CABLE FAULTS
CABLE FAULTSCable fault refers to something that occurs and
current cant flow smoothly. It is due to an open circuit or short
circuit causing the current flow to be of a high value.Most faults
are caused by dampness in the paper insulation of the cable due to
porous or otherwise damaged lead sheath.The causes of damage to the
lead sheath are: crystallization of the lead through vibration;
chemical action on the lead when buried in the earth and
insufficiently protected; mechanical damage.1CABLE FAULTSIn
summary, what makes insulation go bad include:mechanical damage
VibrationExcessive heat or coldDirtOilcorrosive vaporsMoisture or
humidity.These enemies of insulation are at work as time goes on
combined with the electrical stresses that exist.As pin holes or
cracks develop, moisture and foreign matter penetrate the surfaces
of the insulation, providing a low resistance path for leakage
current.2CABLE FAULTSOnce started, the different enemies tend to
aid each other, permitting excessive current through the
insulation.Sometimes the drop in insulation resistance is sudden,
as when equipment is flooded. Usually, however, it drops gradually,
giving plenty of warning, if checked periodically. Such checks
permit planned reconditioning before service failure. If there are
no checks, a motor with poor insulation, for example, will
becomedangerous to touch when voltage is applied, subject to burn
out. What was good insulation has become a partial conductor.3CABLE
FAULTSEquipment to determine cable fault is ohmmeter. Beside that,
a few test can be done to determine cable faults such as:1.
Continuity Test2. Insulation Resistance TestTYPES OF CABLE
FAULTSThe faults which are most likely to occur in the cables
are:Ground or Earth Faults (Break-down of cable insulation) When
the insulation of the cable gets damaged, the current starts
flowing from core to earth or to cable sheath.Short Circuit Faults
When the insulation between two cables or between two cores of a
multicore cable gets damaged, the current starts flowing from one
cable to another cable or from one core to another core of a
multi-core cable directly (without passing through load)TYPES OF
CABLE FAULTSOpen Circuit Faults When the conductor of a cable is
broken or joint is pulled out and there is no current in the
cable.PROCEDURE TO FIND CABLE FAULT1. Testing (Analysis of fault)a.
Continuity Test open circuit faultsb. Insulation Test short circuit
faults
2. Locating cable fault from terminal cables.To find length of
faults from testing station.
3. Confirmation fault point.To find actual location faults.
4. Repair the faultCABLE FAULTS TESTContinuity TestContinuity
test is the checking for open circuit of cable faults. Usually,
this test uses a bell and battery or multimeter. This test is
divided into two types:1. Short alignment2. Long alignment
CABLE FAULTS TESTShort alignment
Short alignment for Continuity TestUses a bell and battery. This
test is used for conduit types. For testing CC1 cables, a
connection is made between C and C1 through a bell and battery. If
a bell rings, the cable is in continues condition, and the cable is
marked with C1 If the bell is not ringing, the cable is not in
continues condition. For A and B cables, the same method is used to
test the cable condition. CABLE FAULTS TESTLong AlignmentLong
alignment for Continuity Test
The method is suitable to use for long cable.For cable A, B and
C, choose a pair of cable, for example cable A and B will be
connected with tester while the end of the same cable with short
circuited. If the test shows an open circuit, the end of the cable
will be changed with another pair and the circuit tested again.If a
low reading is detected, the cables are label with A1 or B1. Then,
connect one of the cables with another cable eg cable A and B, and
test with the same method.
Insulation Resistance Test
Insulation Resistance Test
Megger Insulation testerCable under testInsulation Resistance
between conductors and earthInsulation resistance between
conductorsInsulation Resistance TestSometimes it is helpful to
ground all conductors that are not part of the test. Why? (
Assignment)
Insulation Resistance TestPurpose of IR is to prove the quality
of the insulating materials used in the installationInsulation
should be such that there is no possibility of earth leakage
currents between conductors conductors and the general mass of
earth. The test voltage is d.c not less than twice the normal
voltage of the supply (r.m.s value)For test on medium-voltage
circuits the test voltage need not exceed 500V d.c.The minimum
accepted value of IR is one Megohm.
Insulation Resistance TestIR value of an installation will not
always remain the same.A good deal depends on the amount of
moisture and dirt present at the time of testing.Deterioration of
the insulation is usually due to:agingunsuitable operation
conditions
Machine TestingElectrical Testing is an important factor in
preventive maintenance.A breakdown however small:Cost
moneyReplacement of equipmentLoss of productionLabour in rectifying
the situationFires often accompany breakdownsRisk of electric
shocks to personnelMost Electrical failures are attributed to
Faulty electrical installation
Machine TestingPeriodic testing of machine has yielded valuable
information for assessing trends in the condition of
insulationResult of gradual deterioration of the insulation
materials possibly initiated by mechanical damage which is further
accelerated by the ingress of moisture and dirt into an imperfect
dielectric.
Machine TestingNo.Condition Action1Fair to high values, well
maintainedNo action2Fair to high values, with a downward
tendencyLocate and correct the fault3Low values but well
maintainedCondition is probably satisfactory4Low values,
unsafeClean, dry out or otherwise raise the values before placing
the equipment in service5Fair on high values previously well
maintained but showing sudden dropTest should be made at frequent
intervals until the cause of the low value is located and
corrected; or until values have become steady at lower level but
safe for operation. If the value so low that the equipment is
unsafe, remove it from service and thoroughly overhaul.General
observations on how Periodic Insulation Test can be interpreted
together with the recommended Remedial Action Machine TestingWhen
an apparatus is under test, the current flowing through has three
components:Conductive leakage current (IL )Capacitive charging
leakage current (IC )Polarization absorption leakage current (IA
)
Machine TestingConductive leakage current (IL )Conductive
current is a small (micro-amp) amount of current that normally
flows through insulation, between conductors or from a conductor to
ground.This current increases as insulation deteriorates and
becomes predominant after the absorption current (Figure 1 below)
vanishes.Because it is fairly steady and time independent, this is
the most important current for measuring insulation resistance.
Machine TestingCapacitive charging leakage current (IC )When two
or more conductors are run together in a raceway, they act as a
capacitor. Due to this capacitive effect, a leakage current flows
through conductor insulation. This current lasts only for a few
seconds as the dc voltage is applied and drops out after the
insulation has been charged to its full test voltage.
Machine TestingIn low-capacitance equipment, the capacitive
current is higher than conductive leakage current, but usually
disappears by the time we start recording the data. Because of
this, it is important to let the reading settle out before
recording it.On the other hand, when testing high capacitance
equipment the capacitive charging leakage current can last for a
very long time before settling out.
Machine TestingPolarization absorption leakage current (IA
)Absorption current is caused by the polarization of molecules
within dielectric material.In low-capacitance equipment, the
current is high for the first few seconds and decreases slowly to
nearly zero. When dealing with high capacitance equipment or wet
and contaminated insulation, there will be no decrease in the
absorption current for a long time.Machine Testing
Machine Testing The total current representing the three
component currents will decrease with time and will reach a steady
value only if the absorption current component also becomes
steadyBecause the capacitive charging current falls to a low value
in a comparatively short time, the effect often can be attributed
to absorption current components.It is the effect of this component
which will give an assessment of the state of the dielectric.In
order to make use of this phenomenon, Resistance Test are carried
out.Machine Testing Types of insulation resistance tests.Proof test
(Assignment)Short time/spot reading test (Assignment)Time
resistance test.Step voltage test.Machine TestingTime resistance
test.This test method is based on taking successive readings at
fixed time intervals, and then plotting the readings. This is an
especially effective method when moisture and other contaminants
might be present. As noted earlier, absorption current starts out
high and gradually decreases over time as voltage is applied. In a
machine with healthy insulation, this trend will continue for
several minutes and show an increasing level of resistance. (See
Fig below). On the other hand, if the insulation is poor, the level
of resistance will flatten out after an initial burst.(See Fig
below). Machine Testing
The best way to quantify the results of a time resistance test
is through a dielectric absorption ratio. The dielectric absorption
ratio consists of two time resistance readings. A commonly used set
of intervals is a 60-second reading divided by a 30-second
reading.Another frequently used set is a 10-minute reading divided
by a 1-minute reading. This resulting value is referred to as the
polarization index. The information summarized in the Table above
providing general guidelines for interpreting dielectric absorption
ratios.
Machine TestingStep voltage test. A step voltage test involves
testing the insulation at two or more voltages and comparing the
results.Good insulation will show a relatively consistent
resistance reading regardless of the voltage applied.On the other
hand, when the resistance level drops as the voltage level
increases, it's usually an indication that the insulation is aging,
contaminated, or brittle. This occurs because small imperfections
like pinholes and cracks reveal themselves under increased
electrical stress. When performing a step voltage test, it's
important that you start with the lowest test voltage and then move
to a higher voltage level. Test duration is typically 60 seconds.
Direct Current Armature FaultsLOCALIZATION OF CABLE FAULT
TESTSimple and basic methods to localize cable fault: Murray loop
test, Varley loop testPulse Echo test These methods used basic
equipment that are easily obtained.These tests are performed for
the location of either an earth fault or short circuit fault in
underground cable.Murray Loop test and Varley Loop Test employ the
principle of Wheatstone bridge.
LOCALIZATION OF CABLE FAULT TESTAll the localisation tests
normally involves some calculations of some kind and slide-rule
accuracy is sufficient for tests on short length cables.Where
cables are long, log tables or hand calculations are best for a
really accurate pin-pointing of the fault position. Most common
fault in a cable is a fault to earth, affecting either one or more
cores.Open circuit faults and short circuit faults are much less
common.The first step in the fault location procedure is to
determine the characteristic of the fault,Murray Loop Test
Murray Loop Test Circuit.The circuit diagram to locate fault by
Murray loop test method is shown in the figure above ( earth fault
).P and Q are two ratio arms consisting of resistors, G is a
galvanometer, E is a battery and S1 is a battery key.Murray Loop
Test Murray loop for location of ground fault.A known good
conductor is joined to the faulty conductor at a convenient point
beyond the fault but at a known distance from the test connection.
One terminal of the test battery is grounded. The resulting
Wheatstone bridge is then balanced by adjusting RB until a null is
obtained. Ratio RA/RB is then known. For a circuit having a uniform
ratio of resistance with length, circuit resistance is directly
proportional to circuit length
Murray Loop Test Fig. Murray loop for location of ground
fault.
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