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TM 5-685/NAVFAC MO-912
(b) Metal-en c osed fuse. Fuse enclosed in an
sion of gases during operation. A dropout fea-
oil filled metal housing and used (up to 7,500 volts)
ture that automatically disengages the fuse when it
for protecting transformer banks and other distri-
operates may be used. These fuses usually have
bution elements. Refer to the manufacturer's litera-
seals to keep moisture out of the interrupting cham-
ture for details.
ber. Refer to manufacturer's literature for instruc-
(c) Glass-enclosed fuse. Fuse enclosed in a
glass tube filled with arc quenching liquid. Carbon
( f ) Replace fuse holders and clips which
tetrachloride is the liquid frequently used. Refer to
are worn or make poor contact. Remove oxidation
the manufacturer's literature for details.
and corrosion from fuses, holders and clips. Deter-
(d) Expilsion fuse. Fuse enclosed in a fiber
mine the causes of overheating and correct as re-
tube filled with dry (powdered) boric acid. When the
fuse element blows, the boric acid produces a gas
i. Synchroscope. A synchroscope, usually in-
which aids in promptly deionizing the arc. Used on
stalled on a switchgear control panel, is used to
circuits up to 138 kV. Refer to the manufacturer's
determine the phase difference or degree of syn-
literature for details.
chronism of two alternating current quantities or
(3) Checks and examinations. Examine fuse
two generators. The synchronism always indicates
terminals and holders for discoloration caused by
the condition of the incoming machine with respect
heat from poor contact and/or corrosion. Checks to
to the bus. If the frequency of the incoming machine
be performed are determined by the type of fuse and
is higher than the bus frequency, the synchroscope
point revolves in the "fast" direction. If the fre-
fuse holder, proceed as follows:
quency of the incoming machine is lower than the
(a) Inspect fuse and fuse holder contact sur-
faces for pitting, burning, alignment, and spring
bus frequency, the synchroscope pointer revolves in
the "slow" direction. If the pointer stops at a posi-
pressure. Badly pitted or burned components must
tion other than 0 degrees, it indicates that the in-
be replaced.
(b) Examine the fuse unit, and renewable el-
coming machine is at the same frequency as the bus
but out of phase. Correct the phase error by adjust-
ement if the fuse type is used, for corrosion. Check
ing the prime mover governor of the incoming ma-
for signs of discharge tracking on the fuse. Replace
components that show deterioration.
chine for higher speed. The synchroscope pointer
(c) Verify that all attaching parts are in-
should revolve slowly in the "fast" direction. The
stalled and tightened to proper torque value.
machines are paralleled when the pointer reaches
the 0 degree position while traveling in the "fast"
(d) Check fu se tubes made of fiber or other
organic material. Refinish the fuse tube as required.
direction. When paralleled, the pointer will stay at 0
Refer to manufacturer's literature.
degrees. Refer to manufacturer's literature for spe-
(e) Check vented expulsion fuses. Some fuses
cific operation and inspection information on indi-
may have condensers or mufflers to restrict expul-
vidual equipment.

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