MV cable buried uphill 1

[AusLee]

Hi,

I have a project where one of the options of directly laying an armoured XLPE MV cable (3x95) is at 3 feet depth in an uphill terrain with 60 degrees angle.

I know that the terrain should be compacted underneath but i imagined the cable in a concrete trench and all this weight pushing down and causing strain. When it's direclty burried the resistance forces with the ground would be much higher and the starin smaller. I thought of installing a few cable ladders fixed to the soil and tying the cable once every 300 feet or so, but my question is: what can you call this? It's not horizontal and it's not vertical. Can you please give me some recommendations and/or point me to some references ?

 

[RalphChristie]

Contact the manufacturer of the cable, they can give some recommendations.

In my opinion:
You will have the least strain on the cable when it is burried. But it can be a problem to locate faults (if any) after a few years.

I don't think it is a good idea just to lay the cable in a trench without any support every few metres, (without consulting the manufacturer) due to the very steep angle.

I would go for a cable ladder (if possible), and strap the cable every few metres. Just remember to left the cable slightly "snaked" so that any expansions or contractions can be accomodated.

How long is the cable?


Regards

Ralph

 

[BJC]

I think you have a good ideal with the ladders. I think the cable will eventually work it's way down hill if you didn't anchor it in some way.
Cyclically loading, the changes in soil moisture and temperature will over time put a strain on the cable and it will try to creep down hill.
I have never seen a cable burried on a steep sloap, we have allways installed them in conduit and used cable supports in vaults.

 

[busbar]

My guess would be that asking a cable-tray producer if his product is intended for direct burial may push him 'over the edge.'

 

[BJC]

busbar
I think he's in Europe or down under. I have seen Europeans burry "shopping cart" tray directly in earth. I think it was made for that purpose.
If I were doing it without a duct bank I would use Kellums grips and drive some rebar stakes ever 40 meters.

http://www.hubbellcatalog.com/wiring/catalogpages/page_T35.pdf

I think you can get them in stainless, but bronze should last until AusLee retires.

 

[RalphChristie]

Just another thought:

How are you going to pull the cable in? Winch? Manpower?

I've been reading through a booklet of Aberdare cables, (a South African manufacturer,

http://www.aberdare.co.za)

regarding mechanical forces on cables during installation:

Any cable has a max pulling force which should not be exceeded during installation. The cable construction imposes the limitation on the pulling-in force. when a cable stocking is used the max force can be related to overall cable dia in mm as follows:

Armoured cables F = 0.94d^4 x 10^-6 kN
Unarmoured lead sheath cable F = 0.39d^4 x 10^-6 kN

Attemps should be made to limit the pulling force required to a minimum to avoid stretching the outer layers of the cable.

An increase in the pulling force is permissable when the cable is layed by means of a pulling eye attached to the conductors. As a rule of thumb, the following forces may be applied to a conductor:

copper 4.9 x 10^-2 kN/mm2
aluminium 2.94 x 10^-2 kN/mm2

Then, for example, the max force that should be applied via a pulling eye to a 95mm2 3 core cable is:

95 x 3 x 4.9 x 10^-2 = 13.965kN

Generally when cables are installed using well oiled rollers and jacks the following forces can be expected:

straight route 15-20% of cable weight
2-90° bends 20-40% of cable weight


Regards

Ralph

 

[busbar]

BJC — Kellems grips (and stakes) look sound a good solution, particularly using the ‘lace-up’ type.

I have installed a lot of cable tray in a research environment, and just couldn’t quite imagine the stuff in a trench covered by dirt.

 

[jghrist]

One problem with the cable ladder solution is that there will probably be air voids after backfilling. This may reduce ampacity by limiting heat flow from the cable. You also create pressure points on the insulation because the weight of the soil plus the cable is much higher than the weight of the cable alone (the normal situation when using tray). You like to have a nice smooth bedding for the cable.

I like the Kellums grip solution. If you install them in spacings short enough to provide some slack before backfilling, there should be no problem after backfilling because the backfill will keep the cable in place.

 

[Shavner]

What bout providing a messenger cable burried alon with the cables and securely anchored at the top of the slope and allowing some snaking in the power cables. I belive that the required depth of installing the MV cables IS going to place a lot of strain on them from not only the slope of the run but also the down dforce of the cover earth. Also wht is going to keep the cover on the cables during heavy rains or typical erosion over time. possibly consider using a greatly oversized conduit (like a drainage tile) and anchor it at both ends to keep from shifting. What is the overall length of run up the incline? is a vault at both ends out of the question?

 

[AusLee]

Hi, thanks for the ideas.

I'm well below Europe - my first year in Angola and haven't seen the rains yet but they say it's heavy showers. I thought compacting the ground above and below can solve the issue but you are right: time is a very important factor.

The hill measure with the wheel is 132 meters. Actually the original project was to go by the side of the road - it's an armoured cable so no problem - but that hill can bring the total length from 1700m to 1200. On drum less. I'll see what Aberdare have - we get quite a lot of equipment from Zambia and South Africa. But what i really want to know is isn't there any code - NEC, EN, IEC or BS IEE wiring regulations? the cost of the code section will cover either the saving or the responsibility of either project option. Thank you for pointing out any relevant International code about this and i'll post again if i find something new. Thanks again.

 

[GOTWW]

BJC, I have been told that stainless steel corrodes underground, the stainless apparently needs be in air to maintain its corrosion resistance. Or at least that is what they say about stainless steel ground rods versus copper clad steel types. But, for some reason stainless steel ground rods are specified for substaions??

 

[BJC]

Stainless steel is subject to stress corrosions. Copper or copper clad ground rods will corrode evenly over the whole surface. A stainless steel ground rod may corrode as to form a single large void possibly completely through the rod. I didn't know anyone specified stainless steel ground rods for substations. I would not unless I has a corrosion engineer tell me it was ok in a particular location.
There has been a lot written about the problems with stainless steel ground rods.
For the underground that is the subject of this thread I would consult a corrosion or soils specialists.

 

[mc5w]

Stainless steel that is in an anaerobic environment will corrode like HE double toothpicks due to internal electrolysis. Aluminum will do the same thing. Burial tends to remove oxygen which also removes the protective oxide layer. Corrosion of threads where a stainless steel bolts goes into a nut or tapping is also a problem in a food plant where there is plenty of oxygen.

Have you thought about directional boring instead of trenching? You could put conduit inside of the bore and then strap your cable to a messenger cable and pull in going downhill. If you need periodic handholes for messenger cable support provide them. You will need to provide slack cable at the bottom and at the handholes to compensate for thermal expansion and contraction.

Make sure that you use a good grade of steel for your messenger and rebar. Low impurity steel rusts more slowly than aluminum. I can show you an abandoded 104 year old street car rail that has hardly any rust even though it was abandoned 74 years ago and has had road salt poured on it since.

A possible alternative is to drive a tunnel horizontally and then connect with a vertical shaft. You will need forced cooling such as forced air, refrigerated water in adjacent plastic tubing. If you build your tunnel right you might be able to use gravity circulation of air. For air cooling your tunnel and shaft would have to be big enough to periodically clean out insect and other animal nests. If you have a landslide potential the maintenance for a tunnel and shaft would be trivial in comparison.

Mike Cole, mc5w@earthlink.net

 

[AusLee]

Hello again,

Just to report the manufacturer's recommendations. I will use what we discussed here though. Thanks!

"We do not have an installation procedure for installing XLPE cables, etc. However, there should be no problem with your installation. MV cables have a standard burial depth of 800mm. MV XLPE cables are installed vertically in mine shafts around SA, and wedges are used to support these cables. From your installation, a 60% slope will pose no problems at all. I would suggest that you keep with the direct burial over the 132m.

One point to keep in mind would be that the ratings for the cable be based on the worst case scenario, i.e. installed in a duct in concrete. This will have the worst thermal effect on the cable."