of a relay uses an electromagnet to close or open the contacts, but other operating principles have been invented, such as in solid-state relays which use semiconductor properties for control without relying on moving parts. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called protective relays. Latching relays require only a single pulse of control power to operate the switch persistently. Another pulse applied to a second set of control terminals, or a pulse with opposite polarity, resets the switch, while repeated pulses of the same kind have no effects. Magnetic Latching relays are useful in applications when interrupted power should not affect the circuits that the relay is controlling.