RCBO Unit for shower

[dunny123]

There currently is a 40A MCB on a 6mm circuit clipped direct for a shower circuit in a CU without RCD protection. I wish to provide for a new shower (9KW) a small CU unit housing an a 40A RCBO. Can I simply install this in the adjacent airing cupboard downstream of the 40A MCB ?

i.e. 40A mcb--------40A RCBO (30ma)----45A DP SWITCH-----9KW Shower

 

[Pensdown]

The only problem you will have is discrimination, i.e. in the event of a fault, which one of the 40amp devises will open first?

The RCBO serves no purpose, all you need is an RCD

 

[luminaire]

Can you not simply swop the 40A MCB for a 40A RCBO?
Then you'll have RCD protection and no discrimination to worry about.

 

[dunny123]

Unfortunately cant change the existing OPD as its an old wylex board which has had the rewiarbles replaced with mcbs, as far as i'm aware they dont make rcbo's for them. Also, 40A is I believe the maximum that the board is designed to take.The position and situation of the board and supply does not lend itself to fitting an additional CU.

Does it matter that there is no discrimination of the 40A mcb and 40A RCBO ? A direct fault will cause one of the MCBs to trip, whichever it is the onward cable/circuit will be isolated and therefore safe, and if its an indrect fault then the RCBO will trip.

 

[luminaire]

As it stands at the moment, although desirable it doesn't have to be RCD protected. Of course it does mean it must be supplementary bonded.

 

[sparkybird]

Couldn't you just use an rcd unit instead of RCBO - then no discrimination to worry about

However, you might need to check the main switch max load on the board as it might be quite small (60A?) and you'll be close or over it...

 

[securespark]

Yup! SB's right. If you have an elderly board with a 60A switch (or unmarked), then you cannot run a 40A circuit off it. Fit a new 1 way CU with an RCD incomer & suitable mcb.

 

[luminaire]

Yup! SB's right. If you have an elderly board with a 60A switch (or unmarked), then you cannot run a 40A circuit off it. Fit a new 1 way CU with an RCD incomer & suitable mcb.

But the OP said earlier: "The position and situation of the board and supply does not lend itself to fitting an additional CU."

 

[JohnD]

he also said "its an old wylex board"

they are 30A max per way, not 40A.

h'mmm, the inconvenience of doing it properly, versus the inconvenience of clearing up the smoke damage after the Wylex overheats and burns away round the 40A breaker?

 

[ban-all-sheds]

Which is not fanciful - didn't someone post a photo here fairly recently of one which had done just that?