Wednesday, September 7, 2011

Testing of Cables

       The cables are tested both before and after installation.
       Before installation the cables are tested for the following ;
       1. Continuity test      2. Insulation test
       Both the tests can be performed with the help of megger. The values of inductance, capacitance and resistance of the cable are also recorded before installation. These values help in locating the faults in future when the cable is installed.
       When there are faults in the installed cable then quick identification of location of the fault is very much necessary for quick repairs. For this the following tests are conducted on the cables,
1. Murray loop test
2. Fall of potential test
3. D.C. charge and discharge test.
1.1 Murray Loop Test
       In this test, the principle of wheatstone's bridge is used to locate the ground faults. In ground fault, one or more cable cores get earthed. This is the most accurate test.
       In this test, one sound return wire of the same cross-section as that of cable is required. The connections are shown in the Fig. 1.
Fig. 1
       The perfectly sound cable is looped with faulty cable. The balance of the bridge is obtained by varying the resistances.

       Where           a = Resistance connected to faulty cable
                            b = Resistance connected to sound cable
       Loop length = x + y i.e. 2 times the route length
1.2 Fall of Potential Test
       The arrangement used for the test is shown in the Fig. 2.
Fig. 2 Fall of potential test
       The voltmeter used is high resistance low reading type. The battery, variable resistance and ammeter are in series. Once the reading of voltmeter is taken by making connection to the faulty cable and is obtained as V1. Other reading is taken by making connection to the second cable and is obtained as V1.
       Then the fault distance d is given by,

        Where           L = Total equivalent length of the loop
It is necessary to adjust the ammeter reading to the same value for both the readings.
1.3 D.C. Charge and Discharge Test
       This test is used to locate discontinuity in the core of the cable, with high resistance to earth.
       The connections are shown in the Fig. 3.
Fig. 3   Charge and discharge test
        The two way switch S is used along with the battery and the galvanometer G.
       The switch position is selected as 1 and cable is charged first with the help of battery for 15 seconds. Then it is discharged through the galvanometer with switch position as 2. The galvanometer deflection is observed. Similar readings are taken at the other end of the cable. Then the distance d of the fault can be obtained from the end A as,

        In this test, it is necessary to earth all the broken cores at far end and and all cores at far end and all cores except core under test at the test point, to avoid any false reading.
       Nowadays electronic cable fault locates are available which give the reading directly on a scale. The principle used in such instruments is impressing voltage impulses on the cable under test. These impulses get reflected from the fault location. Then reflections are projected on C.R.O. in the image from. From this, the distance and type of the cable fault is determined.
Read solved examples

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