Megger Test of Unshielded Medium Voltage Cables 1

[CuriousElectron]

Hello,
For an existing MV cable that is unshielded, if it is desired to perform a Megger test to check the insulation resistance of the cable, would it be advisable to megger it to ground or test from a phase to other phases grounded?
I think if test setup has one lead of the megger device connected to phase conductor and the other to station ground, then we'd be including the insulating air as part of the test. Are there any advantages/disadvantages between both setups?
What about a setup where one lead is connected to the conductor and the other lead is connected to the insulation of the cable? Wouldn't this be a preferable setup over the two above?
The test voltage is 2500VDC. We'd also be conducting a PI test.
I appreciate any feedback, I seem to get conflicting info regarding the test setup.
Thanks,
EE

 

[RRaghunath]

If it is single core cable, you wrap tightly an aluminium foil over the insulation and test the cable IR between conductor and the foil, with foil connected to earth additionally.
If it is multi-core cable, connect all other cores to earth and test the IR between the conductor and earth.

 

[CuriousElectron]

Thanks for the response, RRaghunath,
What if it's a long run of cable, can one test the insulation resistance to ground? If there is any voltage leakage along the length of the cable, it would be picked up that way. I realize that it is not as effective measurement as if the cable were shielded, but there is still some merit to performing this test.
Thanks,
EE

 

[waross]

Define "medium voltage".
Depending on the voltage rating of the cable a 2500 Volt megger test may be a continuity test.
You want to check phase to phase to verify the phase to phase insulation.
A high-pot test on 750 Volt cable will be 2500 Volts.
600 Volt rated cable should withstand a 2500 Volt megger test easily.
A long, installed single core non-sheilded cable may often be meggered to earth.
Acceptance megger testing of shielded or armoured cable on the reel is common.
Acceptance megger testing of non-shielded cable on the reel may be futile.
Be aware that your test is more suitable to identifying existing cable faults than to determining the cable condition.
2500 Volts is too low for an insulation leakage test on all but the lowest medium voltages.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!