LOCATION OF CABLE FAULTS INTRODUCTION: Cable fault is referring to something that occurs current can’t flow smoothly. It is because open circuit or short circuit that means current flow is 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; and mechanical damage. Main equipment to determine cable fault is ohmmeter beside that, a few test can be done to determine cable fault such as : 1. Continuity Test 2. Insulation Resistance Test

TYPES OF CABLE FAULT : The fault which are most likely to occur in the cable are : 1. Ground or Earth Faults (Break-down of cable insulation) : Whenthe insulation of the cable gets damaged, the current start flowing from core to earth or to cable sheath. 2. Short Circuit Faults : When the insulation between two cable or two cores of a multi-core cable gets damaged, the currents start flowing from one cable to another cable or from one core to another core of a multi-core cable directly(without passing through core). 3. Open Circuit Faults : When the conductor of cable is broken or joint is pulled out there is no current in the cable.

Producer to Find Cable Fault : To find the cable faults, it has four steps, that is : 1. Testing: A. Continuity Test – open circuit faults B. Insulation test – short circuit faults 2. Locating cable fault test from terminal cable: To find length of faults from testing station 3. Confirmation fault point test: To find actual location faults 4. Troubleshooting

Cable Faults Test : . Continuity test: Continuity test is to checking open circuit of cable faults. Usually, this test uses a bell and battery or multimeter. This test is dividing to two types, that is : (A). Short alignment (B). Long alignment   1. (A). Short Alignment :

Fig. 1is an example method by using a bell and battery Fig.1is an example method by using a bell and battery. This test is used for conduit types. For testing CC1 cables, a connection is jointed between C and C1 through a bell and battery. If a bell ringing, the cable in continuous condition, and cable is marked with a C1, if the bell not ringing, the cable is not continuous condition. For A and B cable, the same method is used to test condition. 1. (B). Long Alignment : Fig.2 , 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 connected tester while theend of the same cable with short circuit. If the test show an open circuit, the end of the cable will change with another pair and circuit will test again. If the low reading is detected, so the cables are label with A1 or B1. Then, connect one of the cables with another cable for example cable A and B, and test with the same method. If reading is low, so cable A and C will detect and right. Thus, the cable will label clearly. The same method can be done

Insulation Resistance Test: Insulation resistance is resistance that measured in ohm units between live part of the equipment and earth for installation. Resistance will measured through insulation that covered conductor. Insulation Resistance Test is used to test short circuit and thickness of insulator. One of the equipment must be made before connect to supply. Voltage Testneed to test the voltage that used for insulation resistance test. If the used voltage is DC voltage, it must less than two times normally DC voltage and if the used voltage is AC voltage, it must less than two times normal RMS voltage; but more than 500v for medium voltage circuit. Earth Installation Test is test made all fuses stay at their place. The main switch must be switched OFF. Switch off all lamps and test it by separately. Insulation that measured to earth must less than 1MΩ and range for the equipment made in difference must less than 0.5MΩ to the earth and 0.5MΩ between polar and phase.

Fig.3 shown connection for insulation resistance to earth testing 2 wires installation that finish with other lamp and feel off other equipment. Both of the cable to main switch twist together and connected to line terminal at ohmmeter. Earth terminal for ohmmeter connect to user earthling. The terminal for two way switches must be connected together for a few times. Heater must be tested separately. Fig.4 on example tested of kettle electric.

Testing conductor make between conductors that connect to pole or phase supply and between conductor to other pole and phase supply. Insulation resistance must be less than 1MΩ. Fig.5 is an example of insulation resistance test between conductor.

location Cable Fault Test Murray loop test, Varley loop test and pulse Echo test are simple and basic method to localize fault testing. This method used basic equipment that obtained easily. These tests are performed for the location of either an earth fault or short circuit fault in underground cable. In these test the resistance of fault of fault does not affect the result obtained except when the resistance of fault is very high. There are two loop tests usually used and are known as Murray loop test. These tests emply the principle of Wheatstone bridge.   5.1 Murray Loop Test: The connection diagrams to locate earth fault and short circuit fault by Murray loop test method are shown in fig.6. As already said Wheatstone bridge principle is used in this test. P and Q are two ratio arms consisting of st6ep resistors or slide wire, G is galvanometer, E is a battery and S1 is a battery key. In this method, sound cable is connected to faulty cable for find actual happen fault. Fig.6 is connection method of Murray Loop Test.

Procedures: End of the faulty cable (assume F is a location fault) was connected with sound cable from F to b location test through D connector. At location test, end of cable connected to b through a galvanometer and s switch. Bridge is balanced by adjusting the resistance of ratio arms P and Q until the galvanometer indicate zero. Assume the sound and faulty cable have some cross-section and length thereby r Ω/m is same. In balance condition of bridge : Q × (cable resistance aF) = P × (Cable resistance bD + DF) Q(rLx) = P + [ rL + R(L – L x)] rQLx = rP( 2L - Lx) rQLx + rPLx = r2PL rLX (Q + P) = r2PL   If L be known, so Lx can be calculated, that is a distance of cable fault from test station. If L and r of sound and faulty cable is not same, so the formula above will be change. Assume r’ = resistance / length unit of sound cable L’= length of sound cable So (rLx) (Q) = P×[r(L - Lx)+r’L’] rQLx = Pr(L-Lx)+ Pr’L’ rQLx – rP(L - Lx) = Pr’L’ rLx (Q + P) = PrL+Pr’L’ rLx (Q+P) = r Lx =

Varley Loop Test: In this test also a sound cable is required in the existing cable. The circuit diagrams to determine the location of ground fault and short circuit fault by method of Varley Loop Test are shown in … disparate connection method that is

Earth Over Lap Test:

In case of earth overlap test are two measurements which are carried out as follows : (a) The resistance M1 is measured between line and ground from the testing end with the far end earthed. (b) Then the resistance M2 between lineto ground is measured at the far end with the testing end earthed. then one can write as, M1 = r2 + ……… (1) M2 = r1 + ……… (2) Also R = r1 + r2 By elimination we have : r2 = M1 r1 = M2 {} Thus in this test also the distance of the fault from either end is obtained by using above relation. The accuracy of these two method namely Blavier test and earth overlap test is not very high mainly because it is often difficult to make connection to the earth at the end of the cable, having a very low resistance and therefore the result are not very accurate.

Voltage Drop Test: This test can be used when there are two cable out of which one is healthy and other one is faulty and both are running parallel to each other.

The healthy and sound cable is used as part of current circuit or as a potential lead as shown in Fig.13 and Fig.14. As shown in Fig.13 a large steady current is passed through the loop which is formed by the healthy cable and faulty cable joined together at the distance and as shown by a lo0w resistance connection. The current is measured by using a resistor R and an ammeter A. By using throw over switch S, the voltmeter V whose one terminal is earthed is connected first across the section cd of the loop and then connected across the section abcd. Let the two reading of the voltmeter be V1 and V2. As the same current flows through both of the loop sections we can write, OR

If the cross sectional area of the cable is same throughout then we can write, From this relation position of the fault can be found. If the cross section area of the cable is not uniform then correction has to be applied to take care of the same. The resistance of the voltmeter should be large compared with the resistance of the fault because it forms a part of the voltmeter circuit. Another reason for high resistance voltmeter is that instrument otherwise taken an appreciable current and hence introduces errors in the measurement. In the circuit as shown in Fig.14 the fault resistance does not entered into resistance of the voltmeter circuit. The length ed is calculated from the known resistance per unit length, the passing through length ed should be low enough such that it should not produce appreciable heating as the resistance per unit length will then be different from that used in the calculation of the length. In this method the voltmeter resistance should be high as compared with the resistance of the both length of section ed and section length abcd. In case of electrostatic voltmeter accuracy is batter.