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Cable fault is referring to something that occurs current can’t flowsmoothly. It is because open circuit or short circuit that means thecurrent flow is high value.

INTRODUCTIONCABLE FAULTS

Most faults are caused by dampness in the paper insulation of thecable due to porous or otherwise damaged lead sheath. Thecauses of damage to the lead sheath are: crystallization of the leadthrough vibration; chemical action on the lead when buried in theearth and insufficiently protected; and mechanical damage.

Main equipment to determine cable fault is ohmmeter. Besidethat, a few test can be done to determine cable faults such as:

1. Continuity Test2. Insulation Resistance Test

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The faults which are most likely to occur in the cables are:-

1. Ground or Earth Faults (Break-down of cable insulation)When the insulation of the cable gets damaged, the current starts flowingfrom core to earth or to cable sheath.

2. Short Circuit FaultsWhen the insulation between two cables or between two cores of a multi-core cable gets damaged, the current starts flowing from one cable toanother cable or from one core to another core of a multi-core cabledirectly (without passing through load)

3. Open Circuit FaultsWhen the conductor of a cable is broken or joint is pulled out there is nocurrent in the cable.

TYPES OF CABLE FAULTS

PROCEDURE TO FIND CABLE FAULT

To find the cable faults, it has four steps, that is:-

1. Testinga. Continuity Test – open circuit faultsb. Insulation Test – short circuit faults

2. Locating cable fault test from terminal cables To find length of faults from testing station

3. Confirmation fault point test To find actual location faults

4. Repair the fault

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CABLE FAULTS TEST

Continuity Test

Continuity test is to checking open circuit of cable faults. Usually, thistest uses a bell and battery or multimeter. This test is dividing to twotypes, that is:

1. Short alignment2. Long alignment

Short Alignment

Using a bell and battery. This test is used for conduit types. For testing CC1cables, a connection is jointed between C and C1 through a bell and battery. Ifa bell ringing, the cable in continues condition, and the cable is marked withC1, if the bell not ringing, the cable is not in continues condition. For A and Bcable, the same method is used to test cable condition.

Figure 7.1: Short alignment for Continuity Test

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Figure 7.2: Long alignment for Continuity Test

Long Alignment

Figure 7.2, the method is suitable to use for long cable. Forcable A, B and C, choose a pair of cable, for example cable Aand B will be connected with tester while the end of the samecable with short circuit. If the test shows an open circuit, theend of the cable will change with another pair and the circuitwill test again.

If a low reading is detected, so the cables are label with A1 orB1. Then, connect one of the cables with another cable forexample cable A and B, and test with the same method. Ifreading is low, so cable A and C will detect and right. Thus,the cable will label clearly. The same method can be done foranother cable in the same alignment.

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Insulation resistance is resistance that measured in ohm unitsbetween live part of equipment and earth for installation.Resistance will measured through insulation that coveredconductor.

Insulation Resistance TestUsed to test short circuit and thickness of insulator. One of theequipment to short circuit test is insulator resistance tester. In thistest, a few tests on installation must be made before connect tosupply.

Insulation Resistance Test

Earth Installation test

The test is made with all fuses stay at their place. The main switch mustbe switch off. Switch off all lamps and test it by separately. Insulationthat measured to the earth must less than 1MΩ and range for theequipment made in difference must less than 0.5MΩ to the earth and0.5MΩ between polar and phase.

Voltage test

Need to test the voltage that used for Insulation resistance test. If theused voltage is DC voltage, it must less than two times normally DCvoltage and if the used voltage is AC voltage, it must less than two timesnormal Vrms; but not more than 500v for medium voltage circuit.

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Figure 7.3: Insulation resistance test forfinish installation

Figure 7.3 shown connectionsfor insulation resistance toearth testing 2 wires installationthat finish with other lamp andfeel off other equipment.

Both of the cable to mainswitch twist together andconnected to line terminal atohmmeter. Earth terminal forohmmeter connect to userearthing.

The terminals for two wayswitches must be connectedtogether for a few times.

Figure 4: Insulation Resistance Testing of Electric Kettle

Heating equipment shall be testedseparately, and Figure 4 shows anexample of tests made on theelectric kettle.

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Figure 7.5: Testing between conductors for 2 cables of single installation

Testing between two conductor.

Connect to pole or phase supplyand between conductors to otherpole or phase supply. Insulationresistance must be less than1MΩ.

Figure 7.5 is an example ofinsulation resistance test betweenconductors.

Murray loop test, Varley loop test and Pulse Echo test are simpleand basic method to localize cable fault testing. This method usedbasic equipment that obtained easily.

These tests are performed for the location of either an earth fault orshort circuit fault in underground cable. In these tests theresistance of fault does not affect the results obtained except whenthe resistance of fault is very high.

There are two loop tests usually used and are known as MurrayLoop and Varley Loop Test. These tests emply the principle ofWheatstone bridge.

LOCALIZATION CABLE FAULT TEST

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The connection diagrams to locate earth fault and short circuit faultby Murray loop test method are shown in Figure 7.6 respectively. Asalready said Wheatstone bridge principle is used in these tests. Pand Q are two ratio arms consisting of step resistors or slide wire, Gis a galvanometer, E is a battery and S1 is a battery key.

In this method, a sound cable is connected to a faulty cable for findactual happen fault. Figure 7.6 is connection method of Murray LoopTest.

Murray Loop Test

Figure 7.6: Murray Loop Test Circuit.

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Procedures:

End of the faulty cable (assume F is a location fault) was connectedwith a sound cable from F to b location test through D connector.

At location test, end of cable a connected to b through agalvanometer and S1 switch. Bridge is balanced by adjusting theresistances of ratio arms P and Q until the galvanometer indicateszero.

Assume the sound and faulty cable have same cross-section andlenght, thereby r Ω/m is same.

In balanced condition of bridge:

If L be known, so Lx can be calculate, that is distance of cablefault from test station.

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If L and r of sound and faulty cable is not same, so the formulaabove will be change. Assume

r’ = resistance/lenght unit of sound cableL’ = length of sound cable

Example:

Murray loop is a variation of the Wheatstone bridge is used to determinethe location of the shorted two wires between them.

(i) Refer to the diagram below, show the following equation whenequilibrium.

(ii) Calculate the distance of fault from a test station where the balance isfound when L1 = 50km, L2 = 100km, R1 = 3.3Ω / m, R2 = 10 Ω/m, A/B =8/1

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In this test also a sound cable is required in addtion to the existing cable. Thecircuit diagrams to determine the location of ground fault and short circuitfault by method of Varley loop tests are shown Figure below,

Varley Loop Test

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When the switch is in position (a), S is adjusted so that the bridgeresistors in a balanced situation. Suppose that in a balanced situation isfound S = S1.

When the switch is in position (b), S is adjusted so that the bridgeresistors in a balanced situation. Suppose that in a balanced situationis found S = S2

Solve equation (1) and (2)

Example:

For cable fault test using Varley loop, when the switch is placed in position1, the variable resistance, S is adjusted to a value of 50Ω to achieve abalance bridge. When the switch is placed at 2, equilibrium occurs when Sis 35Ω. Find the distance fault location from the source if the cable is 25kmlong, given fault cable resistance is 15Ω/km, P = Q = 150 Ω and 200 Ω.

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Cable line in underground has a coating act as absorption ormoisture blocker into insulator cable or main cable conductor.This moisture or underground water will make problem becausethe water usually acided and grounded cable will be fault.

Pulse Echo Test is to find fault cable in grounded coating cablewith identified the location that directly detect with ground.

PULSE ECHO TEST

Procedure:

Connect a battery at end to end faulty cable. (The current will flowalong conductor to location fault, from location fault to earth andthen from earth to other terminals battery. Current can be imaginglike it flowing out from location fault to surround ways.)

Put voltmeter probe at two points such as in Figure 7.10. Positiveor negative value will be obtained, it depend on flowing current inground that fault happen.

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Move the voltmeter along the cable until the value of currentchange ways. the point that the current changes the way shownactual location faulty happen.