One way to categorize electric meters is by the voltage they are required to meter. High-voltage systems require different meter set-ups than the lower voltages that are delivered to residential customers.
Transmission system meter
Meters installed on the transmission system are used to measure large amounts of power flow at generation station switchyards, transmission substations, and grid intertie points where transmission system ownership changes. Although meters used on the transmission system are technically primary meters, it is worth discussing the unique requirements for transmission system meters. Because of the high voltage involved with power metered at these locations, meter installations require current transformers and potential transformers to measure the power flows. These installations are much more sophisticated than the simpler revenue meters generally used at most customer locations and provide a wide range of instantaneous electrical load data and information to remote system operation and control centers. They are often connected to the utility Supervisory Control and Data Acquisition (SCADA) system to transmit the meter data to system operators in near-real time. The accuracy of these data and information meters is critical because even small errors could result in large amounts of inaccurate power flow on the grid or expensive billing errors in wholesale power transactions.
A primary meter is any meter measuring voltages higher than secondary voltages, usually above 480 volts. Primary meters are always transformer-rated installations. Primary meters are commonly used to meter either larger commercial and industrial customers who take transmission or primary distribution level service (and own and operate their own transformers or substations) as well as customers receiving medium voltage at their service. The utility locates the primary meter on the primary distribution system and electricity is measured before it enters the customer-owned service transformers or substation equipment.
The term secondary meter is used to describe any meter installed on the secondary distribution system (behind the secondary service transformer). Installing the revenue meter on the secondary distribution system for residential and small commercial customers is generally the simplest and most economic choice. Secondary meter installations may be either self-contained or instrument-rated depending on the magnitude of the load consumed by the customer. A common metering arrangement for many small residential and commercial customers is to mount the secondary meter on the outside of the building in close proximity to where the electrical service conductors enter the building.