HT Fuse Burn Out & VCB Tripping

"...with the above readings we can say that the PILCA UG cable is fine...", really??? You're looking at a 30 yr old paper insulated lead sheathed cable that has an unbalanced insulation reading, and it just happens to be the same phase that went overcurrent? Time to thump the cable to find the location of the fault to ground and splice in a new piece, though it may cheaper to replace it with some modern XLPE cable.

You're tests were made at a lower voltage than the normal operational 6350V to earth.

A fault with at operational voltage will have enough power to create a flash over in the insulation. At the moment that happens you're reading of 200M Ώ is meaningless. 200M Ώ @6350V the current flow should in theory be 0.00003175A, what size fuse did it blow?

Theory goes out the window when we deal with reality.

20Amps Fuse

Do a hypot test and tell us the results.

Your HT cable must be tseted on min 22kV. Carry out hot test, surly your find the low insulation resistance of conductor "R" with armour and way forward.

Dear shajahan,

As you have not mentioned the 11 kV supply system, it is considered that star connected with neutral grounding; therefore the volttage applied between core and screen is 11kV/sqrt.3 i.e 6350 volt . Therefore IR test by 5kV isulation tester is insufficient. Please use 10kV Insulation Diagonistic Tester to measure Absorption Index( AI), Polarisation Index(PI),Insulation Ressistance(IR),Dielectric Absorption Ratio(DAR) and Dielectric Discharge(DD) values. By analysing the above results, you can see the unhealthyness of the cable. After the above test please carryout High Voltage Surge test (15kV) for cable fault location.It is suspected that there is splice/ joint in the above cable system and there is a formation of very high resistance carbon bridge which is reflected at the IR test result by 5kV Meggar.

Manindra

You must use a test meter that is capable of tesing the cable at the rated 11KV in order to yield accurate results.

While testing with a 5KV meggar will reveal low impedance "short circuit" faults, it cannot identify high impedance faults such as shield leakage or phase-to-ground faults.

I would suggest a DOBLE, DC Hypot, or similar machine that is capable of providing the right voltage value.

You may be suffering from stress cone failure dur to corona damage.