Cable bending radius
Cable bending radius
(OP)
I have triplex cable (3 cables intertwined together) with overlal diameter = 62mm. During pulling, because of the cable run of 350m, we decided to split it ionto 2 pulls from somewhere at midpoint. The trouble is, I failed to account for the manhole at midpoint, and when the cable is inserted through the 600mm dia, the cable min bending radius was exceeded. The cable appears to pass the inuslation resistance tests, and we will soon be doing the hipot. Referring this issue to the manufacturer, they said that they do not have data to show the physical condition of the cable with this level of bending, and cannot guarantee operation after energization. (Think this is a manufacturer warranty statement based upon the condition of the factory testing.)
We take samples of the cables, bend it, straighten it, and then strip open the insulation and examine the condition, and found there was no problem. The cable pass the insulation resistance test, as stated before. And because we are doing a DC-hipot test, many have said DC test will not show failure caused by the excessive bending, even though it shows no physical problem through the peel test on the sample we took. The concern is once the cable is energized, the insulation will swell, and then shrink, and over time, where the cable is bent, problem will develop, and this is not someting visible through peel test before it was energized.
Becasue we bent the cable, the manufacturer recommends to cut the portion that was bent and terminate it with a splice to be safe. My question is, is there any other test, that can be reasonably performed to demonstrate this cable will be ok? Splicing seems a little drastic. What will be other's opinion on this?
We take samples of the cables, bend it, straighten it, and then strip open the insulation and examine the condition, and found there was no problem. The cable pass the insulation resistance test, as stated before. And because we are doing a DC-hipot test, many have said DC test will not show failure caused by the excessive bending, even though it shows no physical problem through the peel test on the sample we took. The concern is once the cable is energized, the insulation will swell, and then shrink, and over time, where the cable is bent, problem will develop, and this is not someting visible through peel test before it was energized.
Becasue we bent the cable, the manufacturer recommends to cut the portion that was bent and terminate it with a splice to be safe. My question is, is there any other test, that can be reasonably performed to demonstrate this cable will be ok? Splicing seems a little drastic. What will be other's opinion on this?






RE: Cable bending radius
Depending upon local standard and the presence of a shield the bending radius may be from 6 to 12 times the overall diameter.
There are manufacturers which require 15 times during installation and 10 times after then.
A modest requirement for a non-shielded triplex cable may 12 times core dia. and 6 times overall dia. [the bigger of them]
"Erikson nkt cable" for 11 kV [shielded cable] recommends 7 times the overall dia. that means 7*62=434 mm. See:
ht
If this triplex cable of 62 mm overall dia. is pulled with 600 mm bending radius I think it is not so bad.
If no sign of damage happened nor on the inner side of the cable neither on the outer side I could say that the cable is ok.
RE: Cable bending radius
I have had to fail installations based on the visual inspection and it tends to take quite a bit of discussion to explain why the damage is not electrically evident. Extensive bending radius damage will more than likely decrease the life expectancy of the installation.
RE: Cable bending radius
To get the "bite" throught thr vault lid requires 2xMBR.
So if indivual conductors is 30 MM and the MBR is 10x you are OK.
Check with the cable vendor, Some have MBRs for pulling (under tension ) and another for Training. the Training radius is for cable being bent whil not under tension.
RE: Cable bending radius
Tom
RE: Cable bending radius
RE: Cable bending radius
1. maximum pulling tension
2. minimum bending radius (as the multiplication of the overall diameter not individual conductors)
3. maximum sidewall pressure (combination of both)
The first two were discussed but the 3rd is also important as the cable can be squeezed on a bend so much that the conductor insulation thickness can be reduced or damaged. This would lead to the problem described by "Tscott8201".
RE: Cable bending radius
Premierpower is correct. You need to be concerned about your metallic shield tapes and the insulation layers. If the metallic tape cuts through the outer semiconducting shield, if a void is created between the insulation and the semiconducting layers, or if a void is created within the cable insulation, you will likely significantly shorten the life of your cable. The manufacturers understand this issue very well. This is the reason that their type test requires the cable to be put through mechanical bending and thermal stresses and then be subjected to a 50/60Hz elevated voltage partial discharge (PD) test. An elevated voltage PD test is the only test which can 'see' inside the cable, detect, locate, and characterize this type of defect. This is fresh on my mind as I just finished reviewing the latest draft of one of the ICEA cable manufacturing standards yesterday.
The only way to prove that your cable still meets the manufacturer's specifications is to repeat the factory 50/60Hz off-line PD test in the field. This industry best practice process would be an alternative to splicing your cable.
Please let me know if you need any additional support.
Cheers
Benjamin Lanz
Past Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants