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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.

    A printed circuit board (PCB) is a laminated sandwich structure of conductive and insulating layers. PCBs have two complementary functions. The first is to affix electronic components in designated locations on the outer layers by means of soldering. The second is to provide reliable electrical connections (and also reliable open circuits) between the component's terminals in a controlled manner often referred to as PCB design.  Each of the conductive layers is designed with an artwork pattern of conductors (similar to wires on a flat surface) that provides electrical connections on that conductive layer. Another manufacturing process adds vias, plated-through holes that allow interconnections between layers. PCBs mechanically support electronic components using conductive pads in the shape designed to accept the component's terminals, and also electrically connect them using traces, planes and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive substrate. Components are generally soldered onto the PCB to both electrically connect and mechanically fasten them to it. Printed circuit boards are used in nearly all electronic products and in some electrical products, such as passive switch boxes.

    A potential transformer (PT) is an instrument used to transform voltage. However, the purpose of the transformer to transform voltage is to facilitate the transmission of electric energy, so the capacity is very large, and it is generally calculated in kVA or MVA; and the purpose of the voltage transformer to transform the voltage is mainly used for measuring instruments and relays. Protection devices supply power and are used to measure the voltage, power and energy of a line, or to protect valuable equipment, motors and transformers in a line in the event of a line failure. Therefore, the capacity of the voltage transformer is very small, generally only a few VA, tens of VA, and the maximum does not exceed one thousand VA. The basic structure of the voltage transformer is very similar to that of the transformer. It also has two windings, one is called the primary winding and the other is called the secondary winding. Both windings are mounted or wound around the iron core. There is insulation between the two windings and between the windings and the iron core, so that there is electrical isolation between the two windings and between the windings and the iron core. When the voltage transformer is running, the primary winding N1 is connected to the line in parallel, and the secondary winding N2 is connected to the instrument or relay in parallel. Therefore, when measuring the voltage on the high-voltage line, although the primary voltage is high, the secondary is low-voltage, which can ensure the safety of operators and instruments.

    A current transformer is an instrument that converts a large current on the primary side into a small current on the secondary side for measurement based on the principle of electromagnetic induction. A current transformer consists of a closed iron core and windings. Its primary side winding has few turns and is connected in the line of the current to be measured. Therefore, it often has all the current of the line flowing through it, and the secondary winding has a large number of turns. It is connected in series in the measuring instrument and the protection circuit. When the current transformer is working, its secondary circuit is always closed, so the measurement The impedance of the series coil of the instrument and the protection circuit is very small, and the working state of the current transformer is close to a short circuit. The current transformer is to convert the large current on the primary side into a small current on the secondary side for measurement, and the secondary side cannot be opened.

    Liquid crystal display is an active matrix liquid crystal display driven by thin film transistors. It mainly uses current to stimulate liquid crystal molecules to generate dots, lines, and surfaces with the back light tube to form a picture. IPS, TFT, and SLCD are all subclasses of LCD. Its working principle is that under the action of the electric field, the alignment direction of the liquid crystal molecules is changed, so that the light transmittance of the external light source is changed (modulated) to complete the electro-optical conversion. Then use the different excitations of the R, G, B three primary color signals to pass through the red, green, and blue three primary color filters to complete the color reproduction in the time domain and the space domain.