Quantcast Cable moisture tests

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TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
Table 5-l. Typical power factor ranges for various cable
to detect a localized fault diminishes as the length
insulating materials
of the cable under test increases.
c. Effectiveness of power factor tests. Power factor
Cable insulation
Power factor
0.001 to 0.002
Polyethylene
tests can be effective in detecting defective insula-
0.001 to 0.002
Cross-linked polyethylene
tion for short cable runs which are common in elec-
Oil and paper
Less than 0.005
trical substations and industrial complexes, and for
0.005 to 0.04
Rubber
indicating general deterioration and/or contamina-
0.04 to 0.08
Varnished cambric
tion of longer lengths of cable insulation runs. This
At 20 degrees centigrade
test can be performed at any voltage that does not
exceed the line-to-ground voltage rating of the
f. Temperature correction. Temperature has an
cable. In addition to checking cable insulation, this
influence on the power factor values. However, at
test can be used to find:
the operating temperature normally encountered in
(1) Any defects in the shield circuit which can
the field, this influence is minimal for modern insu-
lead to localized problems. The measurements
lation systems. Older forms of insulation may re-
should be performed on each end of the cable. The
quire a temperature-correction factor. It is difficult
shield should be grounded for the test only at the
to obtain accurate field cable temperature measure-
end where the test connections are made. A n in-
ments; hence, most utilities evaluate the condition
of the insulation of their cables based on test data,
crease in power factor can indicate discontinuities
uncorrected for temperature. If it appears that high
or breaks in the shield.
cable temperature may have influenced the results
(2) Any defects in cable terminations, particu-
it is recommended that a cable having a high power
larly compound-filled potheads, can be determined
factor be retested at a time when a lower cable
by using collar tests as covered in chapter 3, section
temperature will occur.
VII.
5-30. Cable moisture tests.
d. Power factor tip-up testing. Normally, power
factor should be independent of voltage as shown by
Tests for moisture may be made on paper and
the formula of figure 5-3. An increase in power
varnished-cambric insulation by removing one or
two layers of the insulation and dipping it into oil
factor at an increased test voltage is usually an
heated to a temperature of 260 to 285 degrees F
indication of insulation voids. These voids are ion-
(125 to 140 degrees C). If the insulation contains
ized at the higher voltage and act as resistors re-
moisture, a concentration of bubbles will be emitted
sulting in a greater resistive current and therefore a
from the paper. If there is no moisture present,
greater power factor. This increase in voltage is
there will be little or no bubbling. Generally, the
called power factor tip-up test.
outer layers of insulation are tested first, as they
(1) Two to five measurements should be made
are the most accessible for testing and the most apt
at voltages with an overall 5 to 1 ratio in order to
to show moisture. Where moisture is indicated in
determine whether there are any significant differ-
the insulation, successive layers should be removed
ences in the measured power factors.
and tested until there is no evidence of absorbed
(2) Generally differences are not a concern un-
moisture. All moisture-damaged cable should be re-
less the higher value exceeds 25 percent of the lower
placed with new cable.
value. This much change indicates that further in-
5-31. Cable test records.
vestigation of the cable insulating quality is re-
quired.
It is very important that cable records be made for
e. Cable test data. Power factor data obtained
any inspection or test on any circuit. Such records
from the field tests should be compared with any
should flag when the next inspection or mainte-
nance outage is to be made. Since these tests re-
available previous test data in order to detect any
changes. Lacking any initial test data, evaluation of
quire taking the cable out of service, advantage can
the condition of the cable insulation may be made
then be taken of the maintenance outage, rather
by a comparison of the field data with tabulated
than taking a cable out of service for tests only. The
power factors obtained for similar insulated cables
trend of the reading obtained will determine
known to be in good condition, or with the manufac-
whether the cable is stable, slightly aging, or rap-
turer's published specifications. Table 5-1 indicates
idly deteriorating. Slight decreases in the insulation
typical acceptable power factors for various cable
resistance each year are to be expected as the cable
insulations which may be used if no other data is
ages. Tests should be made more frequently if more
available. Ranges given should not be used to justify
than the usual decrease indicates that deterioration
variations in tip-up test values.
is approaching a critical state.
5-17





 


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