Use of an RCD

[craig1000]

Hi all, this post is going back to a previous one by myself,

In short though after quoting to install a split load cu for a domestic property I also needed to incorporate a supply running upstairs to another CU in a self contained flat (loft conversion)!
In trying to prevent nucience tripping yet still rcd protect the 16mm T& E running up to the flat I added a an additional RCD in the CU making a it a 3 way split load CU & fed the flat from this additional 63a 61008 RCD (tin hat time). This at least left the cable supplying the consumer unit protected & the circuits off the flat CU also protected. I went through many permatations to decide the best one & at the time on here it was agreed i was caught between a rock n a hard place.
Anyway when filling in my test sheets what would my max ZS permissable be, as per table 41.5 30ma RCD-1667ohms? Shall i change it so there is a mcb after the rcd? RCD is 63amp & the upstairs CU has a 50 amp shower on it, & cant get a 63amp mcb to go after the rcd downstairs, if i went to 50amp(highest i can get) then it could cause a prob IF shower was on & ran right up along with some lights & sockets in use, doubtful i guess but possible.

 

[trev]

Mate what are you doing about discrimination? From what I can make out you have your standard split load CU to which you're adding another RCD, why I have no idea, and that is going to feed the 16mm to the flat where there'll be another RCD protected CU. All of these will (possibly) be 30ma so you'll have no idea which will trip in the event of a fault. Might be all of them.

 

[Darkwood]

Sounds to me like you upstairs circuit has no overload protection can you confirm its a 63amp rcd and not a 63amp rcbo, you do realise 63a on a rcd is the max current it will tolerate and not a overload rating that it will trip on.... rcd's dont afford overload protection, get you college notes out and remind yourself of this area.

 

[craig1000]

Sorry to have been do confusing! This is work in progress which I am trying to make sure correct! As per previous posts by me I am new to domestic!
Correct rcd doesn't offer overload protection but of I rcd it & then Mcb the biggest available is 50a which is the same size as the biggest Mcb in the upstairs cu, which along with a 32 & 6 if all were on which is feasible could overload & pop!
Do I use a Rcbo for the supply cable!
I wasn't going to alter the flat cu as it isn't part of my works! As you say I am aware if I then put an rcd on the flat cu I would have no discretion between 30ma rcds/Rcbo
Looked into time delay rcd downstairs but that then would t meet refs either as it's covering a concealed cable etc( previously had this discussion when I first posted about it a few weeks back) I am looking to add the extra rcd or separate the supply to upstairs cu to prevent anything from this cu tripping & knocking out either of the house split load rcds!
I oils install a separate switch unit next to cu but part of the idea of works was to condense the previous 6 various cu's & switches in the garage so want to minimise where I can & feed upstairs cu from main cu

 

[trev]

6 CUs already in this install mate ? What do your max demand calcs say? This is starting to scare me Craig, can you not get a separate supply installed for this flat

 

[Darkwood]

i would have personally run a 16mm swa up as you are feeding multiple circuits so your earth wont be big enough in a 16mm t&e, this wouldn't then require rcd protection at the front end but could be afforded in the flat, tripped rcd would then be within the flat to reset, i would have put an 100amp isolator in feeding an Henly DP block with the house fuse board fed of it and a little 80 amp metalclad 1way fused isolator feeding the flat.
Loading calcs would need to be addressed and diversity worked out as by the sounds of it your going to adding alot more heavy load circuits and may worse case require a supply upgrade but at minimum a request for the cutout fuse size is needed so you can work it out, consider if your cutout only had a 40 or 60amp fuse in it then you could be walking into big problems.

I not too sure you get the complexity to ensure the design and installation is safe and correct alot of it isnt just a matter of asking questions on here, an indepth knowledge of the whole installation is needed and if you dont ask the correct questions the important issues may be missed.

 

[craig1000]

No not 6 cu for install, there was previously a total of 6 covering the house!
I have condensed as part of the works to one 14 way split load for house & 1 6 way for garden externals! Then they have the self contained flat! It is nothing more than a loft conversion but with a locked door & metered separate cu! Shower, ring main & lighting circuit!
The problem is trying to rcd protect the cable going up to thelindt & also the loft cu itself & maintain discrimination. If protect both no idea which will trip under fault, if time delay I break regs! Technically as long as supply cable is protected that would also leave its final circuits protected?! As said want it on seperate rcd so that if a fault occurs it doesn't take out either rcd covering rest of house!

 

[craig1000]

I also would have run a swa but it is an existing install I am working on! It is 100amp supply & I have installed a main isolator feeding dp henlys blocks, split to main house cu & a small 6 way for graden!
I am trying to prevent another cu/isolator on wall, either way I am stuck with the 16mm t&cpc so rcd protection is needed!
I am stuck making the best & safe an existing install!

 

[Darkwood]

Ive just explained the use of a swa cable means you dont need to protect the supply cable up to the flat and you can then change the main switch in the flat fuseboard for a RCD main switch, the front end would be into a 80a fused isolator metal clad to allow easier glanding and this is fed direct from an henley block which also feeds the 14way split board, but to comply you need a main isolator up front that shuts the whole installation off this is then tailed to the meter.
This will have no discrim' issues as its effectively an independent circuit not from the 14way board, but will re-iterate your 16mm t&e wont comply as it is unless you have run a 10mm earth with it additional but is still likely to need rcd protection up front where as a swa cable dosn't require rcd protection for your situe

 

[trev]

deleted for now

 

[craig1000]

I know u explained that about the swa & I know that myself but as I have explained I have not ran in the supply cable to the flat cu, it is an existing install!
I have positioned a main isolator so that the whole install can be shut down as per regs, tailed from the meter! As I have said the issue is making good of an existing install!

 

[Darkwood]

getting it a bit more now, either way you still have to run a 10mm earth unless its already has one run in addition to the 16mmT&E? so thats why i suggested putting in swa in which will get around alot of the issues you face, if you going to have to run a earth cable to make the 16mm comply the you might as well forget that and do an swa which would clear all the other points you have concern with.

 

[telectrix]

just a thought. as the feed to the flat is existing, is it essential to RCD protect it, as you have not installed it yourself? IMO, that is applying regs. retrospectively, and , IMO, could be noted on the cert as a non-comlpiance with current regs.

 

[Mark_Burgess]

I know u explained that about the swa & I know that myself but as I have explained I have not ran in the supply cable to the flat cu, it is an existing install!
I have positioned a main isolator so that the whole install can be shut down as per regs, tailed from the meter! As I have said the issue is making good of an existing install!

Have you taken in what Darkwood has said about the earth supply to the flat?

 

[craig1000]

Yes I have taken into account t what dark wood has said & when I meet with custormer it is something I will be discussing! Ultimately I cannot force a customer into changing something especially from te customers point if view it has been working fine!
I shall be letting them know I cannot certify it in its current condition as conforming to regs!
As lee telectrix has said with that item not being my work & only the cu change being mine if the customer will not allow or be prepared to change it all I can do is make sure they were aware that part does not comply & note it on the cert! The rest of the install is fine & shall comply when I complete the works!