Quantcast Visual methods of cable fault locating

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TM 5-684/NAVFAC MO-200/AJMAN 32-1082
5-12. Visual methods of cable fault locating.
long underground feeder will blow the associated
cable riser fuses within a time period that limits to
Since visual inspection can be the easiest and quick-
one reclosure, the damaging effects of automatic
est way to locate a cable fault, it should be tried
reclosing on the faulted cable feeder.
first. Visual inspection may require checking sec-
c. Check of associated equipment and lines.
ondary effects, such as leaks from the cable-
Blown fuses and open circuit breakers may be
insulating medium. If visual inspections are not ef-
caused by a cable fault or by faults on equipment or
fective, then testing devices will need to be used.
other lines connected to cables. Preliminary tests
a. Faults in exposed cable and splices. A quick
should be made to determine that the fault is actu-
check may be made by driving over the route of the
ally in the cable and not in associated equipment.
cable and looking for such things as a displaced
d. Faulted circuit indicators. As noted, reclosing
structure cover, smoke coming from a structure, or
on faulted cable circuits stresses the circuit ele-
indication of damage caused by digging operations.
ments and can increase potential personal hazards.
A more detailed inspection may be made by exam-
On critical feeders, FCIs are often installed to re-
ining the terminal equipment and the cable and
duce service restoration time by providing a conve-
nient means of determining fault current occur-
splices in the structure. Look particularly for the
rences, location, and direction on underground
presence of compound on the cable sheath, smoke,
and odors of a burnout. Observe the requirements of
(1) Operation. An FCI can be a single or
section II.
multiphase device which senses fault current has
b. Faults in submarine cable. Oil slicks may occur
passed through the line conductors at the point
on the surface of the water near the location of the
where the FCI is installed. The FCI is designed to
fault, or bubbles may appear where the cable is
provide a fault current indication by a flag, a light
faulted. Applying high current from a low-voltage
emitting diode (LED) display, or other means. The
source to the faulted cable may cause bubbles to rise
current sensing is done by detecting the magnetic
to the surface near the fault, thus determining the
field strength generated by the circuit's alternating
approximate location. For short submarine cables,
establishing a line of sight between the terminal
(2) Location. Most FCI s are installed on under-
ends and patrolling this area may aid in locating a
ground distribution current-carrying elements such
point of failure. Maps used in laying the cable will
as cables, switch and transformer elbow termina-
be helpful in establishing the cable route.
tions, and separable connectors. They are also used
c. Faults in gas-pressurized cable. When a fault
on aerial lines.
occurs on gas-pressurized sulfur-hexafluoride (SF,)
(3) Application. The proper application of FCIs
cable, do not re-energize the cable until the follow-
is crucial to their correct operation. Units must be
ing steps have been taken.
correctly designed for indicator trip and reset meth-
(1) Low gas pressure. Gas pressurized cable is
ods. Inrush restraint, time delay, and coordination
may be necessary. Other considerations when se-
usually pressurized at 20 to 80 pounds per square
lecting FCIs means that their initial provision re-
inch gage (135 to 550 kilopascals gage). A record of
quires engineered design.
the installed gas pressure should be kept for all
gas-pressurized cable sections.
(4) Maintenance use. Use FCI sets to locate dis-
tribution faults. Normally where installed, the
(a) Checking pressure. Check the gas pres-
number of FCI sets will be one less than the number
sure at terminations and splices with a tire gage. If
the pressure has dropped to zero or is dropping, the
of cable sets which can be sectionalized. The faulted
cable section will be between a "fault" and a "nor-
fault damaged the conduit or jacket containing the
mal" indication.
gas. Since gas is electro-negative, a gas detector
similar to that used for refrigeration gases can be
(5) Concerns. If units are damaged they must
used to locate the leak at terminations, splices, and
be replaced with like units having the same fea-
other points.
tures. Their operation in regard to trip and reset
(b) Detecting leaks. To detect a gas leak along
must be understood. If loads are changed and the
unit does not have an adaptive trip (tripping on a
a buried duct or conduit line, a pipe can be driven
sudden increase above the nominal current followed
into the ground above the line and the probe in the
by a loss of current) then the trip setting must be
pipe may detect the gas. Another method to detect a
changed. The reset may require manual means or
leak in the line is to inject gas at one end and
may be reset by other actions such as predeter-
measure the pressure drop at access points. Dry
mined time, current, voltage, or other sensing meth-
nitrogen can be used for this method. Then plot the
pressure reading to locate the spot in accordance


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