Quantcast Cable overvoltage tests

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TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
if desired, some details can be found in the "Ameri-
cable insulation can still withstand over-voltages
can Electricians' Handbook.
caused by normal system surges. As noted,
e. Factors affecting insulation resistance. Person-
alternating-current tests are reserved for factory
nel making and interpreting the results of insula-
tests to determine whether the insulation had any
discontinuities, voids, or air pockets. Less destruc-
tion resistance tests should consider the following
tive, direct-current tests are used for installation
factors which affect the test readings:
proof testing. Also, most equipment for direct-
(1) Temperature. Insulation resistance varies
voltage testing is smaller and more readily portable.
with the temperature, and the effect of temperature
A 115-volt alternating current power supply is rec-
depends on many other things, such as type of insu-
tified to provide direct-current for testing. Several
lation, amount of moisture in and on the surface,
commercial types are available. Each type comes
and the condition of the surface. All spot-test read-
complete with transformers, rectifiers, instruments,
ings should be corrected to a base temperature such
and controls.
as 40 degrees C.
(2) Moisture. The amount of moisture in the
a. Voltage. Normally, the maintenance proof tests
insulation has a large influence on its resistance.
performed on cables are at a test voltage of 60 per-
For meaningful results, tests of insulation resis-
cent of the final factory test voltage for new
tance should be made under as near similar condi-
cable/equipment. Determination of voltages for ac-
tions as practical. A long cable can be exposed to
ceptance and proof tests should be made by quali-
different conditions along its length so a comparison
fied electrical engineers, and such tests should be
made only when specifically directed by an engi-
of readings not made at the same point may be
neering activity having jurisdiction over the instal-
misleading.
lation. Tests should be performed in accordance
f. Interpretations. Usually, because of the stored
with ANSI/IEEE 400. It is always appropriate to
capacitance of the cable there will be an initial am-
conduct the insulation resistance measurement test
pere dip toward zero followed by a steady rise. The
first; and, if the data obtained is within acceptable
spot-test reading should be taken after a 60-second
limits,  to proceed  with the direct-current
voltage application. A cable 1,000 feet (300 meters)
overpotential test.
long will have an insulation resistance of one-tenth
b. Procedure. For each cable, the test should be
of that for a l00-foot (30-meter) cable, provided all
other conditions of both tests were identical. A
made between each conductor and every other con-
gradual decline in resistance with age is normal;
ductor and between each conductor and ground. For
however, a sudden decline means insulation failure
the test to ground, all conductors may be connected
is imminent and a continued downward trend indi-
together. There is no need to disconnect other equip-
cates insulation deterioration, even though mea-
ment from the cable, but caution must be observed
sured resistance values are above the minimum ac-
to ensure the test voltage is not greater than recom-
ceptable limits.
mended for any of the equipment. Some preas-
sembled or premolded cable accessories may have a
g. Dielectric absorption test. This test is usually
basic insulation level lower than the cable tested,
conducted at higher voltages for extended periods of
and the lower voltage should be taken from IEEE 48
from 5 to 15 minutes. Since the current is inversely
test limits or the manufacturer's test limits, which-
related to time, insulation resistance will rise
ever is smaller. If a test shows poor results, items
gradually for a good cable but will flatten rapidly
and/or conductors should be retested separately un-
otherwise. Periodic readings should be taken and
til the defective portion is identified. Specific in-
plotted against time. The ratio of the lo-minute to
structions furnished with the tester being used
the l-minute resistance is known as the polariza-
should be carefully followed. Additional information
tion index. A polarization index of two or higher
is contained in chapter 7, section II.
indicates good insulation, while a polarization index
of less than one indicates cable deterioration
c. Safety. When making high-voltage tests, all ap-
the need for immediate maintenance.
plicable precautions regarding live electrical con-
ductors should be observed to avoid dangerous elec-
5-28. Cable overvoltage tests.
trical shock. After testing, the terminals should be
short circuited before disconnecting the tester from
A hi-pot or overpotential test is an overvoltage test
the equipment. The short circuit should be main-
used to check a cable for its relative condition after
tained for at least as long as the time the proof
it has been repaired or otherwise worked on. Nei-
voltage was applied.
ther the insulation resistance test nor the dielectric
absorption test can determine the dielectric
d. Test data. There are three types of direct-
strength of cable insulation under normal use. A
current hi-pot tests commonly performed. In all
hi-pot test is the only way to gain proof that the
cases, leakage (conduction) current is measured and
5-15





 


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