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TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
for very small capacity substations. The importance
sure that the equipment is de-energized, and
ground the equipment prior to the start of any
of the substation determines the need for ground
resistance tests.
(10) External inspection. Provide an external
(4) Surge arresters. Surge arresters are used to
inspection on a semi-annual basis. Check the tank,
protect aerially supplied transformers from light-
radiators, the tap changer, and all gasketed or other
ning and other surges. They should be inspected for
openings for leaks, deposits of dirt, or corrosion.
looseness, broken parts, dirt, and other deposits.
Inspect connections for signs of overheating and
Clean, tighten, and replace parts as necessary an-
corrosion. Inspect bushing insulating surfaces for
tracking, cracks, or chipped skirts. Inspect bushing
(5) Pressure-relief devices: Most transformers
gasketed bases for leaks. Leak repair, cleaning, and
are equipped with pressure-relief devices to relieve
painting should be done as required. See chapter 3,
excessive pressure in the tank due to internal arc-
section VII for bushing maintenance. Check louvers
ing. This device is set to open at a pressure of 10 to
in the enclosures of ventilated dry-type transform-
15 pounds per square inch (69 to 103 kilopascals). A
ers for clogging by dirt or other obstructions. A high
quarterly inspection of pressure-relief devices
noise level or change in the noise level could indi-
should include checking for leaks around joints, dia-
cate improper installation, loose windings, or mis-
phragm cracking, and the like. A cracked or leaking
aligned barriers.
diaphragm should be replaced at once.
(11) Internal inspection of liquid-immersed
(6) Breathers. Many large transformers have
transformers. On an open-type liquid- immersed
breathers of either the open type or dehydrating
transformer, the look-in port cover can be removed
type. The function of the dehydrating agent is to
to examine for evidence of moisture or rust around
prevent moisture from entering the transformer
the bushing supports and transformer top cover. To
tank. Most dehydrating breathers contain silica gel,
examine the tank and core, the liquid can be
which will change from blue, when dry, to pale pink
drained out. Examination of the core should be
when wet. Inspection can be made through a glass
made to check for sludge deposits, loose connec-
window provided for that purpose. The breathers
tions, and any damage to the transformer parts.
should be checked monthly and the dehydrating
Evidence of carbon may indicate internal problems.
agent should be replaced or reconditioned if it is
Windings should be checked for damage to terminal
found they restrict breathing or are wet.
panels, barriers, and loose connections. The need to
(7) Auxiliary equipment. Auxiliary equipment
untank a transformer for internal inspection should
required for cooling, such as fans, oil pumps, control
depend on the age of the transformer and its over-
devices, and wiring, should be checked on an annual
loading and/or trouble history. The frequency of this
basis. The equipment should be cleaned and dam-
inspection should be 5 to 10 years or more.
aged parts replaced.
(a) Contamination. Contamination or im-
(8) Load tap changers. Load `tap changers
pairment of the insulating liquid during examina-
tion should be carefully avoided. If the humidity is
should be thoroughly inspected and the insulating
high, exposure should be avoided entirely, unless
oil tested at the end of the first year's operation.
the work is absolutely necessary and cannot be
Subsequent annual inspections should include test-
postponed, in which case special humidity-control
ing of the insulating oil based on the number of
steps should be taken.
operations, the condition of the oil, and the condi-
tion of the contacts. Maintenance of the mechanism
(b) Liquid addition. If liquid is to be added, it
will vary with the type and manufacturer. The
should be given a dielectric-breakdown test. The
manufacturer's recommendations should be fol-
liquid to be added should be at least as warm as the
liquid in the transformers. If a large amount of
liquid is added, the transformer should remain de-
(9) Visual inspection safety measures. If a
energized for 12 hours or more to permit the escape
transformer is given an external visual examina-
of entrapped air bubbles. A desirable method is to
tion, the case of the transformer should be regarded
add the liquid with the transformer tank under a
as energized until the tank ground connection is
vacuum. (Check the manufacturer's instructions
inspected and found to be adequate. If any proce-
and IEEE C57.106 for further information.)
dure more extensive than an external visual exami-
(12) Internal inspection of dry-type transform-
nation is to be performed, de-energize the trans-
ers. Enclosure covers of ventilated dry-type trans-
former, using an approved positive lockout or tag-
out procedure to ensure against an unplanned re-
formers should be removed carefully. Check for ac-
energization and resulting hazard to personnel or
cumulations of dirt on windings and insulators,
restriction to cooling airflow, discoloration caused
equipment. Before doing anything else, test to en-


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