SWA for shed **PME** EARTH?????????????

[Andy Ellis]

New supply for a shed (32A for hot tub + 16A power circuit)

Shed is 30 meter cable run from CU.

TN-C-S EARTH AT CU

Tub located in timber shed at bottom of garden, no services, water ect, will be filled by hosepipe when required.

Supplied from existing CU on own circuit, 60A 30mA RCD, Supplying 2 Way IP65 CU in shed.

I selected 2 core 6mm PVC SWA, assuming 25 degree ambient temp, clipped on its own to exterior wall. Max demand 47A or 11.04Kw (though cable would alow up to 50.5A) over 30M run I get drop to 10.5V or 4.6%

63A IP65 DP rotarty isolator on outside of shed.

This is as far as I got!

Struggling with the earthing and the PME. Lot of info on web that says it can not be exported to shed blah blah its endless!

I wondered if the best way would be to connect SWA to PME at the CU only, then TT the shed with a rod(s)... cant imagin that would be fun though... hmmm... and suppose it needs to be reliable with a hot tub involved!!!!?

Can you see any advantage to 3 core and using a core as earth, even if not connected on shed side and still TT shed

 

[mickys86]

Why would u use a core as a cpc if u intend to TT the shed?? As long as the sheath on the swa is earthed u wouldn't have any use for the 3rd core

 

[nikmet]

use 3rd core as an earth cpc?

 

[Andy Ellis]

Yea got mixed up there. Id rather not have to TT the shed, but cant see any way around it.

 

[BertUK]

i would use a 3 core swa and use a metal adaptable box at each end to continue the earthing of the armouring if both CU,s are plastic and its tight glanding into them,then use one of the cores of the swa as your main earth.im almost sure that all sub main feed main earths should be equal size to the phase conductors up to and including 16mm csa.

 

[ukpablo]

http://dl.dropbox.com/u/27772944/Garage-Shed%20Supplies.pdf

this may help u out?

 

[Andy Ellis]

So im now wondering, as there are no extraneous parts, no supplys ect, timber construction, could the PME be exported to the shed?

 

[rallyandy]

Hi Andy please see this thread A good guide For 'exported PME' and outbuildings! this is a guide posted on here from the IEE this should clear it up for you, hope this helps, this was cleared by IQElectrical, here is a link for you to view

http://www.electriciansforums.net/a...ted-pme-outbuildings-garage-shed-supplies.pdf

 

[Liamsparks]

Hehe do we have another exporting pme topic on our hands?

http://www.electriciansforums.net/e...159-good-guide-exported-pme-outbuildings.html

 

[Andy Ellis]

absolute minefield as far as i can figure. hate using tt

 

[rallyandy]

Lol looks like it Liam

 

[rallyandy]

seems were all posting the same link lol

 

[malcolmsanford]

New supply for a shed (32A for hot tub + 16A power circuit)

Shed is 30 meter cable run from CU.

TN-C-S EARTH AT CU

Tub located in timber shed at bottom of garden, no services, water ect, will be filled by hosepipe when required.

Supplied from existing CU on own circuit, 60A 30mA RCD, Supplying 2 Way IP65 CU in shed.

I selected 2 core 6mm PVC SWA, assuming 25 degree ambient temp, clipped on its own to exterior wall. Max demand 47A or 11.04Kw (though cable would alow up to 50.5A) over 30M run I get drop to 10.5V or 4.6%

63A IP65 DP rotarty isolator on outside of shed.

This is as far as I got!

Struggling with the earthing and the PME. Lot of info on web that says it can not be exported to shed blah blah its endless!

I wondered if the best way would be to connect SWA to PME at the CU only, then TT the shed with a rod(s)... cant imagin that would be fun though... hmmm... and suppose it needs to be reliable with a hot tub involved!!!!?

Can you see any advantage to 3 core and using a core as earth, even if not connected on shed side and still TT shed

Under these circumstances I would be inclined to read the manufacteres instructions for the tub that may want the earth to the tub tied down to a rod ...................

Even if they don't ask for a rod I would prefer to TT this, as with all that water in the area, and the likely chances of water lieing around I think I would not want to be exporting an earth from the house, I would rahter keep all potentials local.

 

[Engineer54]

Most of these hot tub manufacturers are in the USA/Canada, and is why they sometimes even provide a rod in the installation package. It's there main earthing system over there, outside of the cities/built-up areas.... It's basically, an additional rod to help bring the installations Ra values down. Remember in both the USA and Canada the recommended Ra value is 20 ohm, not the crazy 200 ohm it is in the UK!!! ...lol!!!

Extend that PME supply, the tub is in a wooden shed ...Right?? If the floor is wooden too, then even less to be worried about ...lol!!! If those of you are that worried, ..nothing stopping you adding a rod at the Shed and connecting it to the CU's MET along with the supply cables separate core's PME earth connection and the earthed SWA armouring. You'll not be gaining much, ...if anything mind!! lol!!! At the very least you will then be using any RCD device(s) in the shed's CU, as ''Additional'' protection and not relying on them being your ''Total'' protection for earth faults!!!

 

[robskiing]

Cant you just do a loop test when everything is up and running and all extraneous metal earthed by 10mm back to the met in the dwelling

 

[Engineer54]

Cant you just do a loop test when everything is up and running and all extraneous metal earthed by 10mm back to the met in the dwelling

Already been stated by the O/P that there is No extraneous metalwork/services within the shed...
Therefore no requirement for any 10mm bonding conductors on this installation!!
But i would personally still install a 3 core cable as a CPC, and still use the earthed armouring too....lol!!

 

[Chr!s]

Im with E54 on this one.

If the spa is housed and an equipotential zone is created, then it no different to a hot tub in the house.So i would install it to "Section 701" .

If the hot tub is not housed and an equipotental zone can not be created then i would install to "Section 702". As for PME, i would either TT the installation at the supply end of the Distribution Circuit, or place an Electrode at the "MET" with a predetermined Ra which would limit the touch voltage on a broken pen and also reduce the possible sensation of an electric shock through the difference in the two "Earth Potentials".

Chris