HV Cable Testing

[timm333]

I have a question about the insulation testing of the oversheath of single core 138 kV cables. The oversheath has other metal layers below it, and the oversheath is really not an insulation as it has a conductive layer which is extruded in it. What do we exactly test when we test the insulation of the oversheath? Thanks for help!

 

[Ingenieur]

you are testing impedance from phase to ground (the shielding) in the typical test
the center conductor is insulated from the ground/shield (obviously)

 

[timm333]

Actually this test is not for the conductor and insulation. This test is only for the outermost layer (oversheath) and the layer directly below it (armor). The oversheath has an extruded semi-conductive compound in it which acts as conductive layer and is connected to the ground terminal of tester. The armor is connected to the other terminal of tester. I am trying to figure out that why they perform insulation resistance test between these two layers, whereas both of these layers are conductive and they directly touch each other. What do you think? Thanks

 

[Tony S]

This is a standard test for buried cables to check the outer serving hasn’t been damaged when the trench has been backfilled. Any damage will allow water ingress and result in a possible failure at some time in the future.

I’ve always used 500V DC (Megger) between the armour and a reliable earth point.

 

[Bugman1400]

Damage can also occur when the cable is pulled through conduit and pullboxes. Many times what most think is insulation is actually a semi-conductive material.

 

[timm333]

Thanks. The test is between two surfaces (oversheath and armor). How should these two surfaces be discharged after the completion of the test; should the charge on the two surfaces be allowed (for few minutes) to drain off through the leakage of the tester, or should the two surfaces be connected together so that they get discharged?

 

[Bugman1400]

Thanks. The test is between two surfaces (oversheath and armor). How should these two surfaces be discharged after the completion of the test; should the charge on the two surfaces be allowed (for few minutes) to drain off through the leakage of the tester, or should the two surfaces be connected together so that they get discharged?

I think its typical to wait 15mins for bleed off then, test for absence of voltage with gloves on.

 

[Tony S]

A Megger should be left connected to cable once the test is completed, the meter will discharge the cable.

 

[timm333]

What should be the earth resistance between the armor and the earth point, should it be 5 ohms or less?

 

[Ingenieur]

What should be the earth resistance between the armor and the earth point, should it be 5 ohms or less?

This may help
lower mv range but sheath is tested in the 1-5 kv range
pass 10-100 M Ohm min

https://www.google.com/amp/s/electr...w-voltage-and-high-voltage-cable-testing/amp/

A sheath integrity test (e.g. 1000 V minimum insulation resistance tester) applied between theouter-most metallic layer and earth can identify after-installation damage to the non-metallic outersheath.
The measured value should be read after application of the voltage for 1 minute. Ideally themeasured value should be corrected for temperature to a standard value at 20°C if correctionfactors are available. A rough guide is that the insulation resistance decreases to one half of thevalue for a 10°C rise in temperature. The cable temperature should be recorded along with themeasured values.


Measured values of Insulation Resistance for the sheath should be greater than calculatedvalues. Calculated values for new cable range from 1.5 M/km to 4.0 M/km @ 20°C for PVCsheaths and from 120 M/km to 300 M/km @ 20°C for PE sheaths. Values are highest forsmall cables & thick sheaths and lowest for large cables & thin sheaths. (Factory tests show thatmeasured values are up to an order of magnitude greater than the calculated values.)


Earth the screens after an Insulation Resistance Test on a sheath for at least 5 minutes beforehandling or performing other tests

 

[timm333]

I am actually not much a fan of the articles like this published from India, but thanks anyway.

 

[Ingenieur]

I am actually not much a fan of the articles like this published from India, but thanks anyway.

I looked at several sources
several mfgs
IEC stds
all in the M Ohm range

the text was from this http://www.nexans.co.nz/NewZealand/...for Tests after Installation on MV cables.pdf
a major cable mfg
it is for mv cable, but hv of the same construction type should be = or >
definitely not 5 Ohms

 

[Phil Corso]

Ingunear,

Finally, an excellent non-Pittman-like contribution!

Phil

 

[timm333]

Sorry I did not explain clearly. The insulation resistance would be in M-ohm range, but what should be the ground resistance of the ground electrode which is used for testing of cable on the reel? If the cable is on the reel then it is not in contact with the soil and so a separate temporary ground electrode in the field would be required in order to ground the outer sheath of the cable. What should be the ground resistance of this temporary ground electrode, should it be 5 ohm?

 

[Ingenieur]

I doubt you will get 1 short test rod to 5 Ohm
our medium station beds (30' x30') with 40 rods and connected by 4/0 are only 4 Ohm

how do you measure on the reel?
I thought the purpose was to test the integrity of the sheath for leakage from the shielding through the sheath to ground (and vice versa) for water leakage etc?
for damage caused by installation/backfill/etc?

once installed should not matter since
rod is 10's of Ohms and sheath is MOhms

 

[Tony S]

The only way of testing with the cable still on the reel is by immersion in water, the manufacturer should have done this or some similar test. The main reason for a sheath test is to find any damage caused during installation and backfilling.

 

[timm333]

Yes the test is for the integrity of the sheath to ensure that no water enters into the cable. In addition to water, if the sheath is damaged then air can also touch the cable and so damage the cable. Cable can be damaged during transportation, so on-reel tests are performed. I think it is possible to test the cable on reel in the field without immersion; it can be done by connecting the sheath to ground-electrode by something like a "pipe grounding clamp", or by wrapping a "braided conductor strap" around the cable.

 

[timm333]

One of the things I am still trying to figure out. When we do the insulation resistance test of insulation between two conductive surfaces, we connect one conductive surface to the system grounding electrode. Is grounding part of the insulation resistance test, or is it only for the safety of life? Thanks.