The single-phase electric meter is a ubiquitous sight on nearly every home. Day after day it keeps track of how much energy each utility customer uses. By comparison, three-phase meters are typically reserved for commercial and industrial customers because of their larger power requirements. However, both meter types share several common features and functions.
Utilities rely on three-phase power transmission systems to efficiently deliver power to customers. The three phases, or circuits, are commonly referred to as A-phase, B-phase and C-phase in a polyphase system. Service is provided to single-phase metered customers by using only one of the three phases. In contrast, all three are supplied to commercial and industrial customers with three-phase meters because polyphase service is capable of delivering much more power at higher efficiencies.
The majority of single-phase meters are self-contained. The term refers to the fact that 100 percent of delivered power actually goes through the meter itself. By contrast, transformer-rated meters use devices that allow only a fraction of the actual power through the meter because allowing 100 percent through would destroy the meter. Typical single-phase meters have an ANSI (American National Standards Institute) class rating of 200 amps. This means the meter can safely handle a continuous load of 200 amps of current without exceeding its rating.
The metrology of a meter is the part responsible for accurately measuring power and energy consumption. In older electromechanical meters that have a rotating metal disc, the metrology is made of heavy iron coils, which use electricity to generate a mechanical torque to spin the disc. In newer solid-state, digital meters, the metrology is made of lightweight electronic sensors that perform the same function. Single-phase meters have only a singular metrology. Three-phase meters require separate metrology circuits for each phase. This explains why three-phase electromechanical meters are bulkier and heavier than their single-phase counterparts.
In math and science, constants define a fixed relationship between two or more quantities. Likewise, meter constants are used to define the relationship between metered quantities. One of these is called the watt-hour constant, known as Kh, which is stamped on the faceplate of the meter. In electromechanical meters, the watt-hour constant tells you how many watt-hours of energy it takes to make the disc spin one revolution. A typical single-phase residential meter has a Kh of 7.2 watt-hours per revolution. Knowing this, an estimate can be made of your energy consumption rate by counting the number of complete revolutions the disc makes in a given interval multiplied by the Kh constant . Although solid-state meters do not have spinning metal discs, the legacy value of the Kh watt-hour constant has perpetuated in electronic form.
Single-phase meters have display registers that provide a visual indication of the cumulative kilowatt-hour usage. Electromechanical meters have four or five analog registers that look like miniature clock dials with pointer hands. Solid state digital meters are typically equipped with liquid crystal display (LCD) registers that show numerical readings in a seven-segment digital format like those found on digital clocks and other electronic devices. Registers for single-phase solid-state meters can be programmed to display additional energy quantities for advanced metering applications, such as net energy and time-of-use metering.