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 TM 5-685/NAVFAC MO-912
(9) Electrical analyzer: An instrument for mea-
(2) Installed lightning arresters can retain a
lethal electric charge. Accordingly, lightning arrest-
suring the various parameters of AC circuits. It
ers must be considered as loaded to full circuit po-
power factor meter. The analyzer also includes two
tential unless disconnected from the circuit and
grounded.
current transformers and switches necessary for
f. Special maintenance tools. Always use the
use. It can be used for testing insulation.
proper tool for the job being done. Avoid the use of
(10) Certification. Test equipment should have
valid calibration certification.
improvised tools or tools in poor condition. Store
h. Fuses. Fuses detect circuit overload conditions
tools not in use properly.
and open when there is too much current flowing.
(1) Hand tools include the following: screw-
drivers, pliers, wrenches, wire insulation strippers,
Fuses are the safety valves of the installation's elec-
trical system and provide the most economical type
and wire cutters.
(2) Powered hand tools include the following:
of circuit protection.
hydraulic, pneumatic and electrical. Unless an elec-
(1) Application. There are many types of fuses
trical tool is battery powered or double insulated,
with various characteristics. Always verify that a
make sure the tool has a line cord with a grounded
fuse, whether a new or replacement unit, is of the
conductor and polarized grounding plug. Make sure
proper type and rating before installing. Never ar-
the receptacle to be used is properly grounded.
bitrarily replace one type of fuse with another fuse
(3) Machine tools include grinding wheels and
of the same physical size just because it fits the fuse
cutting tools.
holder. The fuse used should have the correct cur-
g. Test equipment. Before using any test equip-
rent and voltage ratings, proper time delay and
ment make sure that it has valid calibration certifi-
current limiting characteristics and an adequate in-
cation. Test equipment required for switchgear
terrupting rating to protect the circuit and its com-
ponents. Fuse holders should never be altered or
maintenance usually includes many or all of the
following items.
forced to accept fuses which do not fit.
(2) Construction. A fuse consists of two main
(1) Multimeter. The multimeter is sometimes
called a volt-ohm-milliammeter or VOM. It is a
parts: the fusible link and the enclosing housing or
body. The link is a metallic alloy that melts when
single test instrument with a number of different
excessive current flows through it, thereby breaking
ranges for measuring voltage, current, and resis-
the circuit. When the current heats the alloy to its
tance.
melting point, the link breaks and an arc forms.
(2) Voltmeter. An instrument used for measur-
ing voltage. Its scale indicates microvolts, millivolts,
Melting continues rapidly until the resultant gap is
volts or kilovolts.
too wide for the arc to span. A fuse usually can carry
(3) V o l t a m m eter. An instrument used as either
a 100 percent load indefinitely and will blow in a
specified time at 150 percent overload. The follow-
a voltmeter or an ammeter.
(4) Ohmmeter. An instrument used for measur-
ing fuse types are usually used.
ing resistance. It consists of a DC milliammeter, a
(a) Current limiting fuses. Current limiting
DC source, and a resistor network.
fuses are used where necessary to limit the amount
(5) Ammeter. An instrument that measures the
of fault energy flowing through a fuse to the circuit.
A fuse must clear a fault in less than
amount of current in amperes. Its meter shows cur-
cycle of the
rent value in microamperes, milliamperes, or
fault current sine wave to be considered a current
kiloamperes.
limiting fuse. If the fault current is allowed to flow
(6) Frequency meter. An instrument for mea-
for cycle or more, the maximum (peak) fault cur-
suring the frequency of an alternating current. Its
rent is passed through the fuse. A current limiting
scale shows Hertz (cycles per second), kiloHertz
fuse must act quickly to limit the energy let through
(kilocycles), or megaHertz (megacycles).
the fuse to the protected circuit. The total clearing
(7) Wattmet er. An instrument for measuring
time of a fuse is made up of two components; the
electric power. Its scale is usually graduated in
melting time, and the arcing time. The fault current
watts or kilowatts.
reaches maximum at the conclusion of the melting
time, much less than cycle. An arc is established
(8) Megohmmeter. A device that is a high range
inside the fuse at the conclusion of the melting time.
consists of a hand driven, motor driven, or battery
The arc presents a high resistance to the flow of
driven generator as the DC source, and a meter. It
fault current and the current decays to zero, clear-
is used to measure insulation resistance and other
ing the fault. Whenever possible de-energize the
high resistance. It can be used to check for continu-
fuse-holder circuit before removing or installing a
ity, grounds, and short circuits.
fuse.
5-21
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