TT system sub main design

[Markblox]

Hi all, a little stuck with a couple of things.

house on a TT system with a 16mm main earth and a 100A main cut-out.
the board has a 30mA 2 pole RCD main switch and all the circuits are 60898 protected.
One of the circuits supply's the garage with a 6mm 70 degree SWA. The garage, however is large and about to be converted into living accommodation with 250mm loft insulation and cellotex on the internal walls. There is a lounge, bathroom and kitchenette. The circuits are 32A shower circuit for a max 7.36 KW shower using 10mm cable because of the insulation. A 20a radial socket circuit for the lounge sockets wired in 4mm and the same for the kitchen sockets. Lighting will be low energy and will be 6A in 1mm.
There may also be an oven fitted so the worst case will be another 32A circuit.
The Cu will be 6 usable ways and have a 30mA RCD main switch.

I intend to replace the supply cable with a 90 degree thermosetting SWA 3c 10mm, protected by a 63 A C/B
If there is a fault in the garage there will be no discrimination and the whole lot will go out and the reverse is also true
I am going to strongly recomend they change the main CU for a 17th edition type with two 61008 30mA for the house circuits and as it's a TT system a 100mA S type main switch. How should I protect the supply cable? Do I run it from a separate non RCD protected way in the CU and rely on the 100mA S type of the main switch?

 

[Engineer54]

Is this an attached garage?? If so how far is the garage area from the house CU??

 

[rattlehead85]

You are correct with your theory. Personally i would as you say recommend a board change to the house to permit adequate discrimination.
Place the supply cable on a 63A MCB with 100mA S RCD. (Note you may need to up the cable to 3x16 SWA depending on the wiring method, distance and correction factors)
Terminate the garage supply into an all insulated consumer unit in the garage with 30mA RCD protection and run a separate T.T system rod. Terminate the bonding to your services if copper at your garage board.
Ensure you have no overlapping earth paths between your Rods for the house and garage.
Job is a gooden.

 

[Markblox]

Thanks for the quick replies, the garage is detached and only three meters away the supply circuit is to be buried and will only be about 6m long. The SWA will not run through insulation save for about 60mm.
rattlehead, are you saying the 100mA S type will cover the sub main and I should use a 60898 cb in a way separate from the 30mA part of DB1?

 

[Markblox]

One other thing, I spoke to the nic the other day and thought I should put in an extra earth rod and he said no, which when I thought about it makes sense. Are you sure it needs one as it is just another circuit. After all and the earth part of any fault current will goes back via the existing.

 

[HandySparks]

...and run a separate T.T system rod. Terminate the bonding to your services if copper at your garage board.
Ensure you have no overlapping earth paths between your Rods for the house and garage.

Why would you separate the earthing between the house and the garage? An additional rod may be a good supplement to the main rod, but why not keep all the earthing connected together?

 

[kingeri]

Only install a second rod if necessary once you've done everything else. You may find that you don't need one.

 

[Engineer54]

One other thing, I spoke to the nic the other day and thought I should put in an extra earth rod and he said no, which when I thought about it makes sense. Are you sure it needs one as it is just another circuit. After all and the earth part of any fault current will goes back via the existing.

Yet more duff advise from the all knowing NICEIC!! The whole idea of a working TT system, is to gain the lowest stable Ra value you can!! So another Rod position which will be linked back to the existing rod position either via the sub-main earth conductor, or by a hard wire direct link between the new and existing rods will help in achieving lowering the Ra value. Now that makes far more sense doesn't it?? lol!!

 

[Engineer54]

Only install a second rod if necessary once you've done everything else. You may find that you don't need one.

When have you ever come across an existing TT system, that couldn't benefit and be improved on, from an additional rod position??

 

[johnboy6083]

I totally agree with the others. Whilst your on site, the extra cost of another rod, pit & cable is nothing in the scheme of things.

 

[Guitarist]

Ask the local power network operator if you can have it PME'd to begin with. Many are doing so free of charge now.

 

[Markblox]

Yes, but that's why it has an RCD. It only needs to lower than 200 ohms.

 

[Engineer54]

Yes, but that's why it has an RCD. It only needs to lower than 200 ohms.

Oh dear, another brainwashed BS7671 pilgrim!! lol!!

So what is the present Ra value you have for the existing rod?? Is it one of those short thin twigs, or decent deep driven rod(s)??

 

[Guitarist]

Yes, but that's why it has an RCD. It only needs to lower than 200 ohms.

Here we go....

 

[Markblox]

Prospective earth fault is 2A