Quantcast Section VII -Cable Testing

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TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
losses are concerned. Installation of other termina-
b.
fireproofing. Where cables of more than
tions should be in accordance with the instructions
one circuit pass through structures, any fireproofing
of the termination manufacturer for the type of
material around cable sheaths should be main-
cable involved.
tained to prevent damage from adjacent cables.
Section VII - CABLE TESTING
5-26. Cable tests.
sulation resistance test will measure the insulation
resistance of all items connected together, the cable
Tests are made on installed cable for two
to be tested must be completely disconnected from
reasons-to check the condition of the cable and for
all other cable and equipment.
a cable requiring maintenance to ensure that the
a. Measuring equipment. All measurements re-
repair was properly made.
quire a direct-current source which can be a hand-
a. Type of voltage tests. Insulation resistance
cranked generator, a motor-driven generator, a
measurements and direct-current, over- or high-
battery-supplied power pack, rectified alternating
potential test (direct-current hi-pot test) are the
current, or its own internal power source. The mea-
usual direct-current voltage tests for cables. A di-
suring devices can be as follows.
electric absorption test which takes longer than a
(1) A megohmeter which is a contained instru-
standard insulation resistance test may aiso be ap-
ment (commonly called "megger," although "Meg-
propriate. A fourth test is the power factor test.
ger" is the trade name of a tester of this type made
b. Testing frequency. The periodic testing of in-
by James C. Biddle Co.) consisting of an indicating
stalled medium-voltage cables is known as proof
ohmmeter and an internal source of direct-current
testing, since its purpose is essentially a means of
voltage.
proving that weak spots in existing cables have
(2) A resistance bridge.
been recognized before failure occurs. Cables nor-
(3) A voltmeter.
mally have a higher failure rate in the first 2 years
(4) A voltmeter and micro-ammeter.
of service, which is the period when manufacturing
b. Megohmeter. The most convenient and com-
defects will show up. An alternating-current test is
monly used way to measure resistance is to use a
used for factory testing of new cable. Acceptance
self-contained instrument giving a direct readings
and proof testing utilizes direct-current testing
in ohms, kilohms, or megohms. Measurement is ob-
which, while not as effective as alternating-current,
tained by connecting one instrument terminal to the
is less liable to damage cable. Direct-current testing
cable conductor or the equipment terminal and the
provides extremely valuable historical data and al-
other instrument terminal to the metal sheath,
lows comparison of the acceptance testing value to
frame, container, or support of the insulation under
the periodic proof testing values. A yearly overvolt-
test. Instruments are available in voltage ratings of
age test for the first 2 or 3 years, and then testing
500 to 2,000 volts or more. Care must be taken to
every 5 or 6 years, is the optimum and can reduce
use a voltage which does not exceed the insulation
in-service failures by a factor of about nine to one as
rating of the item being tested. Follow specific in-
opposed to not having a proof testing test program.
structions provided with the instrument being used.
Insulation resistance tests should be based on the
This type of instrument is not very accurate or sen-
importance of the circuit; once a year is usually
sitive in very low ranges and should not be used to
adequate.
measure a few ohms or fractions of ohms, such as
c. Other tests. Varnished-cambric and paper-
resistances of conducting paths.
insulated cables may require moisture tests. Gas-
c. Resistance bridge method. A self-contained in-
pressurized cable may require leak tests. Refer to
strument, called a Wheatstone Bridge, containing a
section V.
battery, a galvanometer, and known resistances, is
5-27. Cable insulation resistance tests.
used to compare an unknown resistance with a
Insulation resistance is the resistance which the
known resistance. Each instrument contains de-
insulation presents to a flow of current, from an
tailed instructions for its use. While very accurate
impressed direct-current voltage. An insulation re-
results can be obtained, a bridge is basically a labo-
sistance test is a short-time test made to indicate
ratory instrument and requires a fairly skilled op-
the suitability of the insulation for the purpose in-
erator. It is not recommended for field work.
tended, or to indicate whether an overpotential test
d. Other methods. Both the voltmeter method
can be made without damaging the insulation. It is
and the voltmeter/micro-ammeter method require
not a dielectric strength test, but will give an indi-
laboratory type instruments and a separate source
cation of the insulation's condition with respect to
of direct-current voltage. They are inconvenient to
moisture and other contamination. Because any in-
use in the field and are not recommended. However,
5-14





 


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