Quantcast Cable fault locating test methods

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TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
(4) Open circuit. The continuity of the conduc-
with the manufacturer's directions. After a leak is
tors is determined by grounding the conductors at
located, a repair can be made in accordance with
the far end and then testing between each conduc-
section VI.
(2) Satisfactory gas pressure. If the pressure is
tor and ground. If the conductors are continuous,
the resistance reads low; and, if an open circuit
satisfactory, do not re-energize the circuit until the
exists, the tester will indicate a very high resis-
fault has been located and repaired, since the high
fault current reimposed on the failed cable can fur-
tance.
ther damage the cable.
5-14. Cable fault locating test methods.
5-13. Determining type of cable fault.
The methods generally used may be separated into
Use fault locating equipment when a check of asso-
two major classifications: terminal measurement
ciated equipment and lines confirms that the fault
methods and tracer methods. Except in the case of
is actually in the cable, and visual methods fail to
faults on series lighting circuits (which usually re-
locate the fault. Since no single test will locate all
sult in considerable carbonization because of the
types of faults, the type of fault must be determined
constant-current system involved) the resistances of
in order to use the best test method to locate it. To
faults are often quite high, ranging from several
determine the type of fault, any source of direct-
hundred ohms to megohms when measured at a
current voltage can be used with a voltmeter or a
low-voltage level.
suitable low-voltage lamp. A portable testing set,
a. Terminal measurement methods. Terminal
such as a multimeter of the volt-ohm meter (VOM)
measurement methods involve determining the cho-
type is most commonly used. The section of cable
sen electrical value of the faulted conductor from
under test must be disconnected from feeders,
one of the cable terminations to the fault, and com-
buses, and equipment. Alternating current should
paring this value with the same electrical value on
not be used, because the charging current of the
unfaulted cable. The proportions of the electrical
cable is sufficient to prevent accurate indications of
values in regard to the length of the unfaulted cable
the condition of the cable.
provides the fault distance. The effectiveness of all
a. Types of faults. Cable insulation failures result
terminal measurement methods is dependent upon
in low- or high-resistance faults, because one or a
_
the accuracy of installation records. While most of
combination of the following conditions occur.
the work is done at one terminal, access to the other
(1) One or more of the conductors may be
terminal may be necessary to connect or disconnect
grounded.
conductors as required. Terminal methods include
the Murray loop, the capacitance bridge measure-
(2) Two or more conductors may be short
ment method, the quarter-wave or half-wave reso-
circuited.
nance methods, and the pulse (time domain
(3) One or more conductors may be open
reflectometer) method.
circuited.
b. Checking for fault types.
b. Tracer methods. These methods require test
equipment at the cable terminal but rely on checks
(1) Grounded conductor. In checking for a
along the cable tracer to locate the fault. Tracer
grounded conductor, the VOM is successively con-
methods include the modulated direct-current
nected between each conductor and ground with the
method, the modulated alternating-current method,
far end of the cable open circuited. A good conductor
the impulse (thumper) method, the audio frequency
will indicate a resistance commensurate with that
(tone tracing) method, and the earth gradient
of its insulation. A grounded conductor will show a
method.
very low resistance.
(2) High-resistance grounded. Some installa-
(1) Tracer method warning. Some of the tracer
methods of fault locating can ignite residual gas in
tions are grounded through a high resistance. These
the vicinity of a fault and cause explosions. The
systems operate like an ungrounded system and the
likelihood of such an occurrence, while extremely
first ground fault does not trip out the system, but
remote, cannot be ignored.
only sounds a warning. The ground fault can be
traced using an integral system pulser and a detec-
(2) Structure testing. Normal gas tests with
tor furnished as a part of the system.
combustible gas detectors should be made prior to
(3) Short ci rcuit. In checking for a short circuit,
entering structures during all fault-locating opera-
tions, regardless of the urgency of the situation or
the VOM is successively connected between each
possible combination of conductors. Far ends of the
the type of fault-locating equipment being used. It is
cable must be open-circuited. A low reading indi-
also advisable to use a carbon monoxide (CO) tester
cates a short circuit between the conductors under
to check the atmosphere in structures where fault
repairs are to be made, particularly in cases where
test.
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