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 (b) Direct-writing oscillographs record the phenomena or transients directly on a paper
chart using an inking pen. Due to the pen inertia, this type of instrument has a limited frequency
range in the order of 0.5-100 Hz.
5.2.10 Accuracy. The accuracy of an instrument is based on standard conditions at 25C in a
normal mounting position with no stray field (other than the earth's) and no DC ripple. The AC
power source should have a pure sine wave and normal frequency. Accuracy class is now stated
as the limit expressed in percent of fiducial value (usually full scale value) at which errors will
not be exceeded under standard conditions.
Switchboard instruments are usually 1.0 percent class. Panel instruments (2-1/2 to 5-1/2 inch
sizes) are usually 2.0 percent class, however, in special cases panel instruments may be 1.0
percent. Smaller panel instruments may be as much as 5.0 percent; although some of the
cheaper, but larger types, also fall into this category. Portable instruments have recognized
accuracy classes of 0.25 percent, 0.5 percent, and 1.0 percent. These are the most common and
practical classes for general maintenance work. Manufacturers mark instrument dials with a
number (0.5 or 1.0) to denote their accuracy classification.
5.3 METERS. This section provides descriptions of the various meters used to monitor power
system operation conditions.
5.3.1 Watt-hour Meters. Watt-hour meters measure the amount of electric energy used by a
load. Alternating current watt-hour meters employ the induction-disk type of mechanism; the
disk revolves at a speed proportional to the rate at which energy is passing through the meter.
The number of revolutions, through a gear train, is recorded on a dial in kilowatt-hours. The
watt-hour meter may be used to calculate the power used by a load. Count the number of
revolutions of the disk for any number of seconds and use this formula:
power (kilowatts) = 3600 x R x Kh
1000 x S
Where:
Kh = the meter constant (marked on the meter disk or nameplate)
R = the number of revolutions
S = the number of seconds
If the power is being measured through instrument transformers external to the meter, the meter
constant must be multiplied by the instrument transformers ratios (Kh x PT ratio x CT ratio).
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