MV Cable Kinking Experience - Multiple Bends in a Short Distance 1

[JezNZ]

Hi All,

Recently on an MV design-build project I had an unscheduled design review with the cable installing team after change of personnel.

For all our MV cable installs we calculate the minimum bend radii, maximum pulling tension, sidewall pressure, expected tensions and pressures etc. to ensure that the install will be to the manufacturer's requirements and best practice.

In a particular cable run (3C300AL PVC/MDPE Cu Wire Screen), the cable install team lead gave the feedback that they had concerns over multiple bends in a short section (approx 6 90 degree bends over 30 meters) that could lead to insulation kinking and premature failure, this being based upon collective years of install experience, despite compliance with the manufacturer's installation requirements.

Being unplanned, but not one to ignore practical experience, this feedback forced a post construction design change which was undesirable. So my question is, have people experienced this before, and is there any engineering method of calculation to account for this next time we perform such a design, or general rules of thumb to adhere to?

Thanks

 

[Palletjack]

I’m going to assume this isn’t an NEC installation.
More than 360 degrees of bends in any single pull is asking for trouble. I will be the first to admit, that rule is a poor substitute for calculations, but some manufacturers like to include it in their recommendations.

 

[JezNZ]

Hi Palletjack,

No, not NEC world, although if there are stipulations in the NEC I would love to see them, after all, I don't believe we go about making rules for no or silly reasons!

Interesting note on the maximum of 360 degrees of bends, I've personally never seen this in a manufacturer's recommendation before, do you have a reference, it would be interesting to query the manufacturer about their experience I suppose.

For reference, we are not planning to pull the cable through all the bends, as this particular section of the run is in cable ladder, most of it can be pulled to destination relatively straight and the formed into place later, however the installers' concerns still remained.

 

[Palletjack]

https://www.southwire.com/medias/sy...241897502/MC-Installation-Guide-Southwire.pdf

Page 2

 

[davidbeach]

The NEC says no more than 360 degrees of bend. The NESC says don't damage the cable. There are single 90 degree bends that pass the NEC test and fail the NESC test. I've also done pull calcs that showed 720 degrees of bend would have been a piece of cake; but in that case the contractor couldn't get past the NEC prohibition (not the governing code on the project) and installed an extra vault in the run.

Mentioned that one, with about that much information, here years ago and some smart arse know-it all engineer wannabe tried to ream me a new one. That 720 degrees would be impossible to pull and that I just didn't know what I was doing. Bah. There wasn't an elbow or other curved fitting in the run, just straight PVC pipe under a mountain road with a minimum bend radius of around 200ft. Road was on about a 4% grade and quite curvy. The pull would have been essentially impossible pulling uphill. The pull downhill, though, required using a reel carrier with brakes to ensure that the cable didn't try to run away down the pipe. It was also one conductor per conduit so no worries about jams. And that was when I limited the pulls to 2500 ft since there was an assurance that the cable warehouse could supply replacements in that length. There were places that a 10,000 foot run could have been pulled around 1440 degrees or more with no real problem, as long as it was pulled downhill.

The point is, calc matter much more than some flat prohibition against exceeding 360 degrees. But the NEC likes flat rules with minimum calculations while the NESC puts much more confidence in the Engineer doing the design.



I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[bacon4life]

For some applications my employer even has limits of no more than 270 degrees of bend. I certainly would not do more than 360 degrees without really talking it through with an experienced crew.

When utilizing an elevation difference, be positive that the cable will be pulled downhill. Sometimes crews can find it easier to feed cables into a vault and up the pole vs pulling the cables down the riser. Site constraints may also limit where you can park the huge cable trailer versus the much smaller cable puller. For special cases like David mentioned, utilizing the elevation difference makes a lot of sense.

Also keep in mind whether the cable will ever need to be removed and then conduit reused. A clean cable with lots of lube is going to pull into a new conduit much easier than an ancient cable will pull back out of a old conduit packed with decades of accumulated grime.

 

[JezNZ]

In this case, this cable will be pulled through a mostly straight run of underground ground conduit it one direction, then a remaining length unwound from the drum and pulled in the other direction through above ground cable ladder (a relatively torturous path) to the existing substation approximately 50m away. It's a classic, congested industrial site, but as mentioned in the OP, all of the pull calculations checked out.