SWA going into a trench

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springcrocus

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I will be taking a run of 6mm 3-core SWA down an outside wall and into a trench to supply a summer house at the bottom of the garden.

I am thinking of protecting the cable at the point it enters the ground (and at the other end, on exit) by placing the cable in a hockey-stick shaped piece of gas barrel or some other kind of tube. The opening of the tube would be sealed with a good dollop of sealant and/or a SWA boot. :)

Reasoning behind this is to protect the cable from lawnmower wheels etc. :_|

My questions are:

Would you bother or am I being too fussy?

If you would, what sort of pipe would you use and how high above ground level would you set the top of the pipe?

Note for Steps: The supply is TNS and I will be taking an earth connection from the house, connecting the armour at the house end but not at the shed end. Wondering about banging a rod in at the other end as well. ]:)

 
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Note for Steps: The supply is TNS and I will be taking an earth connection from the house, connecting the armour at the house end but not at the shed end. Wondering about banging a rod in at the other end as well. ]:)
Applaud Smiley

Don't wonder about too much - you might fall in your trench ;)

 

steptoe

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TBH I wouldnt worry too much about a sleeve/pipe, although its possibly a vvery good idea if you're concerned about lawnmowers etc.

its fine to export the earth on a TNS system :D :D :D

ps, if you do export the earth, then do NOT put a rod in as well.!

 

springcrocus

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Many thanks for your input, Steps B-). Have one of KME's Guiness Drink

I have followed the discussions corncerning the exporting of the earth in TNC-S systems, but don't really understand why you should not add a rod to a TNS system.

Is it because if the earth clamp on the cable sheath comes adrift, your new earth rod becomes the main earth and the cable would, therefore, need to be in 16mm ? ?:|

 

steptoe

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Many thanks for your input, Steps B-). Have one of KME's Guiness DrinkI have followed the discussions corncerning the exporting of the earth in TNC-S systems, but don't really understand why you should not add a rod to a TNS system.

Is it because if the earth clamp on the cable sheath comes adrift, your new earth rod becomes the main earth and the cable would, therefore, need to be in 16mm ? ?:|
well at least then you would have an earth, ]:)

it shouldnt be a clamp on the sheath, should really be a sweated joint IMO.

its to do with parralel earth paths, and really I would suppose there would be no good reason for you not to also spike the shed, (on considering it again).

its just not something I would do, sometimes you can get diy bob doing some silly things with earth spikes so I tend to avoid recommending anyone uses them unless on a TT system, (or on PME outside the zone).

I have saw water mains earthed via a rod on PME systems many times, :eek: :O

I have come to the conclusion that a few of these heating companies doing grant work are doing this to avoid running bonds through the whole house.

my my the occupiers just love it when I explain the dangers to them!!! :eek:

to shorten, yes, if you feel the need to, then I can see no really good reason not to spike the shed as well as using your TNS system.

see, I can admit to having a change of mind and not always being right. ;) :^O :^O

 
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I usually fit a 1mtr length of galvo conduit if I think the SWA may get damaged.

What on earth is in that shed that needs a cable big enough to carry 50 amps. Most sheds I do need a light and a plug ! 2.5mm x 3core SWA I'd have thought.

Oooops ! Better get my tin hat on and back down that trench !

Deke

 

steptoe

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I usually fit a 1mtr length of galvo conduit if I think the SWA may get damaged.What on earth is in that shed that needs a cable big enough to carry 50 amps. Most sheds I do need a light and a plug ! 2.5mm x 3core SWA I'd have thought.

Oooops ! Better get my tin hat on and back down that trench !

Deke
I would think just good practice, you never know what the client will plumb in after you have left.

would never have thought 6mm SWA would carry even close to 50A on a TNS down the garden buried!! I could be wrong (again) tho.

maybe I should stop posting in this section before I have to change my mind again!?!? :_| :_| :_|

ps, why the 3core SWA tho? on something as tiny as 2.5 you would never ever need the 3rd core for a simple single phase domestic.!

 
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I would think just good practice, you never know what the client will plumb in after you have left.would never have thought 6mm SWA would carry even close to 50A on a TNS down the garden buried!! I could be wrong (again) tho.

maybe I should stop posting in this section before I have to change my mind again!?!? :_| :_| :_|

ps, why the 3core SWA tho? on something as tiny as 2.5 you would never ever need the 3rd core for a simple single phase domestic.!
Maybe to use the 3rd core as 'earth'? (now you've given permission to export it ;) )

Has OP seen mr Sworld's link re outer sheath as cpc? http://www.talk.electricianforum.co.uk/showthread.php?t=1310

 

springcrocus

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What on earth is in that shed that needs a cable big enough to carry 50 amps. Most sheds I do need a light and a plug ! 2.5mm x 3core SWA I'd have thought.

Oooops ! Better get my tin hat on and back down that trench !

Deke
The lady has indicated that she wants to run a mini-cooker down there, and she might wish to use an electric fire.

Total run of the cable is 50 mtrs, allowing for 17.5 amps max gives a volt drop of 9.57 volts (4.16%) so getting a bit close. 6mm allows some headroom.

As Apache ;) has pointed out, third core is for earth. SWA (third core) will be electrically connected at the main dis board only, not the summer house.

It might be a bit over the top BUT better safe than sorry, I think:)

Anyway, thanks for all your input. Anybody else got any thoughts on protecting the cable at point of entry to the ground?

 

steptoe

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I would agree with your sizing springcrocus, future use, as for the 3rd core,,,,, well, not required.

I have generally found that allowing for volt drop(for a given cable length) any cable under 25mm does not require any futher earth other than the sheath.

I would think that as your protective measures are an addition then perhaps even so much as a 2" waste pipe should suffice.

 
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Don't forget when sizing cable you lose 10 percent of load carrying capacity because cable is buried. Batty
Why is that? I can see if cable surrounded in insulation heat can't dissipate but if surrounded in earth that surely acts as a heat sink?

 

septiclecky

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Why is that? I can see if cable surrounded in insulation heat can't dissipate but if surrounded in earth that surely acts as a heat sink?
Check the de-rating tables in the regs for burying a cable in the ground and don't forget the 'Danger Warning' tape over the top of cable before back filling the trench.

Guiness DrinkGuiness DrinkGuiness DrinkGuiness DrinkGuiness Drink

 

steptoe

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aye apache, im not too sure of regs tables etc, but after 2'6" then ground dont freeze(rule of thumb) so logically it must heat.....

and surely cable must be buried 600mm min, ie, 2', so we getting close to non frost point and holding heat, thus current capacity for insulation.

hope I make sense. :)

 

springcrocus

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Not too sure what the regs specify, but double-digging (i.e 2 x fork depth) is usually cosidered to be 18" so if you bury 20" (500mm) deep, and use the warning tape, then you should be OK. After all, it's all about potential damage to the cable, not so much as whether it can dissipate the heat or not.

17.5 amps down a 6mm cable for the 50 mtr run (35 mtrs buried), with all factors allowed for, is well within the tabulated values. :)

I just hope the lady hasn't got an extra-long garden fork. :p

 

steptoe

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springcrocus, you are well within your capacities either way so I wouldnt have any worries on that score whatsoever.

:)

why dont you bring up something contentious like wanting to export a TT system earth? :D :D :D :D

 
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