SWA 2 core or 3 core?

[reidmaguire]

I dont do a lot of external works, however, I have been asked to install some power for general lighting and powerto an outbuilding with no extraneous conductive parts 15 meters from the main board and my thoughts were;

The supply is tt, so 10mm 2 core swa from spare way in main board to CU in outbuilding - outher sheath eathed to main board only. the outbuilding will be seperate tt and rcd protected.

I have been asked why I am not using 3 core SWA, and when I say it is not required, I am starting to question this myself!!

Any though on this appreciated

 

[nrbspark]

Your thoughts using the 2 core are exactly the advice that was given to me by the NICEIC a couple of months ago when I posed myself with the same question and was not 100% sure myself. The reason that I was given was Parallel earth paths would be present if there is a spike at either end and also a cpc connected at both ends within the SWA.

So they said 2 core SWA with armoring connected at mains distribution end and divorced at new DB, then an earth spike at the new DB with the obvious RCD protection.

Hope this helps.

 

[Guest123]

I dont do a lot of external works, however, I have been asked to install some power for general lighting and powerto an outbuilding with no extraneous conductive parts 15 meters from the main board and my thoughts were;

The supply is tt, so 10mm 2 core swa from spare way in main board to CU in outbuilding - outher sheath eathed to main board only. the outbuilding will be seperate tt and rcd protected.

I have been asked why I am not using 3 core SWA, and when I say it is not required, I am starting to question this myself!!

Any though on this appreciated

Spot on.

 

[Davey101]

I've never really understood why you cant use the earth from the main building (not saying you can't, just that I've never had it explained properly)
Can anyone give a practical explanation please?

 

[AA410]

So could you still used a 3 core in this situation what comes to mind is if the armoured was damaged in anyway some time ago i came across a job were a 2 core cable had been pulled out of the gland hence no earth

 

[Staffie]

I've never really understood why you cant use the earth from the main building (not saying you can, just that I've never had it explained properly)
Can anyone give a practical explanation please?

To my knowledge, something to do with any extraneous stuff in the out-building being at a different potential to that in the main building, hence the need for a seperate earth.....I think...

 

[AA410]

I dont get that if you used a 3 core and earthed the outer steath with the cpc it is at the same potential

 

[Davey101]

So could you still used a 3 core in this situation what comes to mind is if the armoured was damaged in anyway some time ago i came across a job were a 2 core cable had been pulled out of the gland hence no earth

I think the point is that you DONT WANT an earth at the outbuilding, the earth must be provided by a seperate earth rod, the earth from the existing building should be insulated from the outbuilding, or have I got it totaly wrong?

 

[pennywise]

Your not suppose to export the earth from a PME system to an area outside of the equipotential zone ie the house.

 

[nrbspark]

How do you determine the area of a buildings equipotential zone, does anyone have an equation?

 

[Staffie]

How do you determine the area of a buildings equipotential zone, does anyone have an equation?

I don't think there is one?

Basically, if the building to be supplied is not part of the fabric of the building where the supply is coming from, then the earth is not to be imported from the PME and a new earth should be utilised, an earth rod.

 

[nrbspark]

This suggests that if you run a supply to a detached garage from the house that a spike should be used every time and not the 3rd swa core as the cpc. Also should you use a spike if you are supplying an outside light in your garden.

I think that I may be confusing myself but I have seen the 2 items listed above many times on a TNC-S system.

 

[Toonlad]

just been looking through my course work and found a similar example to the ops question.I hope this helps.If not my lecturers are talking ****e.

bs 7671 542.1.8 -If the protective conductor forms part of a cable, the protective conductor (ie the swa) shall be earthed ONLY at the installationcontaining the associatedprotective device. This by default the HAS to be the main house end, utilising the supply companys earth,but for the supply cable only.

Additionally you would not bond both ends.For this would then connect the outbuilding earthing system to the suppliers earth , which would cause circulating eddy currents in the supply cable swa, causing nuisance tripping of the outbuilding main switch rcd.

 

[AA410]

What about on a TNS systeam what earthing arangements are for that on an out building

 

[Guest123]

No issues with extending a TN-S earthing system to an outbuilding, providing circuit parameters allow it.