It is common for utilities to reference standards from both the American National Standards Institute (ANSI) and the International Electrotechnical Commission (IEC). Some utilities install meters of each type on the same system, and advances in electronic meter design can allow for some meters to comply with the performance requirements of both standards.
Like the IEC series of electricity metering standards, the ANSIC12 series covers both electricity metering equipment and communication protocols for electricity meters. There are three active standards for metering equipment and three active standards for communications with meters. (See Table 1).
There are additional ANSI communication standards in process, and ANSI periodically publishes revisions to all the standards. A short explanation of each active ANSI standard is given in the next section. As an aid to a reader who is familiar with either set of standards, the descriptions will identify where there are similarities between the IEC and ANSI standards. This article is intended to be a general rather than a detailed technical comparison.
BRIEF DESCRIPTION OF ANSI STANDARDS
ANSI C12.1 is the overall equipment performance standard for electricity revenue meters. It includes the performance and influence specifications for electromechanical meters (also known as Ferraris meters) as well as specifications common to all ANSI meters, such as reference conditions, design acceptance test procedures, surge withstand tests, insulation tests, environmental tests and mechanical tests. In a general sense, this standard is similar to a combination of IEC 62052-11, General Requirement and IEC 62053-11, Electromechanical Meters.
ANSI C12.10 primarily serves to specify the outline and terminal dimensions of the meters. ANSI meters, unlike IEC meters, are round and typically have blades at the rear that are designed to mount into a socket. The ANSI standards allow a number of different terminal connections such as single phase, two-element, three-element, self contained and transformer-rated. This standard specifies the electrical connections that are made to each of the meter’s blades.
ANSI C12.20 specifies the more accurate metering performance and influence limits for 0.2% and 0.5% accuracy meters. This standard is similar to IEC 62053-22, Static Meters – class 0.2S and 0.5S. In particular it specifies the requirements for load performance, power factor performance, voltage variation performance, frequency variation performance, equality of circuits (for multiphase meters), effect of internal heating on metering performance, effect of ambient temperature on metering performance, and the effect of surges. A revision of this document should be published later in 2003.
ANSI C12.18 is a standard that specifies how to transport data. In this standard the language PSEM (Protocol Specifications for Electric Metering) has been designed to provide an interface between the metering device and any other device over a point-to-point communications medium. It is intended to be used over the electricity meter's optical port. It specifies the low level details such as bit rate, error detection scheme, and time-out. It also specifies session log-on/log-off, read, write, and command structures, as well as the dimensions and optical intensities for the meter’s optical port.
ANSI C12.19 is identical to IEEE 1377-1997. It defines the table structure for utility application data to be passed between a meter and a computer. It does not define device language or protocol. It defines structures for transporting data to and from end devices. It defines a set of tables that allow multiple vendors to make products to read, write, and configure a metering device.
A brief description of the tables would include specifications for configuration of consumption (kWh and others), demand, the control of the meter’s display, security, time-of-use schedule, load profile definitions, event logs, and user-defined tables. The standard also specifies the form and format of manufacturer tables to allow innovation and differentiation in meter design, while allowing the data to be transported and interpreted in standardised ways. Manufacturers are able to define new tables without breaking existing systems.
ANSI C12.21 is an extension of C12.18 that allows for the use of a remote point-to-point communications channel, particularly by telephony. It includes additions for authentication, control of channel connect, disconnect, and timing. ANSI C12.22 is still in review; it will specify additions to extend the communication to a network architecture.
COMPARISON WITH IEC STANDARDS
Both the ANSI and IEC metering standards have been used for many years. The first American electricity metering standard was published in 1910. Both standards have evolved over the years in response to newly developed technology. Both standards have supported manufacturers, regulators, and utility users.
Looking at a typical ANSI meter and a typical IEC meter, the most obvious difference is that the ANSI meter is round and is designed to fit into a socket, whereas the IEC meter is rectangular and designed with a terminal block to accept stripped wires. These styles were developed in the early twentieth century and were influenced by the fact that IEC meters are primarily used indoors (or in a protective enclosure) and ANSI meters are primarily used outdoors. Because the typical ANSI meter is intended for outdoor application, it has a wider specified operating temperature range and provides better weather protection. It must be pointed out, however, that the ANSI standards also define non-socketed meter styles (with a terminal block for stripped wires) and the IEC standard has a set of extended temperature limits for Outdoor Meters.
There are many similarities between the standards. Because both IEC and ANSI meters perform the same primary function, the standards specify many of the same tests. They both specify performance tests such as starting current, creep, and accuracy over a range of load currents, voltages and power factors. Both standards also specify immunity to external influences such as voltage surges, current surges, magnetic fields, electro-static discharge, and radio frequency interference. However, because of differences in specifications of test levels and test conditions, IEC and ANSI meters are not tested at identical conditions. Both standards are applicable at frequencies of 50 Hz or 60 Hz.
A significant aspect that is omitted in the IEC standards is the dimensions of the terminal block. The ANSI standards fully specify the size and shape of the external connections to the meter, for both the socket-type meter (S-base) and the terminal blockmeter (A-base).
There are two facts that help a reader familiar with one set of standards to understand and compare the ANSI and IEC metering standards. The first is the method of defining the current rating. The ANSI standards define a small number of values of maximum current (for example 200A or 10A) and all the other load-based performance requirements are based on that classification. The ANSI mid-scale calibration point is called test amps (TA). Conversely, the IEC standards use a mid-scale calibration point on which to base the other performance requirements. The term basic current (Ib) is used for direct connect IEC meters and the term rated current (In) is used with transformer operated meters. In IEC meters the maximum current is specified separately from the basic current or rated current.
The second important fact is the use of the term class. In the ANSI standards, class is the maximum current rating of the meter. For example an ANSI class 20 meter would have a maximum current rating of 20 amps. In the IEC standards, the term class is the accuracy specification. For example, an IEC class 2 meter would have a basic accuracy rating of 2 percent.
There are many communication protocols used in electronic meters. Historically, each manufacturer developed and supported its own protocol. Recently, however, publicly available standards have been developed which allow a single device to read meters from multiple vendors. For example, the basic ANSI communications standards are now supported by multiple manufacturers.
There is some compatibility between the ANSI and IEC communications standards. The recent release of the IEC protocol standard allows the use of ANSI C12.19 tables. For security and safety reasons, both ANSI and IEC meters use an optical, through the cover, communications path. The physical spacing and optical signals are the same between ANSI and IEC metering standards. However the optical transmitter and receiver positions are reversed and the magnetic clasp mechanism is different between the two standards. In spite of these differences, it is possible to make an optical probe and adapter that can be used with either type of meter.
The ANSI C12 electricity metering standards are used in many countries around the world. While the largest markets for ANSI standard electricity meters are in Canada, Mexico and the United States, they are also being used by many utilities in parts of Asia, Central America and South America, among other countries.
Both the IEC and ANSI electricity meter standards provide a set of documents suitable for use by manufacturers, utilities and regulators. While each standard reflects the requirements of its history, they are also updated periodically to reflect new technology.