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 it alerts the meter reader to a possible recording error. Data must either be
corrected or "force" entered, in which case this fact will be noted. The
exact time the meter was read is recorded to the second. All types of meters
can be serviced. At the end of the day, the device is placed into a computer
interface. The interface equipment automatically feeds all the information
obtained that day into computer data files. The system also prepares reports
with format and information selected by the utility manager. Systems similar
to this help eliminate human error.
2.2.6 Field Testing. The accuracy of a watthour meter can be checked using a
simple time test. The number of watthours for any period is the product of
the number of disc rotations times the meter constant (kh). The speed of
rotation of the disc indicates the use rate or watts being used. The kilowatt
is a more reasonable quantity for consumption rate and the following formula
can be used when timing a meter disc:
Kilowatts (kW) =
PTR x CTR x kh x 3600 x REV
1000 x s
3.6 x pKh x REV
Kilowatts (kW) =
S
where
PTR =
potential transformer ratio
CTR =
current transformer ratio
K h =
meter constant found on the face of the meter
REV =
number of disc revolutions during observation period
period of observation in seconds
s=
p Kh =
primary meter constant Kh x (PTR x CTR)
A stopwatch can be used to check meter accuracy using the formula given
above. The calculated kW can be compared against a known load. If load
remains constant, results are quite accurate. However, since accuracy depends
upon a constant load which may be impossible to obtain in the field, it may be
preferable to compare the watthour meter being checked against a watthour
meter standard of known performance. Since both meters will be connected in
the circuit, small variations in voltage, current, or power factor will not
introduce errors as both meters are observing the same loads.
2.3 Demand Meter. A demand meter is a kilowatthour meter with an accessory
clock motor and counter mechanism. The basic type of demand meter indicates a
single maximum demand during a given time interval, using a sweep pointer.
Behind the front plate there is a pointer-pusher mechanism that advances the
pointer. After each demand interval, the pointer-pusher is reset to zero by
the clock while the pointer remains at its last indicated high point.
Therefore, the maximum demand during a single interval is registered
regardless of the number of intervals spanned. The q aximum demand, in
addition to total kWh consumed, is used for billing purposes.
9-21
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