Secondary CU

simonk · May 3, 2012

Hi guys, this is my first post here (though I've been lurking around and reading for a while) and I have a question.

A customer has has a house that used to be two flats. The main CU (downstairs) is fairly new (about 6 years old, I'm told) but I don't think to 17th edition as only a few of the circuits are RCD protected. A 50A NON-RCD breaker feeds a 10mmsq cable to a 'secondary' CU upstairs. They want this secondary CU upgraded from a old rewirable board. Clearly, I'm going to provide RCD protection on all circuits from this new board, so the question is do I have to provide RCD protection at the main CU, thus also protecting the cable feeding this secondary CU and all circuits from it? Or is it within the regs to leave everything as it is down there within the main CU and to simply install the secondary CU with an RCD main switch and a few MCBs.

Also, the main tails looks to be under 25mmsq, do I have to do anything about this if I can get away without altering anything in the main CU?

Many thanks for your replies - hopefully someone can clear this up.

Simon.

telectrix · May 3, 2012

all depends on how your 10mm is routed. if it's buried <50mm deep in walls, then RCD protection is required. if you are fitting a new sub board, you need to determine whether the cpc in the 10mm is adequate for the PEFC . ( i am assuming you mean 10mm T/E )

spark 68 · May 3, 2012

A 50A NON-RCD breaker feeds a 10mmsq cable to a 'secondary' CU upstairs.

Firstly what is the earthing system ?

This cable need not have RCD protection provided it is not buried in a wall <= 50mm etc. and meets Zs and disconnection times etc. (ie. is not on a TT system).

Also, the main tails looks to be under 25mmsq, do I have to do anything about this

This depends on the size of the incoming cut out fuse, part of the confusion in this respect is due to the OSG stating 25mm tails for everything.

We need a bit more info.

Beaten by Tel again Lol, I must learn to type quicker

simonk · May 3, 2012

Blimey that was quick! Thanks for your replies so far.

The 10mmsq T+E from the main CU *probably* finds its way upstairs through a void in the wall and then under the floorboards. Assuming it would require RCD protection if installed now, I was mainly wondering if RCD protection for this cable is necessary at all as it is already there and done to previous regs and I am not changing it, rather like all the other circuits in the house without RCD protection. The final circuits distributed from the new seconday CU will of course be RCD protected.

The earthing system is TNCS, the DNO moved the meter ouside when the last work was done about 6 years ago, I am assuming they sized the fuse and tails to be compatible at that time. I haven't removed the fuse as an isolator has been fitted on this occasion! The bonding looked adequate but the main earth *may* have been only 10mmsq, will have to check that one again.

spark 68 · May 3, 2012

You are only required to certify any work you are doing (ie. the new CU), but still need to check/test that anything you are re-connecting is safe to do so.

As Tel has said you will need to check that the 10mm T&E is adequate for the PEFC mainly due to the undersized CPC in T&E, I take it this is no longer two seperate flats ?

simonk · May 10, 2012

Haven't done this job yet but will probably go there next week. No, it's now one house. Assuming that the disconnection times for the 10mm2 distribution cable with 50A type B are OK, I will probably leave it at that without RCD protection, as I've just realised that RCD protecting this cable would cause all the upstairs circuits including the lights to be coming off this one RCD. A better way I reckon is for the upstairs CU to have one RCBO for the lights and the other few circuits from another RCD + MCBs.

telectrix · May 10, 2012

sounds good. if the 10mm is ok for the load, but the cpc is undersized, you could run a separate 10mm cpc alongside.

simonk · May 10, 2012

Thanks for that, erring off topic slightly now to the adiabatic equation.

Say for instance, the cpc in this cable is 4mm2, would that mean the minimum PEFC would be 250A (to clear the fault instaneously) and the maximum PEFC would be 1450A (=115*sqrt(4^2/0.1)) to meet thermal contraints? Corresponding to Zs between 0.92 and 0.16.