RCBOs on an RCD'd mainswitch

[DomB]

Hi,

If I had a CU with an RCD'd main switch and put in a new circuit on an RCBO, would the RCBO always be the first to trip should there be a fault on that circuit?

Thanks!

 

[tony mc]

Depends on what the RCD is as a main switch.

More info required but if you have a 30mA RCBO and a 100mA type S main switch then the RCBO should trip first.

 

[polo1]

As Tony says; if both 30ma, you cannot guarantee discrimination.

 

[DomB]

Thanks. It's a new CU going in and the customer is hankering after RCBOs for some circuits even tho we have an RCD'd main switch.

If I get a 100mA main switch then I guess we have to RCBO the socket circuits so they're on 30mA per reg's?

 

[tony mc]

If you put in a 100mA main switch it will have to be a time delayed type to have any sort of dicrimanation.

 

[DomB]

p.s. is discrimination a problem if it's happening within the same CU - thought it was only a problem if the user didn't know where to look in the event of a trip?

 

[tony mc]

Sorry Dom but if a cct is covered by a RCBO say down sockets and a fault occurrs then only that RCBO should trip and not the main switch which will cover the entire premises.

So to avoid that you need to design the CU so that would not happen!

 

[ringer]

p.s. is discrimination a problem if it's happening within the same CU - thought it was only a problem if the user didn't know where to look in the event of a trip?

Discrimination is a problem if ALL the power is lost because the main RCD trips before the RCBO kicks in. that is the whole point of discrimination - determining which trips first and considering convenience for the user.

 

[Amp David]

Must be a TT earth?

 

[1shortcircuit]

Why can't you replace the 30mA RCD for a 100A Main switch and RCBO all circuits?

How many ways are in use in this board?

Who's the electrician here? Whilst it's important to listen to the customers requirements it's YOUR job to explain to them how YOU intend on doing it. If you cannot come to an agreement then politely advise them that they should seek someone else's services

Can see the future aggro on this one already, since you fitted my consumer unit exactly as I told you to do it my toilet doesn't flush lol

 

[1shortcircuit]

Must be a TT earth?

Had to go back and read the original post after your reply. Think you may well be right here:thumbsup

 

[Amp David]

Thought i'd ask because what threw me out was the fact its a new CU going in but theres a 30ma RCD as a main switch. Thought i'd mention the earth setup as OP never mentioned it.

 

[Amp David]

Thanks. It's a new CU going in and the customer is hankering after RCBOs for some circuits even tho we have an RCD'd main switch.

If I get a 100mA main switch then I guess we have to RCBO the socket circuits so they're on 30mA per reg's?

Not just your sockets chief. What about circuits supplying any special locations and circuits that have the cable installed <50mm below finished surface.

Sounds like a right old cobbled together mess

 

[DomB]

Thanks all. Agreed re the customer trying to dictate what's done; I just wanted to be able to put him straight without any come-back.

It is a TT system, yes - spotted!

I'd rather not do RCBO's all round mainly cos of the cost.

I was going to keep it simple, get a 17th edition dual RCD CU (15 Way Dual RCD + 100A Mains Switch) and split the circuits wisely across the two.

He's calling for RCBOs for his alarm & heating circuits. I can see his point - should he be away and something trips the main RCD which affects his alarm circuit then that might not be best..

grateful for your advice chaps

 

[1shortcircuit]

Okay so you're going to fit a 17th edition board and separate the circuits, got that bit now. So, if it's TT and already has a 100mA RCD... up front then BIN IT! You will no longer need it and by doing so you will not have discrimination issues to worry about.

If this customer insists on having the smokes on an RCBO then advise him that he will not know if it trips but providing they're mains/battery powered this will not be an issue. You will need a high integrity board that allows you to have RCBO's connected before the two RCD's.

I actually now prefer to fit smokes with a commonly used lighting circuit for obvious reasons but a high integrity board does have advantages for supplying other circuits too such as future power supply to shed/garage or Fridge etc

Have I made sense? I know what I mean but have I put it across okay?

:thumbsup

 

[nickblake]

If your customer want RCBO's do as short circuit says or to reduce the costs a little buy a split load board and go half half

 

[DomB]

Ah that's brilliant - thanks, you pointed me right to it - I see Wylex do a Dual RCD Split load high integrity board which would allow me to put in the RCBOs for those HI circuits.. Perfect.

 

[1shortcircuit]

He's calling for RCBOs for his alarm & heating circuits.

I should read more carefully in future

:thumbsup

 

[DomB]

Thanks Nickblake, I was looking for a way of splitting out the circuits across two RCDs yet still have High Integrity circuits. Wylex's "HIGH INTEGRITY DUAL RCD SPLIT LOAD UNITS" look to be the go

 

[nickblake]

if you get a 15 way hi integrety wylex board they supply bus bars you can cut so you could have a board 5 x 5 x 5 being 5 rcbo's then 2 lots of 5 MCB's protected by 2 RCD's , but you configuer them as you want