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 TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
graphic instruments, revenue watthour meters,
with built-in diagnostic capabilities should be
and similar equipment should receive calibration
checked when their associated power apparatus is
tests at a minimum of every 4 years. Units provided
checked.
Section II-PREVENTIVE MAINTENANCE
12-8. Mounting of instruments and meters.
tion was made. Units should always meet code-
access requirements and be protected from me-
Instruments and meters, designed for permanent
chanical damage.
installation, are available for either flush or projec-
tion mounting. Proper mounting can be a major
12-10. Maintenance of instruments and
factor in reducing the amount of maintenance that
meters.
will be required. A minimum of vibration should be
ensured in the mounting location and design. Vibra-
Accuracy tests, repairs, calibrations, and adjust-
tion is a problem, especially with solid-state units.
ments of instruments and meters should be per-
Shock-resistant mountings may be necessary, if the
formed only by personnel trained and qualified for
panel on which the instrument is mounted is sub-
this type of work, or done under the immediate
ject to shock, as caused by the operation of heavy
supervision of such personnel. Where activities do
switchgear. Locations where strong magnetic fields
not have such specialists on board, arrangements
exist (in proximity of heavy current-carrying con-
should be made with a nearby activity equipped for
ductors), should be avoided if accuracy is to be at-
this type of work; with local electric power compa-
tained.
nies on a maintenance contract basis; or through
use of manufacturer's service shop facilities. It
12-9. Installation of instruments and meters.
should be recognized that accuracy requirements
Check the installation if repeated problems occur.
for meters should be appropriate f o r the use to
Extreme temperatures or excessive moisture may
which readings and records are put, and that the
not have been taken into account when the installa-
cost of high accuracy must be economically justified.
Section III-CALIBRATION AND ADJUSTMENT
12-11. Accuracy check of instruments and
sizes the necessity of selecting an instrument for a
meters.
test such that the readings are on the high end of
the scale.
The measurement of values in ac circuits is some-
b. Meters. The accuracy of an adjusted meter will
times quite complex because of harmonic content,
remain practically unchanged for a long period, un-
phase shifts of voltage versus current, or a combina-
less it is subjected to rough handling, heavy over-
tion of both of these factors. Additional loads on
loads for an extended period, or lightning surges.
various phases may be unbalanced with respect to
The best method of checking meter accuracy is ob-
both loads and phase shifts. Take these conditions
tained in the phantom load tests shown in figures
into account if true rms measuring devices are not
12-1 and 12-2. However, a quick determination of
used and the input source provides harmonics.
the accuracy of a watthour meter under steady load
a. Instruments. The accuracy of observed read-
can be obtained by measuring the time required for
ings depends partly on the construction of the in-
a certain number of meter disk revolutions. For
strument involved and partly on the skill of the
instance, if it takes "t" minutes for the meter disk to
make "r" revolutions, the average power "W" is
observer. It is generally recommended that an in-
given in equation 12-1.
strument with a knife edge pointer be used in con-
junction with a mirror to avoid parallax. The accu-
= watts
(eq. 12-l)
racy of the observed reading of any analog
t
instrument varies with the deflection of the instru-
"K" is the watthour constant and is stamped on the
ment; that is, the larger the deflection, the greater
meter nameplate. The power flow determined by
the accuracy. For example, consider an instrument
this method should be compared with the reading of
with a scale reading of 100, divided into 100 divi-
a digital true rms watthour meter connected in par-
sions, which is accurate to plus or minus one divi-
allel with the watthour meter being checked.
sion. A reading of 20 on this instrument has a mar-
12-12. Adjustment of instruments and
gin or error of plus or minus one division, which
meters.
means that the true value could be between 19 and
21, an actual variation of 10 percent. This empha-
The instrument or meter to be adjusted should be
disconnected from its normal circuit before any
12-2
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