Quantcast Section IV- Insulators

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TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
fill are usual coverings. For very large substations
chemical application for removal of vegetation is
some areas may be seeded for grass.
required, it should meet environmental require-
a. Ground treatment. Removal of vegetation,
b. Housekeeping. Miscellaneous storage should
elimination of low spots in the yard, and control of
grassed areas is necessary. If grass is permitted,
not be permitted except in specific areas reserved
careful maintenance is necessary both for esthetics
for this purpose. Storage should not interfere with
and safety reasons. If allowed to grow uncontrolled,
operations and should be in a protected, tidy, and
weeds, grass, or other plants create fire hazards,
accessible manner. Birds may cause problems re-
are unsightly, impede free action, and may grow tall
quiring removal of nests and possible provision of
enough to contact live parts and cause flashovers.
bird repellant controls such as tape, images, or
Low spots collect debris and stagnant water. Where
sound systems.
3-19. Function of insulators.
foreign deposits, and mechanical damage from ex-
ternal sources also cause deterioration. Evidence of
The function of an insulator is to support a conduc-
such impairments may cause a flashover puncture
tor or conducting device safely. An insulator, being
accompanied by a destruction of insulator parts.
of a nonconductive material, physically and electri-
Workers should be CAUTIONED that equipment
cally separates the supported item from any
must be de-energized unless the procedure in chap-
grounded or energized conductors or devices.
ter 4, section XV is authorized.
a. Composition and problems. Insulators are
a. Ceramic insulators. Ceramic insulators are
composed of porcelain, glass, fiberglass, or a com-
made of wet-process porcelain or toughened glass.
posite compound. Maintenance is necessary to pre-
serve their insulating ability which can be degraded
(1) Construction.
by contamination or other damaging actions. Most
(a) Porcelain insulators. Porcelain insulators
insulator damage will result from gun shots; light-
are manufactured from special clays to produce a
ning, surge, or contamination flashovers; and wind
plastic-like compound which is molded, oven dried,
damage. Defective insulators can also cause visible
dipped in a colored glazing solution, and fired in a
corona or interference voltage propagation.
kiln. The glossy surface of the glaze makes the in-
b. Related material. Apparatus type insulators
sulator surface self-cleaning. Large porcelain insu-
are provided in substations to support devices and
lators are made up of several shapes cemented to-
heavy lines. See chapter 4, section XII, which pro-
gether. A chemical reaction on the metal parts from
vides a discussion of insulation levels.
improper cementing can result in a cement growth
which can be sufficiently stressful to crack the por-
3-20. Tests of insulators.
Radio interference conditions may be detected by
(b) Glass i nsulators. Glass insulators are
using instruments designed for this purpose. Other-
made from a mixture of sand, soda ash, and lime
wise, maintenance tests on insulators are normally
which is mixed and melted in an oven, then molded,
limited to occasional power factor measurements at
cooled, and annealed.
the more important installations, where the loss of
(2) Inspection.
the facilities must be kept to an absolute minimum.
(a) Look for fractures, chips, deposits of dirt,
Bus and switch insulators should be power-factor
salt, cement dust, acid fumes, or foreign matter,
tested in conjunction with similar testing of other
which under moist conditions may cause a flash-
apparatus within the substation. Power factor tests
are described in section VII.
(b) Check fo r cracks in insulators by tapping
3-21. Inspection and repair of insulators.
gently with a small metal object ONLY WHEN DE-
ENERGIZED, about the size of a 6-inch (15 centi-
Switch-and-bus apparatus type insulators are the
meter) wrench. Insulators free of cracks emit a ring-
most intricate type and require the highest degree
ing sound when tapped; cracked ones sound dull
of reliability in service. This is because the several
and hollow. To avoid damaging good insulators, tap
pieces of porcelain and hardware, assembled in a
them; do not hit them hard.
single unit, are usually located at key positions in
(3) Repair.
the systems, where failure is extremely serious.
(a) If the main body of a pin type or post
Switch-and-bus insulator failures occur when porce-
insulator is cracked, replace it immediately.
lain is thrown in tension by any thermal movement
(b) Hone small chips from shells or skirts,
between nested parts, which can cause cracking and
allow the entrance of moisture. An accumulation of
and paint with an insulating paint or varnish to


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