Cable bending radius

[a10jp]

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?

 

[7anoter4]

I agree with people sustaining that the bending radius shall not be less than minimum recommended.
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:

http://www.encv.co.uk/Products/~/media/encv/Files/triplex_cable m al lrt_11_kv 1 pdf.ashx

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.

 

[premierpower]

It has been my experience that damage from exceeding the minimum bending radius is usually not evident from the electrical acceptance tests. However, it will be physically evident in mechanical damage done to the cable. The cable shield, especially if it is copper tape, is more vulnerable to bending radius damage than either the insulation material or the cable outer jacket. The amount of pulling stress and pulling rigging that is involved in exceeding the bending radius is going to have a drastic impact on the type and extent of damage. For instance, if the minimum radius is compromised midway in a cable pull and the pulling tension is more than couple of thousand pounds, the copper tape shield is going to be damaged. The damage can be identified by carefully running you hands along the cable feeling for lumping of the shield. To determine the extent of the damage, you would have to cut an inspection window in the outer jacket. The worst case would be that the stress was adequate enough to actually break the copper tape. If the bending radius was compromised without much tension, it is unlikely that the shield will be damaged. For example, if the bending radius was compromised at the leading end of the pull. The shield will move, but it will work itself back into the proper position.
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.

 

[BJC]

The dismeter that maters is the daimeter of a single conductor- not the diameter of the bundle.

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.

 

[Tscott8201]

I only work with high voltage cable (i.e. 7.2kv to 14.4 kv), so this is all I can speak to. I know our main concern with our cables during pulling is the tension from the pull and the minimum bending radius. Both of these concerns manifest in similar damage to the cable. We see the insulation becoming unbonded from the conductor, and over time this allows moisture into the open void created and will eventuality lead to cable failure. The problem is that initial integrity tests will seldom identify a problem and you usually do not know there is a problem until after the cable fails. This is probably not as great a problem on lower voltage cables.

Tom

 

[a10jp]

HI BLJ, actually I found out that for triplex cables, the mfr listed a separate MBR for the bundle instead of the single conductor is that becasue the cable is already twisted, you cannot bend the cable any more than as if it is the normal single conductor because of the kink. Initial examination show no signed of separation of the copper tape shielding, neither was the sample that we test bent and cut open the sheath for examination. Regardless, I have now learned we should not have bent the cable, what was done has already been done. It is as it is.

 

[cgrodzinski]

I do not have too much experience with pulling low voltage cables but there are three conditions that should be looked after during any pull.
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".

 

[benlanz]

a10jp, You are absolutely correct. An insulation resistance test will not detect even extreme bending radius issues. In order for an insulation resistance test to detect a problem you basically need to cut all the way through the insulation (an insulation failure). This is one of the reasons IEEE no longer recommends a Type 1 DC test for an acceptance test. One of my utility clients just did some experiments and demonstrated that even a dime thickness of XLPE insulation left on the cable resulted in a 'good' DC test! In fact some claim 120V extension cord will pass a typical medium voltage DC HIPOT!

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