Cables in trenches.......

[SparkyMark]

Im about to wire from and existing dist board to a remote location, about 70 metres away.

There will be an RCD to protect the cable, cable will be swa, partially clipped on the surface, but for the greater length, buried underground.

Two questions;

How deep should we dig the trench?
Does the cable need to be in a duct?

Thanks,

SparkyMark

 

[amberleaf]

Useful Junk . …………….. ( The Meaning of a Word ) ………… Seems the More I know the More I need to Find Out ????

● ( Doesn’t “Mechanically Protected “ in the Context of “ Earth Bonding “ mean simple [ That it’s Not Bare Wire ] “Wrong Wrong Wrong “

“ Completely Protected “Mechanically “ is a rather Vague phase as to Speak .

( Plain English )
Mean that the “ Earth “ Cable was/is hidden from Sight Possible - inside Stud Partitions’ / Floorboards / along it’s whole Length not Damage . ( “ Damage“ Attached to Skirting Boards ! or Whatever )

● Definition : “Mechanically Protected “
Is Sheathed within a Coving that does not Yield easily to Applied Pressure or Striking . Yeah .

● Definition : “Protected from Corrosion “
Is where the System is Not allowed to come into Contact with Dampness or Chemical Ingress that may Cause the Conductor(s) or its Insulation to Breakdown & Weaken it Properties of doing it’s Job Properly . Yeah .

 

[telectrix]

dig to a depth whereby the cable is not subject to damage by activities such as digging. ideally about 16". lay on sand and cover with sand. fill trench, burying a warning tape at around 12" deep. you can put in ducting if only to facilitate replacement.

 

[johnboy6083]

agree with above. its downm to what activitiy is above the ground. if its going to have a path/road above it then about 500mm deep is fine. in farmers fields, then you want at least a metre (like the one we dug up yesterday!). as tel said, remove all sharp stones ect, and then cover cable with sand. backfill and then about 300 lay tape. its no good having the tape on the cable, by then youve already damaged it. best practice to have ducts if possible, especially if its likely to have more cables there in the future.

 

[grantr37]

2 ft down, put 3 inches of sand in, lay flexy duct in trench (you can get narrow stuff from builders merchants) gravel up around the edges to just under the depth of the duct, 2 inches of sand on top, roll out yellow electric warning tape on top , 1 more inch gravel,then cover with topsoil...leave a length of cheap blue nylon rope in there from end to end for future changes...

 

[Paul.M]

Please correct me if I say something wrong.

You don't need the cable to be rcd protected due to the cable being swa but no harm will come of it except for walking 70m back to the c/u to reset the rcd. Doesn't need to be in conduit as its swa and is protected against mechanical and corrosion. Just trying to make life easier for ya.

 

[Engineer54]

Standard cable trenches are normally 600mm/60cm. Ducting is always a better choice but can add a good bit to cost of the installation.

If your do decide to direct bury your cable, don't try and pull the cable straight once it's in the trench. It may look better, but you should be looking for a slight snaking to the installed cable, which allows for some earth movement without strain on the cable.

Most of the others suggestions on sand bed depth and cable sand cover is fine, as is the other back filling depths etc. But make sure that sharp stones and the like have been removed from trench floor and back fill material, at least to a height above the cable of around 12''. It is advised that you back fill with the different materials in stages of no more than 30cm, then compact before adding the next 30cm.

If the area in which the trench has been run is subject to further excavations, it's normal to lay cable tiles/briquettes as additional protection, but if not, i'd say it's fine without them. Just use the warning tape as described here by others. If you have no means of compacting the the different fill levels, then water flooding will effect a good compaction medium. Finally before any glanding off, conduct a sheath test...

There is quite a bit more to direct burying a cable in a trench than first meets the eye. The worst thing you can do, is excavate a trench, lay the cable and back fill with the excavated material!! A sure fired way of ending up with at worst a pierced cable, and at least sheath damage...

Remember, the SW armouring is no protection against sharp stones and objects, they can and will go straight through the sheath, between the wire armouring and into the cables conductors...

 

[grantr37]

standard swa is not designed to be buried in wet ground with no mechanical protection, its not making life easier for anybody saying that it doesnt need ducting, its just like saying well I wouldnt do it because I cant be bothered, just dig a hole and chuck it in there...cause I think its ok thats what I would do.
standard small core SWA cable should really have ducting when buried outside, unless you want to push the boat out and buy industrial exterior sub surface grade armour cable at 25pounds a metre...and bury it a metre down to stop it being hit with a shovel..

The exterior cable has a professional moisture barrier built in and the Jacket plastic is about 4 times as thick as the covering on standard SWA cable...

Using normal SWA Cable without a duct will result in it being damaged quickly and failing either through movement in the ground/sharp edges on stones (ground moving or compacting due to walking or driving over surface etc) or breakdown of the plastic coating and armour due to poor soil drainage in the immediate area around the duct that could lead to it being permanently soaking wet...the armour (Earth) would rust away in a matter of months...

 

[grantr37]

on the last post where I typed DUCT in relation to the soil drainage, I meant to type CABLE

 

[Paul.M]

Thanks Grant for correcting me. I believed that normal swa would be fine. What conduit is recommend in this situation?

 

[grantr37]

standard drainage duct from any builders merchants, and they can deliver it, if the jobs for a farmer they will already have loads of the stuff, otherwise its not that expensive...
There is ducting out there with "Danger Electricity" moulded onto it in big letters, but that is a bit over the score and is very very expensive, so only suitable for the Electricity supply companies...

just normal drainage ducting, it provides good mechanical protection and takes away most of the moisture that would otherwise just sit there, it also allows for movement without the cable becoming perforated and has the benefit of easier replacement or additions in the future...

 

[Engineer54]

Grantr37,

I know exactly what type of cable you are referring to above, and it is as you say ''Industrial'' standard, and as such, utilised where additional cable sheath protection is required. Don't get confused here by saying that standard SWA cables can't or shouldn't be direct buried, They Can and Are every day, in every country throughout the world.

I have been involved with literary hundreds of thousands of metres of both direct buried and buried ducted cable installations. from MV to LV, ...from the smallest to 600 sq mm multi core SWA cable, and in all types of soil types. To the best of my knowledge, all those cables were standard grade XLPE/SWA/PVC....

If a cable trench is constructed correctly and the cable(s) pulled in and laid correctly you will rarely if ever have any problems with those cables over a good number of years. Now i am talking about major project installations here, where the contractor, or the designated electrical contractor will have all the necessary equipment in order to correctly install such cable installations such as calibrated electromechanical cable pulling devices, numerous cable pulling wheels/dolly's, and the supervisors and man power to get the job done in a satisfactory manner. A Sole Trader on the other-hand will in most cases not have any such equipment, but can still conduct a good installation if he takes his time and takes care in such a cable installation. His main problem is taking care not to damage the cable sheath, and to immediately repair any such damage prior to back filling.

Oh, and cable ducts are no protection from water ingress either, i've known of several ducted cable systems, along with the underground access rooms, being totally flooded for 3 months of the year, ...every year!!! One such room, had both MV and LV T joints located in there!!! Correct..NO, but for the 5 years or so that i was aware of this situation, there were no problems with any of the installed systems in that ducted installation either!!!

 

[Engineer54]

I suggest you check with the cable manufacturers, none will tell you that there SWA cable is unsuitable for direct buried installations. In fact ....they will assure you that they ARE!!

 

[grantr37]

yes and I have seen 125's split open when they were buried without a duct, they tried to fix with amalgamating tape, the inspection team came a long, failed it, 90 metres ripped out and chopped up with the stihl saw (petrol disk cutter) before being "spirited away" to the scrap man by the electrical manager.....cost thousands to replace it.
why do you think the electricity board are always looking for cable jointers to carry out repairs?

 

[Engineer54]

yes and I have seen 125's split open when they were buried without a duct, they tried to fix with amalgamating tape, the inspection team came a long, failed it, 90 metres ripped out and chopped up with the stihl saw (petrol disk cutter) before being "spirited away" to the scrap man by the electrical manager.....cost thousands to replace it.
why do you think the electricity board are always looking for cable jointers to carry out repairs?

Never seen a buried cable split wide open, from natural causes... Can only surmise that the cable was installed with damaged sheath or installed tightly dead straight!!!

I would never condemn a buried cable of sizable length, i would always repair with either a single joint (where possible) or with a double joint (new/replacement cable section between the 2 joints). Obviously conducting a sheath test on the two jointing ends before jointing commenced. Only if these tests failed would i have the cable replaced.

Do you mean 120 sq mm or 1250 sq mm cables Grant?? If the latter, i would have originally installed smaller paralleled cables, much cheaper, and probably better current carrying capabilities. In fact i think i can say, i've never seen a single cable of that size over that sort of length....