Earth system clarification

[Phil 2125]

At my place of work we are about to run a 3 phase supply from the main factory out to an office building (brick-built, not temporary). We are using 35mm 4 core cable running through underground pipework for around 40 meters. The factory has a TN-C-S earthing system and we were anticipating running a separate earth cable parallel to the main cable. However, an electrician has advised us that as it’s a separate building it must (a) be provided with a TT earth electrode, and (b) the TT earthing system must be isolated from the TN-C-S system. Now I had no problem with the need for a TT earth electrode - although a quick look around some forums indicates this is probably not a requirement, and we probably could have had a continuation of the TN-C-S without problem. But my main question is that other than for initial testing purposes, why would the two systems require absolute isolation? Once their initial integrity had been proved, surely neither could compromise the other? And the greater the number of paths the earth in a fault situation the better? I can find nothing in the 17th to support either claim – though I could have missed it. Some clarification would be appreciated.
Cheers,
Phil

 

[Des 56]

Why not run a 5 core and use the TNc-s supply

TT should be isolated from the Tnc-s because the earthing and bonding of the outbuilding would need to meet the requirements of Tnc-s earthing and bonding if they are linked and not TT requirements

 

[malcolmsanford]

Is this building going to have it's own services such as water for toilets, kitchens etc, or are you going to bring the services from your existing building.

If your going to utilise services from the main building then the electrician may have recommended a TT system becasue these services will need to be bonded back to your main building MET which is 40 metres away. By utilising a TT you can create a completey seperate equipotential zone and thefore bond the services at the local board.

You could though run a 5 core cable 35mm and use the extra conductor for the bonding and the SWA as your CPC.

 

[paturback]

Is this one of those cases where due to distance it would be more economical to install TT system?

 

[Phil 2125]

The building has been in use for several years. It has got a water supply installed for toilets, kitchen etc. although these aren’t taken directly from the factory building. The existing single phase supply isn’t up to the increasing demands, so it was decided to upgrade the system. We were actually thinking of using SY rather SWA as the route requires some flexibility, and 35mm SWA will have some problems with the available radius. Although we can get the SY in 5 core, I don't think the braiding would really pass should it be considered part of the earth system; hence the separate earth wire comment. Economy-wise it may well be cheaper to install a separate TT system, but my curiosity centered on the 'requirement'? to isolate the two earth systems should that scenario arise.
Many thanks for the replies so far
Phil.

 

[Engineer54]

Unless your going to be prepared to make a decent deep driven rod installation, then stick to the TNC-S earthing system.

As this is a place of work, it's not going to be the same as your typical outbuildings rod installation, and the cost equation between rods and bonding conductor, could well be in favour of providing the bonding conductor. Besides why ditch a perfectly good earthing arrangement, for a more than possible lower one??

Find out from the cable manufacturer, the CSA of the braiding provided on the SY cable your proposing to use.

EDIT.... The separate core of the cable is for your bonding needs, the braiding of your SY cable will if of suitable CSA will be your CPC, no problem both being connected to the MET of the CU/DB at the load end. If your using a 5 core cable, the CSA of your braiding is going to be increased, and may well be more than adequate for your CPC needs....

 

[baldsparkies]

Take a look at this Phil,
Its pdf so easy to copy off, and proves a handy referance for swa braiding suitability as a cpc.


http://www.earthingnuts.co.uk/pdf/pvc_xlpe.pdf

 

[Engineer54]

Baldsparkies,

That table is a little misleading, requiring calculation in too many cases, far better to use the tables in this paper, written and published by a respected Institute member.

Using the armour of SWA.pdf‎