Consumer units, 17th ed & RCDs

[acmedone]

Out of curiosity I have just been browsing small CUs. I thought that 17th ed required RCDs to be fitted but when looking at 4 or 5 way CUs there appears to be no provision, eg:

http://www.screwfix.com/p/wylex-5-way-fully-insulated-main-switch-consumer-unit/24223

http://www.screwfix.com/p/5-way-100a-double-pole-isolator/54580

The smallest with RCD seem to be 8w. Do the requirements not specify an RCD then?

 

[EFLImpudence]

They're empty - you can populate them with whatever you need.

You will need an RCD + MCBs or you could use RCBOs.

No point being supplied with an RCD if not needed.

 

[Alarm]

http://www.screwfix.com/p/volex-80a-30ma-dp-rcd/68302

Ta daaaaaaaaaaaaa

 

[acmedone]

They're empty - you can populate them with whatever you need.

You will need an RCD + MCBs or you could use RCBOs.

Ok, an RCD takes 2 slots. So that leaves just 3 available outputs.

So do I understand that something like this
http://www.screwfix.com/p/wylex-8-way-fully-insulated-rcd-board-consumer-unit/99580
which says it comes with an RCD, means the RCD is loose in the kit (so the switch shown in the picture is just an isolator) and the assembled unit actually gives 6 outputs?

No point being supplied with an RCD if not needed.

Under what circumstances?

(Forgive me. CUs are something I have just taken for granted; it's just a boring fuse box. But since busybodies are never happy unless they're changing rules it appears it is time to be more informed.)

 

[sparkiemike]

No point being supplied with an RCD if not needed.

Under what circumstances?

Do you know that not all circuits require RCD protection?
Do you know what a RCBO is?

 

[holmslaw]

..

 

[acmedone]

No point being supplied with an RCD if not needed.

Under what circumstances?

Do you know that not all circuits require RCD protection?

No. That's why I'm asking.

Do you know what a RCBO is?

Yes. But what is the point if an RCD is fitted?

 

[acmedone]

The cu in your link is specifically for use on a TT supply.

What does TT stand for? I used the word "outputs" deliberately to distinguish from "ways"

 

[flameport]

Do you know what a RCBO is?

Yes. But what is the point if an RCD is fitted?

There isn't one.
The units with a main switch could be fitted with 5 RCBOs = 5 circuits. Plenty for smaller properties.
Or MCBs where the circuits did not require any RCD protection.
Or a combination of both.

That 8 way Wylex item linked to earlier is 8 ways + an RCD as the main switch.

 

[acmedone]

Do you know what a RCBO is?

Yes. But what is the point if an RCD is fitted?

There isn't one.
The units with a main switch could be fitted with 5 RCBOs = 5 circuits. Plenty for smaller properties.

There is a massive price difference between say, 1 RCD plus 5 MCBs compared to 5 RCBOs.

 

[flameport]

There is a massive price difference between say, 1 RCD plus 5 MCBs compared to 5 RCBOs.

There is, but why is that relevant?

 

[sparkiemike]

The cu in your link is specifically for use on a TT supply.

What does TT stand for? I used the word "outputs" deliberately to distinguish from "ways"

TT is the type of earthing system, it simply means that there are two connections with earth (Latin: Terra); one at the transfomer and one at the origin of the installation. RCDs are usually always required if you have a TT earthing arrangement. Wikipedia has a good article on earthing systems if you are interested.

"Outputs" is not a recognised term, I think you mean circuits - each circuit will have its own protective device (Fuse, MCB or RCBO) and there may be additional protection from a RCD depending on the design. The protective devices could take 1, 2 or 4 ways in a consumer unit.

 

[sparkiemike]

deleted - double post

 

[sparkiemike]

There is a massive price difference between say, 1 RCD plus 5 MCBs compared to 5 RCBOs.

Yep; and depending on the the type installation would determine which one you would install. Also would you want a single fault to switch off the entire installation, or would you just want the circuit where the fault has occurred to be switched off?

 

[ABCwarrior]

I got this Wylex 5 way CU with 100A RCD for £26.85(reduced from £51.85), still for sale as I write.
http://www.screwfix.com/p/wylex-5-way-100a-rcd-board/44086

Compared to a 2 way(63A RCD) at £34 odds a no brainer...

And given that this seems to be the exact same 100A RCD that comes with it for sale by itself at £55 it's a total bargain.
http://www.screwfix.com/p/wylex-100a-30ma-dp-rcd/57083