Quantcast Chapter 14. Test Methods

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TM 5-683/NAVFAC MO-116/AFJMAN 32-1083
CHAPTER 14
TEST METHODS
14-1. Test evaluation.
insulation, providing a low resistance path for leak-
age current. Sometimes the drop in insulation resis-
The tests listed in this chapter are most commonly
tance is sudden, as when equipment is flooded. Usu-
performed to determine the condition of low voltage
ally, however, it drops gradually, giving plenty of
equipment. If a testing program is to provide mean-
warning, if checked periodically. Such checks permit
ingful information, all tests must be conducted in a
planned reconditioning before service failure. If
proper manner. All conditions which would affect
there are no checks, a motor with poor insulation,
the evaluation of these tests must be considered
for example, may not only be dangerous to touch
with any pertinent factors recorded. The test opera-
when voltage is applied, but also be subject to bum
tor must be thoroughly familiar with the test equip
out. Current through and along insulation is made
ment used and should also be able to detect any
up of three components (fig 14-2): capacitance
equipment abnormalities or questionable data dur-
charging current; absorption current; and conduc-
ing the performance of the test. To provide optimum
tion or leakage current. The total current is the sum
benefits, all testing data and maintenance actions
of the three components and it is this current in
must be recorded. The data obtained in these tests
terms of megohms at a particular voltage that can
provide information which:
be measured directly by a megohmmeter. Note that
a. Determines whether any corrective mainte-
the charging current disappears relatively rapidly,
nance or replacement is necessary or desired.
as the equipment under test becomes "charged".
b. Ascertains the ability of the element to con-
Larger units with more capacitance will take longer
tinue to perform its design function adequately.
to be charged. This current also is the stored energy
c. Charts the gradual deterioration of the equip-
initially discharged after your test, by short-
ment over its service life.
circuiting and grounding the insulation. You can see
14-2. Insulation testing.
further that the absorption current decreases at a
relatively slow rate, depending upon the exact na-
Insulated electric wire is usually made of copper or
ture of the insulation. This stored energy, too, must
aluminum (which is known to be a good conductor of
be released at the end of a test, and requires a
the electric current) conductor with appropriate in-
l o n g e r time than the capacitance charging
sulation for the rated voltage. The insulation must
current-about four times as long as the voltage
be just the opposite from a conduction it should
was applied. With good insulation, the conduction
resist current and keep the current in its path along
or leakage current should build up to a steady value
the conductor. The purpose of insulation around a
that is constant for the applied voltage. Any in-
conductor is much like that of a pipe carrying water
crease of leakage current with time is a warning of
(fig 14-1). Pressure on water from a pump causes
trouble. With a background now of how time affects
flow along the pipe. If the pipe were to "spring a
the meaning of instrument readings, let's consider
leak", water would spout out; you would waste wa-
two common test methods: (1) short-time of spot
ter and lose some water pressure. With electricity,
reading and (2) time-resistance tests.
"voltage" is like the pump pressure causing electric-
a. Short-tin or spot-reading test. In this method,
ity to flow along the copper wire. As in a water pipe,
connect the megohmmeter (para 13-4) across the
there is some resistance to flow, but it is much less
insulation to be tested and operate it for a short,
along the wire than it is through the insulation.
specific timed period (60 seconds usually is recom-
Insulation, with a very high resistance, lets very
mended). Commonly used DC test voltages for rou-
little current through it. As a result, the current
tine maintenance are as follows:
follows a "path of least resistance" along the conduc-
tor. The failure of an insulation system is the most
Equipment AC Rating
DC Test Voltage
common cause of problems in electrical equipment.
up to 100 volts
100 and 250 Volts
Insulation is subject to many effects which can
440 to 550 volts
500 and 1,000 Volts
cause it to fail; such as, mechanical damage, vibra-
tion, excessive heat, cold, dirt, oil, corrosive vapors,
Bear in mind also that temperature and humidity,
moisture from processes, or just the humidity on a
as well as condition of your insulation affect your
muggy day. As pin holes or cracks develop, moisture
reading. Your very first "spot reading" on equip-
and foreign matter penetrate the surfaces of the
ment, with no prior test, can be only a rough guide
14-1





 


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