The Output results of MEGGER TEST and its solution?

Scenario:-
 Distribution Substation. [SMDB-S AREA].
 Electrical Room. [MDB-CAB-4]
 3x(4C-300 mm2 XLPE/SWA Cu Conductor+ 1C-16 mm2 Y/G ECC )
There is a fault between cables which have been connecting form Substation to Electrical room having length of 300 meter approximately. There is a solid link between substation and electrical room having rating 630 A HRC Solid Links.
These cables have been laid by many years ago and did by other group. During excavation the one cable to be found with five straight joint, second with two, and third with insulation damage. (As mentioned in attached pictures).
The cables have been tested by performing MEGGER TEST @ 500 V (using MIT515) by disconnecting the terminals from both sides. (Refer attached photos).


My questions are as follows.
 Which is the minimum resistance in which cable say to be okay or it depend upon the output load?
 When performing the test the MEGGER MIT515 Device shows the output result in 1TΩ while on second time it yields the value of 0.97 GΩ. Why is there any reason?
 All Cables with at least one core have given the fault. (My observation Less than 200 MΩ fault).





What is the best solution based on the situation. [Economical Solution].
 Needs replace all cables?
 Open/Cut cable from Straight Joint and perform again the MEGGER?
 May be some device/ instrument which solve the solution (detect the cable fault at exact location)?
 Other Solution?

Re: The Output results of MEEGER TEST and its solution?

tipu_sultan said:

May be some device/ instrument which solve the solution (detect the cable fault at exact location)?

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There are instruments which can detect an invisible break, and tell you its distance down the cable. The electric company uses such instruments. It sends a waveform into the cable, and listens for an echo.

It may also be possible to detect cable characteristics (including where there is a break) based on capacitance and/or inductance.

one cable to be found with five straight joint, second with two,

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Does 'joint' = 'splice'?

The ground does not appear wet. Did you find you have current leaking into the ground? If not then it suggests the packing and wrapping is intact.

and third with insulation damage.

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This could be packed and wrapped similar to the other two cables. No doubt there are modern materials available, to make such repairs durable and waterproof.

I'm not an electrician, so I could be wrong.

tipu_sultan said:

Scenario:-

What is the best solution based on the situation. [Economical Solution].
 Needs replace all cables?
 Open/Cut cable from Straight Joint and perform again the MEGGER?
 May be some device/ instrument which solve the solution (detect the cable fault at exact location)?
 Other Solution?

Click to expand...

I doubt you will be able to guarantee insulation quality with 500V Megger.

Do you know BIL rating? probably BIL 10 or 10kV

Do you have a Megger Oil Tester or any other HIPOT tester? That's what you need to verify.
Do you have a scope? A fast scope can locate fault from TDR method , cheap but needs skill,
- otherwise you need a portable TDR tester with variable pulse up to 10kV to locate fault. ( not cheap but less skill to operate) cable or near tester or wires.

Other useful insulation tests are Partial Discharge ( early warning to major failure) using cheap scope to detect small impulses every minute or second or cycle as insulation failure escalates...
PD activity will show up as partial impulses if there is insulation weakspot before breakdown occurs.

But if breakdown from line transient occurs, follow-on current till zero crossing can burn thru. cable

Another useful test is swept dielectric test with HV. ( expensive tester unless you have skill to make one )

Then you need to verify resistance in micro/milliohms of conductors.

But for insulation easiest test is 10kV with low current tester.

I don't know think cable can be re-jacketed except at isolated spots. like 5 times before or put into separate PVC tubing that can withstand buried pressure, maybe with sand, not rocks.

Expensive test equip to locate faults... maybe you can borrow one.
https://www.cablejoints.co.uk/sub-product-details/megger-cable-fault-locators