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 5.3.1.1 Instrument Transformers. Current and potential instrument transformers may be
used with watt-hour meters. On three-phase three-wire circuits, two current-element meters are
used. On four-wire circuits, three current-element meters are necessary.
5.3.1.2 Ratings. Watt-hour meters are rated according to voltage, current, and frequency.
Transformer-rated meters are suitable for service at all voltage and current ratings when used
with appropriate instrument transformers.
(a) The voltage rating for a particular application depends upon the feeder connection.
For self-contained meters, applied on a three-phase three-wire system, the meter is rated at
line-to-line voltage. For metering a load, served from a three-phase four-wire wye system, the
meter is rated at line-to-neutral voltage. The standard voltage ratings are 120, 240, and 480 V.
(b) The following two standard curlent ratings are applicable to each meter:
o
The class designation (CL) of a watt-hour meter denotes the maximum load
range in amperes. The standard class ratings are 10 (for transformer-rated
meters), 100, and 200.
o
The test current rating (TA) of a meter corresponds to the value of current at
which the watt-hour meter is calibrated. Standard test currents and their
relations to meter class are: CLIO - TA 2.5 A, CL100 - 15 A, and CL200 - 30 A.
(c) The standard frequency rating is 60 Hz.
5.3.2 Demand Meters. The maximum power demand during a specified period of time is
measured by a demand meter. Both indicating and recording meters are available for this
purpose.
5.3.2.1 Curve-Drawing Wattmeters. Curve-drawing wattmeters, which record the
load-time curve of the system, can be used to determine the maximum demand by averaging the
load over the selected demand interval of time.
5.3.2.2 Integrating Demand Meters. Integrating demand meters totalize the energy used
over the demand interval and either record the average demand for each interval or, by means of
a maximum indicating pointer, indicate the maximum demand that has occurred since the meter
was last read and reset. The most common demand intervals used in commercial metering are 15
minutes and 30 minutes.
5.3.2.3 Lagged Demand Meters. Lagged demand meters usually obtain their demand
interval by thermal time lag. Lagged demand usually indicates 90 percent of the maximum value
of a suddenly applied steady load by the end of the selected demand interval, and 99 percent at
the end of the succeeding interval.
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