Underground cable fault

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-

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