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 TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
(2) Conduit and connection. Tighten all loose
tus with which they are associated is inspected, but
joints in conduit around fittings, terminal boxes and
not less than every 6 months.
supporting clamps. Clean and paint corroded areas.
a. Safety. Before performing any maintenance on
Verify tightness of all bolted connections. Verify
i n s t r u m e n t transformers, they must be de-
that wiring, grounding, and shorting connections
energized and completely isolated from any ener-
provide good contact.
gized source. Isolation may be accomplished by
(3) Drawout mechanisms. Test the proper op-
opening applicable disconnect switches or fuses or
eration of the voltage transformer withdrawal
by de-energizing appropriate circuit breakers. In
mechanisms (tip out) and grounding operation.
polyphase circuits, all phases must be disconnected
to ensure that instrument transformers are not en-
3-30. Tests of instrument transformers.
ergized through interconnected secondaries.
Drawout-type voltage transformers, used in metal-
should receive power factor tests during the sched-
clad switchgear, should be completely withdrawn
uled maintenance period for transformers given in
for maintenance.
table 7-l. Procedure for making these tests is de-
b. Procedure. Maintain bushings and terminals of
scribed in section VII. Other tests, which may be
instrument transformers as described in section
made during or after shop repairs, include:
VII. Maintain fuses, if present, as described in chap-
a. Oil analysis.
ter 8, section II.
b. Ratio.
(1) Case. Inspect case or tank for evidence of
c. Polarity.
corrosion and leaks. Clean and paint as required.
d. Resistance.
Instrument transformers that show evidence of
e. Exciting current.
leaks should be replaced with those of the same
f. Overvoltage.
rating and returned to a shop for repair.
Section VII - BUSHINGS
3-31. Definition of bushings.
(1) Construction, condition, age, and history of
the bushing.
A bushing is an insulating structure which provides
(2) Conditions under which the bushing must
a through conductor or a passageway for such a
operate. Bushings subject to excessive contamina-
conductor. A bushing has a provision for mounting
tion or temperature should be inspected more fre-
on a barrier (conducting or otherwise). The bushing
quently than those that operate under normal con-
insulates the conductor from the barrier and con-
ditions.
ducts current from one side of the barrier to the
(3) Relative importance of service continuity.
other side. The primary function of a bushing is to
(4) Accessibility.
provide an insulated entrance for an energized con-
b. Visual inspection. Bushings should be visually
ductor into an apparatus tank.
inspected for evidence of any condition that will
3-32. Type of bushings covered.
tend to impair satisfactory performance, including:
Information in this section pertains to bushings on
(1) Excessive contamination.
such substation apparatus as power transformers,
(2) Cracked or broken porcelain.
sulfur hexifluoride (SF,) and oil circuit breakers,
(3) Low oil level (oil-filled bushings).
and high-voltage instrument transformers. Al-
(4) Broken or deteriorated seals.
though bushings on low-voltage instrument trans-
(5) Fractured metal parts.
formers ordinarily require little attention, the fol-
(6) Excessive operating temperature.
lowing recommendations for inspection and
(7) Loose or missing parts, such as a power
cleaning can be followed for such equipment as well.
factor test tap cover.
3-33. Maintenance of bushings.
c. Porcelain inspection. When inspecting porce-
lain, the following procedure is recommended:
Bushings are always an integral part of specific
(1) Fractures. Check for fractures and chips in
apparatus and should be inspected along with that
porcelain. The significance of a crack or chip de-
apparatus.
pends on its location and configuration, since a
a. Schedule. External portions of bushings, which
chipped skirt does not affect performance unless the
are easily viewed and form a part of equipment that
effective creepage distance is appreciably reduced.
is under constant supervision, should be visually
If the crack appears to extend into the body of the
inspected on the same schedule as the associated
porcelain, examine it carefully to see whether it is
apparatus. Factors that may increase the frequency
only a harmless surface marking the glaze or some-
of maintenance and inspections include:
3-8
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