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 where:
Rr =
register ratio found on front plate, back plate, or frame of
register
Kh = watthour constant
TF = transformer factor = CT ratio x PT ratio
Gr =
first gear reduction ratio usually 50 or 100 (obtain from
manufacturer)
Self-contained meters, used for domestic service and small industry, usually
have a multiplier of either 1, in which case no multiplier would be shown, or
10. Whenever the multiplier is more than 1, the words "multiply by", followed
by a number appear on the front plate. For transformer-rated meters, which
are used with instrument transformers for heavy industrial and central station
metering, the multiplier can be 10, 100, 1,000, 10,000, etc. In this case the
register is supplied for only one transformer factor. The multiplier can also
be numerically equal to the transformer factor. If so the register will read
in terms of secondary energy and be read on a "secondary-reading register*'.
In any watthour meter, the total energy in kWh is equal to the product of the
watthour constant (Kh) and the disk revolutions divided by 1,000:
Kh x disk revolutions counted
kWh =
1,000
To determine total energy in kilowatt hours as counted by a transformer-rated
meter, the secondary reading register would be multiplied by the ratio of the
instrument transformers which is called the transformer factor. In a system
using current transformers with a rated primary current of 20 amperes, total
energy registered would equal:
Kh x disk revolutions 20
x
=
kWh
5
1,000
If the system also contained a PT with a 7,200 to 120 ratio, the total energy
registered would equal:
Kh x disk revolutions 20 7,200
x
x
kWh =
1,000
5
120
In this example, the numbers 4 (20/5) and 60 (7,200/120) are the ratios of CT
and PT respectively. Their product, 240 (4 x 60), is the transformer factor
for this meter. Whatever value is shown on the secondary register must be
multiplied by the transformer ratio to obtain the correct number of kWh used
in the primary circuit.
2.1.6.3 Voltage Rating.
Meters are available for 120-, 240, or 480-volt
service as required.
9-16
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