burying high voltage cable

[Knobhead]

Have a look at section 14 http://www.westernpower.com.au/documents/ldd/UCIManual.pdf

 

[Knobhead]

Really how long ago was that ?? So where is this sort of training carried out now Tony??

Early 2011 as far as I know. Local DNO jointers are having to travel to Scotland now. Was talking to one of them a few weeks ago.

 

[mechelec]

The DNO I work for (recently taken over) has specified HV (or MV) cables to be buried direct i.e. no ducting at the same depth as LV that is 450mm in footpaths. I would suggest this varies with DNO's and if on a private network would be down to risk assesment. If it was going under a reinforced comcrete carpark 350mm may be deemed deep enough to avoid danger if it was a private network and a risk assesment would be undertaken before any excavation. On the other hand running it down a grass verge I would expect it to be deeper to avoid danger.

 

[Dan the spark]

That sounds a bit shallow to me, when I used to work for my old company we pulled our HV/MV cables into trenches at 1m deep, wouldn't fancy leaving an 11kv cable buried at 450mm that a ground workers gonna come and stick a shovel through. I may be wrong but this is the depth we always worked to

 

[mechelec]

That sounds a bit shallow to me, when I used to work for my old company we pulled our HV/MV cables into trenches at 1m deep, wouldn't fancy leaving an 11kv cable buried at 450mm that a ground workers gonna come and stick a shovel through. I may be wrong but this is the depth we always worked to

It used to be 650 for HV (MV) and 450 for LV it is now 450 for all up to 11KV
It doesnt seem to matter how deep they are buried they still get damaged anyway

 

[Engineer54]

The direct burial depth as well as the ducted burial depth on this project as well as on my previous projects is 60cm to 65cm for both LV and MV for cable distribution. Just before Xmas i had a site meeting with the China power company to discuss our requirements, points of entry and cable routes for there supply cables within our project site. They wanted there cables at a depth of 100cm if direct buried or 70cm if ducted. After some discussion they agreed with our project specifications of 60cm in ducted raceways.

As for trench make up, all our trenches, all have the bases compacted flat and at least a 10cm layer/bed of soft sand on which the duct or cable sits. a further layer of soft sand to around 30cm which is then water compacted. The cable tiles are laid at this level, followed by the first 10cm layer of sifted excavated backfill when the warning tape is then laid. Then the final back-fill, to fill the trench to FGL which is also thoroughly compacted by a foot thumper and topped up as as required!! On our projects major multiple runs of LV cable and all MV cable routes are augmented by surface ground block route markers to show route and changes in direction etc.

One of the main considerations when bringing ducts and cables out of there trenches to an above ground raceway or points of connection, is the slow transition radius required. Bending stress on an MV cable is something you need to avoid as much as possible!!! Generally all our main MV and LV switchboard rooms will have grated open cable gullies around 1.25m deep X 60cm that switchboard connection points will sit over!! This gives a good access, for cable termination procedures etc....

 

[Knobhead]

Over the years I’ve seen them at all kinds of depths, but I’ve always worked on private networks. The normal would be 600/750mm, with 100mm sand above and below. We always used cable tiles. Two of the places I worked used to have their own brickworks so the tiles would even be personalised for the company.

At times it went from the sublime to the ridiculous.
In quarrying what sets off as ground level can soon change, large earthmovers can shave rock as the bucket caches as it drives along. One occasion I was walking along and spotted what looked like a strip of hessian, a kick around in the muck and there is the spiral of armouring of a cable. The cable had originally been buried in a 600mm trench through solid stone. It caused a bit of a panic when I sauntered in to the managers office.
The other extreme was trying to trace an old cable. The CAT signal kept vanishing at the same points, I could find the cable either side of this huge hump in the ground. The cable must have been 10M down at the centre, so we had about 500M of cable we had a rough idea where it was but not really certain.

 

[Engineer54]

Over the years I’ve seen them at all kinds of depths, but I’ve always worked on private networks. The normal would be 600/750mm, with 100mm sand above and below. We always used cable tiles. Two of the places I worked used to have their own brickworks so the tiles would even be personalised for the company.

At times it went from the sublime to the ridiculous.
In quarrying what sets off as ground level can soon change, large earthmovers can shave rock as the bucket caches as it drives along. One occasion I was walking along and spotted what looked like a strip of hessian, a kick around in the muck and there is the spiral of armouring of a cable. The cable had originally been buried in a 600mm trench through solid stone. It caused a bit of a panic when I sauntered in to the managers office.
The other extreme was trying to trace an old cable. The CAT signal kept vanishing at the same points, I could find the cable either side of this huge hump in the ground. The cable must have been 10M down at the centre, so we had about 500M of cable we had a rough idea where it was but not really certain.

Oooops!! should have read what i had posted, before posting!!! I have totally forgot to mention the all important cable tiles!!!!!....lol!! Which should have gone on top of the sand! the cable marking tape, goes on the first layer of back-fill, which i also didn't mention.... Thanks for reminding me Tony...lol

Start again then ...lol!!


Compacted trench floor.
10cm soft sand bed.
Cable laying.
20cm soft sand (compacted)
cable tile placement
10/15cm sifted back-fill
Cable warning tape
Final sifted back- fill (compacted)
Top-up, back-fill as required...

What i also neglected to say, direct buried cables should not be laid in the trench dead straight, they should always have a light snaking arrangement, to accommodate any minor ground movement etc.

EDIT....OK i've now amended my original text in post #36 on trench make-up, to avoid any confusion....

 

[Knobhead]

What i also neglected to say, direct buried cables should not be laid in the trench dead straight, they should always have a light snaking arrangement, to accommodate any minor ground movement etc.

And that’s one I’d forgotten!

Not much of a problem in the quarries but when I moved in to steel it was. The sites were riddled with abandoned coalmines.
It was such a problem around the coalfields that BICC made 11KV expansion joints. I’ve never seen one only read about them.

 

[gizmo]

Sounds more like another six week wonder to me !

Call me stupid, but why would your boss want YOU to find out about the rules and regs regarding MV cabling systems ? Oh yes, thats it, he thinks he can play with electricity, without the having to pay for the experience, knowledge or appropriate training !

If as you say, your boss is asking you to find out about distances etc, then tell him you can find nothing. You or your boss start fiddling with those cables while they're still live, and something does go wrong, then, to put it bluntly, you're toast !

 

[tig]

hi guys thanks for all your help, but just to say i am an apprentice at
a hospital (though not a lad), it is not my boss who is putting the cable in but a private firm, he was just asking me to find out because that is just the kind of tricky questions that employers like to test their lowly apprentices with

 

[Engineer54]

hi guys thanks for all your help, but just to say i am an apprentice at
a hospital (though not a lad), it is not my boss who is putting the cable in but a private firm, he was just asking me to find out because that is just the kind of tricky questions that employers like to test their lowly apprentices with

Ah!!...... all is now revealed ...lol!!

A little tip for you, that you may find useful. When the cable or cables are being terminated, ask the guy's if you can have the off cuts of the final heat-shrink insulation tube, from the termination kit. Use it on hammer handles, or on any other tools that will benefit. You'll be amazed at the balance and overall feel improvement in a wooden hammer handle!! lol!!

Depending on the final shrunken size of the tube, this stuff has an absolute multitude of uses you can apply it too...

 

[mechelec]

Ah!!...... all is now revealed ...lol!!

A little tip for you, that you may find useful. When the cable or cables are being terminated, ask the guy's if you can have the off cuts of the final heat-shrink insulation tube, from the termination kit. Use it on hammer handles, or on any other tools that will benefit. You'll be amazed at the balance and overall feel improvement in a wooden hammer handle!! lol!!

Depending on the final shrunken size of the tube, this stuff has an absolute multitude of uses you can apply it too...

Wooden handled hammers? how old did you say you was lol.
I used to have a club hammer that I reshafted after I broke the original wooden handle, I must have done a bad job fitting it because whenever you hit anything it would twist in your hand. It nearly broke your wrist to use it. Fitted another and had it for a good few years until it dropped down a cavity wall.

 

[Engineer54]

Wooden handled hammers? how old did you say you was lol.
I used to have a club hammer that I reshafted after I broke the original wooden handle, I must have done a bad job fitting it because whenever you hit anything it would twist in your hand. It nearly broke your wrist to use it. Fitted another and had it for a good few years until it dropped down a cavity wall.

Haha.... you would have had a much better grip on your club hammer, had you sleeved it with this heat shrink MV insulation tube!! lol!! Can't see a problem doing the same with a metal hammer to be honest .... it'll also then be insulated to 15000 volts minimum too!!!

 

[The Ranger]

You'll be lucky, it's shut down!

Early 2011 as far as I know. Local DNO jointers are having to travel to Scotland now. Was talking to one of them a few weeks ago.

I was there 8 weeks ago! was defiantly open then and defiantly open in the 2nd week of Dec as 2 lads i work with were there!