CABLE FAULT
LOCATION
Cable
fault location is the process of locating periodic faults, such as insulation
faults in underground cables, and is an application of electrical measurement
systems. Cable fault location had its beginnings in post-war Dresden, when in
1948 the radio manufacturer Radio Mende was expropriated and transformed into a
Soviet-German limited company. Cable faults are damage to cables which affect a
resistance in the cable. If allowed to persist, this can lead to a voltage
breakdown.
Causes
of cable fault
The possible
causes of cable faults are as under:
Mechanical
damage
This fault occurs
when the cable is insufficiently protected or mishandling at the time of laying
of the cable underground, poor workmanship of cable jointing.
Dampness
When the level of
water is just near to the cable laying in the ground, dampness of paper
insulation in cable occurs, which may damage the sheath.
Mechanical
puncturing
It takes place in the
cable, while excavation work goes on by use of crowbar or pick-axe etc.
Crystallization
Special measures
are taken for the lead sheathed cable to prevent vibrations.
Overloading
or temperature effect
Overloading of
cable rises the temperature of cable insulation, so it may be prevented from
overloading. Surrounding temperature of nearby machine like furnace, steam pipe
and hot water pipe line etc may heat the cable.
Chemical
action
In the soil, due
to chemicals etc. causes pitting and corrosion on the cable. For this the
cables are surrounded with minimum 10 cm layer of pure sand.
Leaking
oil
Leaking of the oil
from cable boxes also causes the failure of cable.
There are
different types of cable faults, which must first be classified before they can
be located.
Types of cable
faults such as:
·
Short
circuit faults
·
Open
circuit fault
·
Earth
fault
·
Cable
cuts
·
Resistive
faults
·
Intermittent
faults
·
Sheath
faults
·
Water
trees
·
Partial
discharges
Methods
adopted in locating various types of cable fault
Sr.
|
Nature
of fault
|
Method
of test
|
1
|
Core
to core fault only
|
Fall
of potential test
|
2
|
Core
of earth fault
|
a)
Murray
loop test
b)
Fall
of potential test
|
3
|
Open
circuit only
|
Capacity
test
|
4
|
Open
circuit and earth fault
|
a)
Induction
method
b)
Fall
of potential if metal sheathed cable
|
5
|
Core
to core fault and earth fault
|
Fall
of potential test
|
6
|
Core
to core fault and earth fault and open
|
Induction
method
|
Fault identification
Prior
to locating a fault, it is necessary to determine the nature of fault.
·
Isolate
the faulty cable and test each core of the cable for earth fault.
·
Check
the insulation resistance between the conductors.
·
Short
and earth the three cores of cable at one end. Check the resistance between the
cores and earth, between individual cores (at the other end) to check open
circuit fault.
·
In
case there is any fault, the insulation test of individual cores with sheath or
armour and between the cores is essential. The
test should also be done by reversing the polarity of the insulation
resistance tester (megger). In case of any difference in readings. The presence
of moisture in the cable insulation is confirmed. The moisture in the cable
forms a voltage cell between the lead sheath and conductor because of the
difference in the conductivity of these metals and the impregnating compound
forms an organic acid when water enters it.
Testing of faulty cable
The
cables are tested as per following test for finding fault.
1.
Murray loop test
2. DC charge and
discharge test for open circuit fault location
3. Phase to phase
fault test for short circuit fault location
4. Fall of
potential test for earth fault location
5. Capacity test
6. Induction test
7. Impulse wave
echo test
8. Time domain
reflectometry test
1. 1. Murray loop test
Murray Loop Bridge is a bridge circuit used for
locating faults in underground or underwater cables.
It has been used for more than 100 years. This
method can be used for both low and high resistance fault in circumstances-
·
Fault
in one or two cores
·
When
three cores are faulty, provided that an adjacent cable is used for
measurement.
·
When
three cores are faulted if the contact resistance differs from each other by a
factor more than 500.
·
When
contact resistance does not exceed 500ohms, if working with low voltage bridge
and 1.5 Mega ohm if working with a high voltage bridge.
Murray loop test is
the most common and accurate method for fault localization. In this test, the
principle of Wheatstone bridge is used to locate the ground fault. In ground
fault, one or more cables are earthed. if the fault current is more than 10
mA when battery voltage is 100V, the
fault resistance may be of the order of 10KΩ
. A high gain dc amplifier can be used for high sensitivity.
Working: the faulty core is looped with sound
core of the same cross sectional area
and a slide wire or resistance box with 02 sets of coils are connected
across the open end of the loop. A Galvanometer is also joined across the open
end of the loop and a dc hand operated generator supplies the
current for the test. Balance is obtained by adjusting the slide or resistance.
The fault position is given by the formula;
d
=
a/(a+b)
Where
a = resistance connected to
faulty cable
b = resistance connected to
sound cable
Loop length = x +
y i.e. 2 times the route length
2. DC charge and
discharge test for open circuit fault location
This test is used
to locate discontinuity in the core of cable, with high resistance to earth.
Preparing for the test , charge the cable with a battery for a very short
time say for 15 sec and then discharge
it through a moving coil galvanometer. Test is repeated at the other end for
the similar reading. The distance of the faulty point from end A is given by –
In this test it is
necessary to earth all the broken cores at far end and also other cores except
the core to be tested to take correct
readings.
In these days,
electronic cable faults locators are available which give the reading directly
on scale. The principle used in such instruments is impressing voltage impulse
on the cable under test. These impulses get reflected from the fault location.
Then reflections are projected on CRO (cathode ray oscilloscope) in the image
format. From image type and distance are determined.
3.
Phase to phase fault test for short circuit fault location
The cable is
tested with the help of insulation tester (megger). Testing between two cables,
if short
Circuited, will
indicate zero. If the conductor is earthed then the testing between conductors
to earth will show less resistance in comparison to sound conductor. If two
phases are short circuited, then the faulty point can be located by the formula
4. Fall of
potential test for earth fault location
Ammeter, voltmeter
battery and variable resistance are connected as shown in diagram. Let the
reading taken across the faulty cable be V1and across the sound
cable be V2. Then the fault point distance can be given as
Where =
total equivalent length of cable.
During
the performance of test the same value
of current should be maintained in the circuit. There are many deferent circuit
arrangements but accuracy is not as good as Murray loop test.
5. Capacity test
It
is adopted to locate open circuit fault in a cable when insulation resistance
of the faulty core is hire. The principle of this method is to compare the
capacity of the faulty core with one which is sound or with a standard
condenser. The faulty core is charged to a certain voltage and the charge is
released by discharging through a moving coil galvanometer. The deflection of
instrument is noted carefully. In the similar manner the sound core of the
cable is charged and discharged. The duration of charging is however maintained
same in both the tests. The distance of break can be determined with the help
of the following formula –
Distance
of break = ( a/b) x length of cable
a=deflection
of the galvanometer of the faulty core.
b=
deflection of the galvanometer of the sound core.
6. Induction test
The
induction method can be used for the location of faults to earth in the case of
a cable having no metallic sheath. in this test a high frequency AC or interrupted DC is passed into the faulty core. The cable
rout is then explored with a search coil connected to a telephone receiver , this
coil taking the form of about 200 turns
made of fine wire wound to form a triangle of about 1 meter side fitted
with head phone. The headphone picks up the audible hum sound while carrying it
over the faulty cable. The humming sound stops suddenly as soon as the the
search coil is away from the fault point.
This
method is suitable for locating fault in a non-sheathed cable.
Since
armour of the cable shields the magnetic field, no current will be induced in
the search coil thereby no audible sound is heard.
Sometimes,
the head phones catches disturbance created by other sources. Precautionary
measures have to be taken against such circumstances while carrying out the
fault finding.
7.
Impulse wave echo test
This method is
based on principle that a pulse propagating along a cable will be reflected
when it meets with an impedance mismatch. This effect can be seen on a cathode
ray tube, CRT. The pulse propagation velocity is inversely proportional to the
squire root of the dielectric constant of the cable. For a cable of uniform dielectric,
the pulse reflected at the mismatch is displayed on CRT at a time delay
directly proportional to to the distance of mismatch from the test; irrespective
of the conductor size. The fault position is given by-
X= (t1/t2)
x length
Where,
t1=
pulse time to fault
t2=
pulse time to far end of cable.
This is quickest
and universally accepted. These days portable digital fault
locators are available using wave echo technique. It consists of a unit having
a crystal controlled digital timing method which is simpler and accuracy level.
Fault distances are displayed in meter digitally. The fault, distance upto 25
km can be diagnosed. It can be used for both LT and HT cable.
8. Time Domain
Reflectometer:
The Pulse
Reflection Test Sets IRG Series for cable fault pre-location using the Time
Domain Reflection (TDR) method on low, medium and high voltage cables. It can
also be used on live cables up to 400 V. Further fault location methods are
available with the application of the appropriate coupling device. Its
measuring ranges enable pre-location on cable lengths from 0 m to 65 km (0 to
213,000 feet).
Time Domain Reflectometer
(TDR): The TDR sends a low-energy signal through the cable, causing no
insulation degradation. A theoretically perfect cable returns that signal in a
known time and in a known profile. Impedance variations in a
"real-world" cable alter both the time and profile, which the TDR
screen or printout graphically represents. This graph (called a
"trace") gives the user approximate distances to
"landmarks" such as opens, splices, Y-taps, transformers, and water
ingression.
One weakness of
TDR is that it does not pinpoint faults. TDR is accurate to within about 1% of
testing range. Sometimes, this information alone is sufficient. Other times, it
only serves to allow more precise thumping. Nevertheless, this increased
precision can produce substantial savings in cost and time. A typical result is
"438 ft 5 10 ft." If the fault is located at 440 ft, you only need to
thump the 20-ft distance from 428 ft to 448 ft, instead of the entire 440 ft.
Another weakness
of TDR is that Reflectometer cannot see faults-to-ground with resistances much
greater than 200 ohms. So, in the case of a "bleeding fault" rather
than a short or near-short, TDR is blind.
Conclusion
Using the
combination of a cable analysis system, a surge generator and a surge
detector/fault pin pointer, the process of underground fault locating becomes
more efficient, gets service restored quicker and minimizes the possibility of
programming the cable for additional faults while finding the present fault.
Book reference: a practical guide to cable installation and toolbox talk.
In India-
Available with book
shop and -
Price:
Rs. 375/- excluding delivery charges
