All-RCBO CUs - because we can?

[flameport]

If certain manufacturers have their way, it's won't be all-RCBOs in the future.

It will be all-AFDDs, and the price of those make RCBOs seem like loose change.

 

[JohnW2]

If certain manufacturers have their way, it's won't be all-RCBOs in the future. It will be all-AFDDs ....

Indeed - and I may have to start some new 'questioning campaign' (and a new thread here ) ....

... I haven't yet studied the issue to any extent, so I am at present not be able to have much of a view (although I have distinct suspicions, even now!) on the question of whether AFDDs address an issue which actually needs to be addressed (at least, in domestic installations) - rather than (back to the title of this thread!) perhaps "because we can"?

Kind Regards, John

 

[securespark]

That's an interesting point, which I confess has not occurred to me before - and I agree that it would be a distinct advantage in a few (albeit, I would think, not all that many) cases.

Kind Regards, John

Lots of older houses have narrow cupboards often built-in or semi built-in and very few want the board re-sited or on show.
I've lost 10 or 15 board changes over the years because it's been a deal-breaker.

 

[JohnW2]

Lots of older houses have narrow cupboards often built-in or semi built-in and very few want the board re-sited or on show. I've lost 10 or 15 board changes over the years because it's been a deal-breaker.

In that situation, have you ever considered joining two together vertically (with main switch removed from one, to provide a 'single point of isolation' for the two? I might consider it for myself, but it's probably a no-no for a customer! (there are actually some proper 'stacking kits, aren't there - and maybe even tall CUs with two narrow rows of devices??).

Kind Regards, John

 

[ban-all-sheds]

The devices work in 3P boards when turned through 90°....

 

[SimonH2]

Some manufacturers have a double height board with two DIN rails - though these tend to be on the wide side as they are normally for larger capacity rather than reduced width.
I've read some spec sheets where mounting orientation is given as (to paraphrase) "any way you like".

There is one advantage to a vertically aligned CU ... If you do printed labels along the lines of "kitchen sockets", "upstairs lights", and so on - then the text on the labels can be the right way up I know some label printers can do vertical text, but I tend to find that it's both hard to read, and overly spaced out.

 

[ban-all-sheds]

I used to have a CU with the ways numbered, and a printed table alongside wth a description for each number.

Because the numbers I stuck on were blagged from a piece of IT equipment - I'm guessing some kind of disk array, my circuits were numbered 0 - 7.

 

[JohnD]

tall CUs with two narrow rows of devices??)

I don't know if they still make this sort of thing

View media item 1090
View media item 6822

 

[ban-all-sheds]

Ah, the drawbacks of Type Approval...

 

[SimonH2]

A bit I spotted in the OSG ...
After describing the typical split board, it goes on to say (description under fig 3.6.3(iii)) : "This is not suitable for an installation forming part of a TT system as there is insufficient fault protection of the single insulated conductors which connect the load side of the double pole main switch to the supply side of the RCCBs."
Earlier in section 2.2.6 it has a fuller description and says : "The IET's Wiring regulations Policy Committee, therefore, advises the following ..." The choices being to use a metal CU with all RCBO (where the rick of the busbar dropping onto the metal case is considered to be low) or to use a split board with a time delayed RCCB as the main switch.

So another of those "new reg introduces a risk that now needs mitigating" situations

 

[JohnW2]

A bit I spotted in the OSG ... After describing the typical split board, it goes on to say (description under fig 3.6.3(iii)) : "This is not suitable for an installation forming part of a TT system as there is insufficient fault protection of the single insulated conductors which connect the load side of the double pole main switch to the supply side of the RCCBs."

Hmmmm. What about the tails connected to the supply side of the main switch - aren't they worried about the fault protection (or lack of it) for them? Are they just fussing about the fact that the conductors between main switch and RCD are 'single insulated'? If so, they could be insulated and sheathed - but I don't see how that would alter the degree of fault protection.

So another of those "new reg introduces a risk that now needs mitigating" situations

I suppose that it's not for me to criticise, but it sounds as if a fair bit of 'barrel scraping' has gone into writing that.

However, it's the OSG, not "a new reg".

Kind Regards, John

 

[SimonH2]

The tails are insulated and sheathed - or double-insulated depending on manufacturer's description. I guess the logic (which I can't disagree with) is :
Metal case is a conductive part (that's a new requirement); on a TT system, earthing alone won't provide sufficient protection against shock; single insulation is not acceptable for shock protection; hence there's a risk of the single insulation failing and making the case live. So unless you say "the rules about not relying on single insulation" (which seem to be scattered about sections 4 & 5 of the regs in one form or another) or you say that really we aren't that bothered about shock protection in TT systems; then it's a valid interpretation of the regs.

Previously, the CU case was most likely insulating plastic and hence no problem - you had two layers of insulation.

 

[flameport]

single insulation failing and making the case live.

Not just the case.
It's everything connected to the case.

 

[JohnW2]

The tails are insulated and sheathed - or double-insulated depending on manufacturer's description. I guess the logic (which I can't disagree with) is : Metal case is a conductive part (that's a new requirement); on a TT system, earthing alone won't provide sufficient protection against shock; single insulation is not acceptable for shock protection; hence there's a risk of the single insulation failing and making the case live. ...

As I said, if 'insulated and sheathed' wouldn't be good enough for the connection between main switch and RCD within the CU, why do they apparently think that it adequate for the incoming insulated-and-shed tails feeding the main switch?

However, there may be some confusion here, since what you quoted talked about 'fault protection', whereas what you are talking about above is 'shock protection'. In a TT installation, adequate 'fault protection' (of a cable or component) can only be provided by an upstream RCD. In practice that can on;y really be done by having a stand-alone time-delayed RCD upstream of and external to the CU. Even what the OSG suggested of having such a device as a CU incomer leaves the incoming tails without any fault protection.

The number of layers of insulation don't really have anything to do with 'fault protection' (in the normal sense), although they are obviously relevent to the risk of electric shock.

Kind Regards, John

 

[SimonH2]

As I said, if 'insulated and sheathed' wouldn't be good enough for the connection between main switch and RCD within the CU ...

The internal connections are not sheathed, just single insulated singles.

EDIT: I suppose a third way would be to replace the internal cables with insulated and sheathed singles.