Is this bad practice?

[La Poste]

Upon fitting a split load twin RCD board there are still two circuits left.
Both are for outside and both are on RCBO's. One supplies the shed and one supplies the garage.
The CU is full up and I have bought a small MK board to house the two RCBO's.
Is it bad practice to take the supply for these two RCBO's from the outgoing side of the main switch of the CU above?
If I took the supply from the outgoing side of the 100A switch from the board above then one switch would turn off the whole installation.
I have read about fitting an additional 100A isolator and then splitting the tails and supplying the top main board with one set of the tails the bottom board with only two RCBO's with the other set of tails but this seems a bit excessive considering there are only two RCBO's on this small board below.
So is it bad practice to add two extra cables, one phase and one neutral to the outgoing side of the isolator switch on the main board above to feed this small board below?
The system is TT earth rod.
Thanks.

 

[malcolmsanford]

IMO yes it would not be good practice.

1 getting another fairly substantial cable into a dp isolator that already houses a 16/25mm^ is going to be tight, and you may in a worse case scenario create a bad connection.

2. Space if your existing CU is that crowed it would be a nightmare to work on.

Henley Block the tails from the meter. Then 1 set of tails to your existing CU and then another to your smaller CU. A 63amp DP isolator and those 2 RCBO

 

[Des 56]

You propose to feed the new board from the outgoing/load side of the main isolator so that there is no issue with cramming tails into the incomer, along with existing supply tails
If the new unit can be mounted immediately adjascent to the main board and the supply to the new board tailed/ crimped/nut + bolted to the bus bar ,its then just an extension of that bus bar to then feed the enclosure with the 2x rcbos

 

[malcolmsanford]

You propose to feed the new board from the outgoing/load side of the main isolator so that there is no issue with cramming tails into the incomer, along with existing supply tails
If the new unit can be mounted immediately adjascent to the main board and the supply to the new board tailed/ crimped/nut + bolted to the bus bar ,its then just an extension of that bus bar to then feed the enclosure with the 2x rcbos

Wouldn't the bus bar be on the load side of the RCD and not load side of the 100amp DP swich. Most I worked on the Supply side feeding the RCD are tails, but may be wrong

 

[telectrix]

concur with malcolm's first post. split tails etc. saves cramming more into the main CU

 

[Des 56]

Thank you Malcolm
Yes your right.I read split board and ignored the twin rcd bit

I read through too quickly, assumed a bus bar off the main switch and a single rcd and hit the keys
La poste Ignore my rumblings

 

[malcolmsanford]

Thank you Malcolm
Yes your right.I read split board and ignored the twin rcd bit

I read through too quickly, assumed a bus bar off the main switch and a single rcd and hit the keys
La poste Ignore my rumblings

No probs Des ..................think though your like me an old industrial man bolting into bus bar chambers and all.

 

[oldtimer]

Malcolm I had the same problem and looked at this 15 Way Hi Integrity Dual RCD + 100A Main Switch Insulated Consumer Unit you still get your twin RCD but you have 3 ways
that you can adapt via RCBOs for externals

 

[nickblake]

Citys do a 22 way single phase CDU the M2 , the way i would do it would be to use a circuit from the unprotected side (he has said its a split load CDU not 17th ) of the existing board and use this to supply a 4 way sub board for the RCBO's then move the one circuit you have used into the new board then you can install the RCBO's no fiddling with the tails and one switch for the whole installation , opps mis read its a 17th board well thats easy use the sub board as the main board and supply the existing board from that

 

[olly w]

Did the CU that you fitted not come with three sections of bus bar so that you could split the out going ways to allow you to feed a second smaller CU from a non RCD protected MCB?

Then you could run the two remaining circiuts (and one other to gain a spare way) from the second CU.

 

[La Poste]

Thanks for the replies.
I am veering on the side of splitting the tails and having two boards.
The system is a TT and I have read that TT system must have a single isolator switch for the whole installation, this is why I thought of adding a set of tails from the outgoing side of the isolator switch on the main board above down the sub board. I could then isolate everything from the one switch of the main board.
The board is a 17th edition split load dual RCD board.
The outoing side of the isolator switch on the main board above has two sets of multi-strand cables feeding both RCD's of this main board. I was thinking of adding a third multi-strand cable and running it to the board below hence having three sets of cables running from the outgoing side of the isolator switch.
The bus bars of the main board above are designed to be clamped to the outgoing side of the RCD's of the main board and hence running a supply from an MCB of this main board would haveto pass through one of the RCD's of the main board above before travelling down to the second board below, as the second board below comprises of RCBO's this would be unacceptable as my circuit would then have two RCD's in series.
Another idea is to just install a 100A main switch after the meter and split the tails from this switch, one set going to the board above and one set going to the board below thus giving me one 100A isolator switch to turn off the whole installation although I will then have three isolator switches, one main, one for the board above and one for the board below, this seems a little excessive to me but heh I don't write the regulations.
I am going to have another look at it today.

 

[malcolmsanford]

Can I ask where you saw that about a TT system must having a single isolation point for the supply?

Reg 537.1.4 just calls for main linked switch or circuit breaker as near as practicable at the origin of every installtion, and that if it's to be used by ordinary people it must switch both live conductors ie double pole.

By henley blocking the tails your new board is part of the origin of that installtion and as long as it as a double pole switch in it, IMO you will comply to the Reg. I may be wrong and if I'am then sorry about this, but I don't think I am

 

[La Poste]

Can I ask where you saw that about a TT system must having a single isolation point for the supply?

I will haveto dig it up later.

 

[Spudnik]

Malc,

TT installations should have an isolator that disses L&N conductors in a circuit and not just an MCB like in TN systems.

However, many argue that the main switch, which is normally DP, is sufficient.

I could, of course, be way off topic.

 

[malcolmsanford]

Malc,

TT installations should have an isolator that disses L&N conductors in a circuit and not just an MCB like in TN systems.

However, many argue that the main switch, which is normally DP, is sufficient.

I could, of course, be way off topic.

This is what I love about the Regs like the rcd for use by ordinary persons on sockets they use the same here for disconnection of a supply. It dosen't say what system so assume it means TT or TN . but if your in a factory where it as a maintenance department could you make use of a MCB on a TT or TN system.

No matter where it was I would use a double pole isolator for disconnection of a supply regardless of what system, as would 99.9% of other sparks, so why don't the BRB just make it so instead of phaffing about