An unsatisfactory feature and performance of cable used in the electrical installation are termed as cable faults.
There are various types of cable faults such as earth fault, short circuit fault, and open circuit fault.
Here we will be discussing the types of cable faults, their causes, and tests to be performed to locate the cable faults.
Types of Cable Faults
Following are the types of faults which are most likely to occur in the cables.
1.Ground or Earth Faults:
The insulation breakdown of the cable results in-ground or earth faults. Due to this current starts to flow from the core of the cable to the earth or the cable sheath.
2.Cross or Short-Circuit Faults
The insulation breakdown between the two cables or in a multi-core system cable the insulation breakdown between the two core results in a cross or short-circuit fault. Due to this current starts to flow from one cable to another or from one core to the other.
Open-circuit fault results due to the breakage of cable and no current flow through the cable.
Determination of Nature of Fault
First, we have to determine the nature of the fault, to locate the fault.
Then to determine the nature of the fault we have to measure the insulation resistance of each core to the ground and between the cores of the cables. To measure the insulation resistance we require an instrument known as Meggar.
The indication of ground fault gives a low value of insulation resistance between any core and earth.
The indication of short-circuit fault gives the low value of insulation resistance between two cores.
In the case of an open-circuit fault. To determine the nature of the fault, the far end of the cable is earthed (in the case of single cables). For multi-core cables, we need to interconnect the far end of the cables. Then, an ammeter is inserted in series with the cable and a low voltage supply is connected across the cable and earth or cables. The zero or no deflection in the ammeter indicates an open-circuit fault.
Causes of Cable Faults
Cable faults are mostly due to the dampness in the paper insulation due to penetrable or damaged lead sheath.
The lead sheath is damaged due to chemical action on the lead when the cables are buried in the earth. Also due to the crystallization of lead through vibration and insufficient protection provided to the cable resulting in mechanical damage.
Location of Cable Faults for Ground Fault of Single Cable
For a single cable, to determine to locate the ground cable fault we use Blavier’s test.
This test is performed with a voltmeter or ammeter or bridge network.
The figure below shows the circuit schematic for the test.
Here, r is the resistance of the fault to the earth.
r1 and r2 are the conductor resistances of the lengths of the cable “far rand” to a fault and “test end” to fault respectively.
At first, we measure the resistance between T1 and earth. The reading of this measurement is R1.
Then we have to ground the far end T2 of the cable and will again have to measure the resistance between T1 and earth. The reading of this measurement is R2.
The total resistance of the cable R is
Solving equations (i), (ii), and (iii) will give the value of r, r1, and r2.
If the length of the cable is L meters and the cross-section of the conductor is uniform along the length of the conductor with L1 being the length of the cable between the far end and fault and L2 being the length of the cable between the test end and fault.
From this above expression, we can determine the distance of the fault from the west end.
Cable Faults Test
We need to perform loop tests for locating a ground cable fault or short circuit fault in underground cables.
Unless the resistance of the fault is very high, the resistance of the fault does not affect the results of the loop test. Murray loop test and Varley loop test are two common loop tests method. These tests work on the principle of the Wheatstone bridge.
Fall of the potential test method is performed in the case when a second cable free from the fault runs along the faulty cable. This method of cable fault location is not as accurate as of that of the Murray loop test.
In case of locating a short-circuit fault, we have to measure the capacitance of the faulty cable from both ends by the use of a ballistic galvanometer or by an ac bridge. The essential requirement for this test is that the core of the cable to be tested should have good insulation resistance.