Advice on trench depth for SWA - Answered!

522.8.10 in effect requires buried cable that is not in a duct of adequate protection to be at a sufficient depth to avoid being damaged by any reasonably foreseeable disturbance of the ground and the location marked by suitable marker tape.

Wrapping the marker tape round on on top of the cable isn't good!

Answers that might be suggested will probably be 450 mm, 600 mm or 1000 mm.

Regards

BOD

Or very shallow indeed but under concrete slabs or inside something like a piece of scaff tube.

Avoids the risk of disturbance another way - by bending the thing coming towards  it  ;-)  The actual depth is not as important as how the ground is used - so in the opposite case if you know marquees will be pegged out, or it will be ploughed, then a lot deeper than the usual 600mm for no special measures applies.

If the path is unlikely to be dug up in the next decade or 2 it is less of a concern than say a flower bed.

Thanks for the advice. I could indeed pave over the trench but I also like the idea of scaffold tube. If I went that route could the cable depth be really shallow?

Mike

Well, if you're going to slab over it I can't see why you couldn't just lay the scaffold tube pretty much on the surface. Wrap it in pretty yellow warning plastic so it's obvious what it is. 

Otherwise I would suggest that it needs to be buried below the depth of the first scoop of a digger which is unlikely to be more than 300mm with your warning tape at 150mm.

Grumpy: 
 

Well, if you're going to slab over it I can't see why you couldn't just lay the scaffold tube pretty much on the surface. Wrap it in pretty yellow warning plastic so it's obvious what it is. 

Otherwise I would suggest that it needs to be buried below the depth of the first scoop of a digger which is unlikely to be more than 300mm with your warning tape at 150mm.

Perfect, thank you!

Mike

Would a glassfibre tube be acceptable as a conduit? It's lighter than a scaffold pole, very strong and doesn't rust.

Mike

Would a glassfibre tube be acceptable as a conduit? It's lighter than a scaffold pole, very strong and doesn't rust.

If you can't puncture it with a garden fork heavily stomped on, it might suffice.

I for one am not keen on the likes of scaffold tube, not because of it's mechanical strength but that should there be any relative movement at the end, it could well use it's mechanical strength to shear or certainly damage the cable.

Regards

BOD

It is very much a case of horses for courses, and it rather depends who is most likely to dig it up and with what - if that is just you, well you know it is there, as opposed to a cable in a more public place, and a back garden is more likely to see a garden fork than a JCB - another site like a theme park may not be so lucky. 

 There will be a great many wires in and around the radio shack and the user is likely to be more alert to such things than the average punter. 

I have had great success with scaff  pole method in the past, though I do tend to stuff the ends so the cable is held off the tube lip by being wrapped in  rag during back-fill but I agree it is not in any sense of the word flexible, or forgiving of errors of alignment , but that rigidity is rather the point, in terms of cable damage it has much in common with say earthenware pipe, and there is plenty of cable in that underground as well. It only matters if  the cable gets pulled against it, but in situations where that is credible, then it is the wrong approach.

Mike

Thanks Mike. I'm not planning to move house for many years, and will obviously create a cable plan for the next owner, but ultimately it will be down to the person who certifies the work to be satisfied it is safe for the uninitiated. I'm planning to run the SWA in a straight line between junction boxes on the walls of the house and garden room. These will only be 5 metres apart and presumably carry warning stickers so it should be “obvious”, but you never know! Paving slabs over the top sound like the easiest and best solution.

 Guidance Note 1 Section 5.7 [7th Ed (17th+A3, 2015, pages 83-85) or 8th Ed (18th, 2018, page 85-86)] recommends generally absolute minimum of 0.5 m to meet the “sufficient depth" requirement of Reg 522.8.10.

The reasoning here, is that at a depth of greater than 450 mm, the effects of ground freezing in the UK are less, putting less mechanical strain on the cable over time, plus as others have said, it's below spade, fork or “first scoop” depth.

There are, just to note, some specific Part 7 requirements or recommendations for depth of cables:

• Agricultural and horticultural premises, requires at least 0.6 m with mechanical protection, but cultivated ground at least 1 m (Reg 702.522)
• Caravan parks,  Reg 708.521.7.2 Note 1 recommends at least 0.6 m
• Marinas Reg 709.521.1.7 Note recommends at least 0.5 m
• Electrical shore connections for Inland navigation vessels, Reg 730.521.101.3.2, Note 1 recommends at least 0.6 m

For footways and carriageways of highways (including bridleways, but perhaps I'd also be inclined to use for other public-access areas such as public footpaths) NJUG publication Guidelines on the Positioning and Colour Coding of Underground Utilities’ Apparatus  Vol 1 for covers not only depth, but also the colour of the sheath/duct of cables. It may also be sensible to use these also for driveways, access routes, etc. LV is a minimum of 450 mm beneath a footway or verge, and 600 mm beneath a carriageway, and telecoms 250-350 beneath a footway/verge, and 450-600 mm beneath a carriageway – but minimum depths for street lighting and street furniture may differ.

Also worth noting that most armoured cables are not suitable for long-term or permanent submersion. So, a submersible cable would be required if you are laying below the water table. The soil type you lay armoured cable in should consider drainage etc. For example, if laying in soil with high clay content, a suitable method is to lay the cable on 100 mm sand over over a 150 mm to 300 mm mix of sand and aggregate (such as quarter or half down), and cover with sand to 100 mm above cable before back-filling - give or take use of tiles and/or, as has been mentioned, cable marker tape at a suitable depth so the tape is exposed before the cable is struck.