TM 5-683/NAVFAC MO-116/AFJMAN 32-1083
tested and recalibrated every 2 years. Single-phase
(11) Verify connection of secondary PT leads by
watt hour meters should be tested at least once
applying a low voltage to the leads and checking for
voltage contribution at applicable devices.
every 5 years; self-contained polyphase watt-hour
meters every 2 years. Transformer watt-hour
(12) Check for PT secondary load with second-
meters on the secondary system (600 volts and be-
ary voltage and current measurements. Make sure
low) should also be tested every 2 years; trans-
load is less than volt amps (VA) of PT.
f. Metering. Most of the buildings in a typical
former watt-hour meters on the primary system
military installation are unmetered. Meters (both
every year. Voltmeters and ammeters should be cali-
recording and indicating), relays, and associated
brated at 2-year intervals.
equipment are usually part of a substation, main
g. Alarms. Alarms associated with transformer
overtemperature, high or low pressure, circuit
distribution switchboard equipment, or special
breaker trip, accidental ground on an ungrounded
equipment. Industrial shops and other buildings oc-
system, cooling water flow or overtemperature, or
casionally utilize kilowatt hour demand meters,
other system conditions should be tested periodi-
cally to assure proper operation.
an effort to determine whether the maintenance or
h. Indicators. Circuit breaker "open-close" indica-
operation of these meters is adequate demands that
tors can be checked during their regular mainte-
the metering instruments be of the specified range,
nance. Ground indicator lamps for ungrounded elec-
accurately calibrated, and adequately serviced. In-
tric systems should be checked daily or weekly for
strument accuracy is always expressed in terms of
proper operation. Other miscellaneous indicators
the percentage of error at the full-scale point. Maxi-
such as flow, overtemperature, excess pressure, etc.,
mum accuracy, consequently, is obtainable by keep-
should be checked or operated periodically to assure
ing the rating as high on the scale as possible, and
requires a properly rated instrument. An instru-
i. Protective relaying. While the application of cir-
ment with an accuracy of 1 percent, with a scale
cuit protection as developed in a short circuit and
reading of 100 divided into 100 divisions, is accurate
coordination study is an engineering function, as-
to plus or minus one division. Hence, a reading of 20
surance that this designed protection remains in
has a margin of error of plus or minus one division,
operation is a maintenance responsibility. Applying
which in this instance means that the true value
relay settings and periodically testing them are
could be between 19 or 21--an actual variation of 10
maintenance functions. Relays should be examined
percent. If the maximum indicating meter reading
to ensure the following:
is less than 50 percent of the meter range, indicat-
ing meters should be recalibrated, and ratchets and
(1) All moving parts are free of friction or bind-
dials changed. How often an instrument is cali-
brated depends on its use and the desired accuracy.
(2) All wiring connections are tight.
If calibration standards and equipment are not
(3) All contacts are free of pitting or erosion.
available, instruments of nearly the same rating
(4) solenoid coils or armatures are not over-
may be checked against each other. If wide discrep-
ancies are noted, the suspect instrument should be
(5) Glass, covers or cases are not damaged.
j. For relay testing procedures, refer to chapter 14.
checked by a competent laboratory or returned to
The protective relay circuitry should also be
the manufacturer. For many military establish-
ments, the utility company will best perform this
checked by closing the breaker in the test position
work. Take care to prevent entrance of dirt or lint
and while documenting, closing the contacts of each
into an instrument because this dirt could hinder
protective relay to trip the circuit breaker.
the actuation of the instrument pointer. Clean the
2-9. Switchgear trouble-shooting.
glass with a damp cloth because a dry cloth may
induce a static charge on the glass and affect the
Table 22 provides detailed information regarding
instrument reading. Breathing on a charged glass
trouble-shooting switchgear failures. Probable
discharges it. Never oil instrument bearings. Indi-
causes along with recommended remedies are listed
cating demand and power factor meters should be
for typical failures.