Has anyone experienced Medium volat

[gordonl]

Has anyone experienced Medium volatge cable failure due to excessive Sidewall Bearing Pressure during pulling?

Other comments on max Sidewall Bearing Pressure would be appreciated.

 

[GordS]

The Sidewall bearing Pressure (SWBP), in lbs/ft, is sqrt(Tc/R)^2 + W^2) where Tc is the tension in lbs at pulling end of curved section, W is cable weight in lbs/ft, R is raidus of bend in ft. This applies to one condcutor. If you have a several single conductors, you need to determine an occupancy factor to accomodate for additional frictional forces between conductors. IEEE 576 gives a graph to aid in determining the occupancy factor. Knowing the occupancy factor, you can calculate SWBP using equations also shown in the same standard. IEEE 586 ststes maximum SWBP is 300 lbs/ft where R is radius of curve in ft; I have a Phillips catalogue which recommends maximum SWBP of 250. Sounds like the allowable value is manufacturer specific.

 

[gordonl]

I was looking in a Philips catalog which suggested the same. I have single conductor 750MCM with two foot radius bends so I seem to be limited to 500lbs of pulling tension. The limit for conductor size is around 6000lbs, but I take it as I'm going through a bend the tension on the tugger when the cable hits that bend can't be more than 500lbs?

I hate to admit it, but we pulled a cable two years back where we maxed at 6000lbs with many bends through out, and the cable has been happily energized since. (34.5kV)

Am I misinterpreting the restraint imposed by the sidewall bearing pressure, or was I just lucky with this other pull that it hasn't failed.

All of the bends are 2 foot radius pulleys, and the installation is in convoluted tray through an existing building.

 

[BJC]

Gordonl, it was clean living that kept your cable from going south on you.
Most cable failure occur because of damaged done during installation. Sidewall pressure is a big factor and mechanical dammage during handling ( like getting stepped on kinked or run over ) is another common cause.
I think manufactures are setting the mazimum side wall pressure lower and lower. Ten or so years ago figures like 900 lbs were common and now 300 is the norm. It varies with insulation and shied type. XLP with a wire shield you is almost idiot proof. Some rubber compound with a tape shield are more delicate.
Southwire has the calculation and formulas on there website. If your there get a copy of their cable manual ( useful and free ) Okonite has good technical data on theri site and a "Cable Installation Manual" that you can get from your local rep.
I think I've said it on this site befor that the Polywater "Pull Planner" program is very good and about $100 ( maby free if you buy a lot of lubricant from them). If you have a critical pull it's worth buying. It's also worth buying if you work for an owner who want's documentation that you know what your doing.

What are the parameters of your 35 KV cable - tape or wire shielded? A 2' bend radius is close to the minimum static bend for a 500 or 750 mcm cable with a spiraled tape shield.

 

[gordonl]

I'm using 15kV, 100% XLPE, wire shield, 750MCM copper cable. From the manufacturer's data I've read at Southwire and from Philips they didn't seem to differentiate inbetween cable constructions for maximum sidewall bearing pressures, do you have some documentation or a reference on this I can use?

Thank You,
Gord

 

[BJC]

gordonl at the moment I can't find any thing about differences on cable types vs sidewall pressure. So maby I dreamed it up. I'll keep looking.
I am sure XLP did have a higher sidewall pressure and I believed they reduced the allowabel because of all the trouble they were having with premature failure of URD cable because of "treeing".
The actual configuration of cable - triplexed vs cradeled has a lot to do with the actual sidewall pressure on any particular conductor. The usual methods of calculating this consider the worse case so there is plenty of slop in them. The Okonite Installation Manual and the Soutwire Power Cable Manual have good write ups on this. I cannot find the complete Okonite or Southwire ( southwire you can order on line) manuals on line so you probably have to go to your local rep to get them.
This is a good link from Polywater on the subject.

http://www.polywater.com/techtak1.html

 

[GordS]

gordonl-I don't think the tension on the tugger is to be limited to 500 lbs in the example given - it is the tension going out of the bend. The tension on the tugger is the sum of the tensions required for the entire pull. The tension coming going out of a bend is related to only the forces to pull everything behind the bend.

 

[gordonl]

I was thinking of what the tugger registered as the cable exited (as properly corrected) the pulley, this would be as the pulling eye exits the pulley.