RCBO in office consumer unit

[Stringman]

Hi all
Now being upfront I am not a sparks.
I am about to have some work done by a friend (certified) and as he is doing me a favour I want to as much ground work and speccing up as I can.
So heres the situation.
we have a summer house at the end of the garden which is being fed by swa from a 32a rcb in the house.
This goes to a 2 way garage cu to feed the power and lights. All works well but if some thing trips on the circuit it trips the main rcd in the house.

I am building an office between the office and the house.
I have inherited some 2.5mm swa so my idea was to swap out the 2 way cu in the summerhouse and fit a 4way cu with a 20a rcbo to feed the office.

Now the office has a shed attached to each side and I would like power and lights to these as well.
My original idea was to use the old 2 way cu to feed power and lights , but I think it would be safer to have a separate breaker for the sheds and one for the office.
There wont be a huge load in the office, couple of laptops, router etc , an echo and a mini fridge. Again not much in the shed maybe a fridge and then the odd use of power tool and lawnmower etc?

Now bearing in mind the feed for both i protected by the 20a rcbo is there any point in using individual rcbo?
What I am hoping to achieve is protection such that if do something stupid in the shed and it trips then it doesnt trip the office.
I dont mind paying for the rcbo but if theres no point then I wont

Thanks all

 

[brianmoooore]

RCBOs, RCDs and MCBs connected in series don't quite work in the way you are assuming. If the fault current or earth leakage (as appropriate) is high enough to trip the highest rated device, then they will all trip. Fuses do tend to provide discrimination, as I proved to myself about an hour and a half ago, when I carelessly shorted live and neutral together of an extension lead. 32A MCB turned off, but 13A plug fuse survived.

 

[littlespark]

Leave the speccing up to the electrician friend.

Nothing worse than taking advice from an online source like a forum, just for them to then say, no… I wouldn’t do it like that. It’s all the wrong stuff.

Yes, dig trenches, drill holes, make routes for cables…by all means…. but ask him what is required, and where he wants holes drilled.

 

[Stringman]

Leave the speccing up to the electrician friend.

Nothing worse than taking advice from an online source like a forum, just for them to then say, no… I wouldn’t do it like that. It’s all the wrong stuff.

Yes, dig trenches, drill holes, make routes for cables…by all means…. but ask him what is required, and where he wants holes drilled.

thanks for that, its kind of the answer i was expecting ( as I know its the correct one) but I like to understand things as much as possible, and one part of says "the circiut is protected by the 20q rcbo so the house wont trip" But I also wonder if by having the shed on a rcbo it protects the office from tripping?
In my mind i picture it as a linear thing and by putting a rcbo in before the 20a one if the 1st one tripped would it stop the 20a tripping?

 

[Stringman]

RCBOs, RCDs and MCBs connected in series don't quite work in the way you are assuming. If the fault current or earth leakage (as appropriate) is high enough to trip the highest rated device, then they will all trip. Fuses do tend to provide discrimination, as I proved to myself about an hour and a half ago, when I carelessly shorted live and neutral together of an extension lead. 32A MCB turned off, but 13A plug fuse survived.

Brian That makes perfect sense ( starting to get my head around it) so if there is a fault in , say the shed, the 16a should go before the 20a?
And regardless of if he uses mcb or rcbo it must make sense to have office and sheds on different ones ( can you imagine the look I would get if I tripped her zoom call!!!)
I know that in the summer house tnhere have been the odd occasion where something has tripped and its taken the main house rcd. That is what I want to avoid.

Thanks agai

 

[nicebutdim]

The original post could be clearer, but I believe you're essentially asking if the use of a lower rated rcbo would prevent tripping of an upstream RCD.

Depending on the type of fault that occurs, it's very likely that your plan won't resolve this issue. An rcbo essentially combines the function of both rcd and mcb. Placing one downstream of an rcd will still afford protection against faults, but it's very likely that a fault involving current leaking to earth will cause the upstream RCD to trip.

Make clear to your electrician that you want to take every precaution to avoid this circuit tripping back at the house and they'll probably put forward a very different proposal. You mention that the upstream RCD trips on occasion and extending the existing arrangement is only going to increase that likelihood.