TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
INSTRUMENTS AND METERS
12-1. Electrical instruments.
volved. Power quality is influenced by generated
harmonics to the extent that the total harmonic
Industry standards define electrical instruments as
disturbance affects utilization equipment. There is
devices used to measure the present value of electri-
more at chapter 16, section III.
cal quantities under observation. An instrument
c. True rms values. A variety of true rms sensing
may be an indicating instrument or a recording
devices are in use for voltage and current measure-
instrument. By this definition ammeters, voltme-
ments. They are also recommended for power qual-
ters, and frequency meters are instruments, not
ity measurements. Such units will be increasingly
installed in switchgear as recent trends in power
measurements have been toward digital true rms
12-2. Electrical meters.
display devices in the interest of accuracy, ease of
Industry standards define electrical meters as de-
readout, and accumulation of data.
vices used to measure and register the cumulative
12-4. Fixed installations of instruments and
value of electrical quantities with respect to time.
For example, a watthour meter is used to measure
and register the amount of average electrical power
Fixed installations of instruments include amme-
over a period of time.
power factor meters, wattmeters, and watthour
12-3. Validity of electrical measurements.
meters. In general, these may be single-phase or
The basis for measurements should be understood.
three-phase devices, suitable for mounting on
a. Measurement techniques. Many units measur-
switchboard panels, consoles, or switchgear cu-
ing ac values will do so using average, peak, or
bicles. They are available as either indicating, re-
effective values, based on the assumption that the
cording, or integrating meters and instruments.
system provides a pure sine wave. With the growing
Older installations will probably be the electro-
use of solid-state equipment, the waveforms being
mechanical type. New installations will increas-
measured are increasingly less like a pure sine
ingly be provided with solid-state digital units with
wave. Resulting measurements can be misleading,
but such results are not necessarily the fault of the
12-5. Portable instruments and meters.
Portable instruments and meters are used for cali-
b. Equivalent rms value. Sine-wave measure-
bration of fixed instruments and meters, trouble
ments have been traditionally based on the effective
shooting, and maintenance work. They should be
value or root-mean-square (rms) value. This ac volt-
compared periodically with laboratory standards for
age or current (also known as the effective value) is
accuracy. True rms sensing units should be used. If
the value of direct current or voltage that will pro-
not available, they should be requisitioned.
duce the same heating effect. A peak ac value of
1.414 per unit will produce the same heating effect
12-6. Inspections of instruments and meters.
as a 1.0 per unit dc value, if the ac waveform is a
Instruments and meters should be visually in-
pure sine wave. Therefore, instruments and meters
spected periodically to determine that they are
may measures the highest instantaneous, (peak)
clean and that their contact surfaces are free of
value or the half-cycle average value. These values
corrosion; that nuts and binding posts are firmly
are converted by calibration into an rms value.
tightened; and that wire, cable, and leads are ad-
None of these techniques work for accurate mea-
equate, neatly arranged, and properly insulated.
surements of distorted voltage and current
waveforms resulting from the action of increasing
12-7. Tests of instruments and meters.
power electronic l o a d s . Since the distorted
The schedules for the calibration and tests of instru-
waveforms can be represented, mathematically as
ments and meters are dependent, to a great extent,
the imposition of harmonic frequencies (integral
upon the particular installation. When precision is
multiples of the fundamental 60-hertz frequency)
not essential, the period between tests is not critical
these waveforms are considered harmonically dis-
and may be assigned as convenient. Switchboard
torted in relation to the number of harmonics in-