Rcd- mcb

[davidcaulfield]

Hi guys in installing a 16mm submain with a 10mm earth and I'm going to install a 80amp time delay RCD with a 63amp MCB to protect it. Some one said I could just use a 63amp RCD to protect it? Never done this before and wouldn't think you could.

Also should I just use a 63amp RCD with a 63amp MCB

 

[Darkwood]

Deleted OP whipped my butt then i realised i mixed this thread up with another Oopps!:bucktooth:

 

[davidcaulfield]

Yea I could get round that.

It's just my old boss use to use time delay RCDs all the time to stop nuisance tripping

When he did a periodic report he would list two 30ma RCDs inline.

On an existing insulation with cables installed is there Any way of protecting the consumer unit and submain without discrimination?

Where should a time delay RCD be installed then?

 

[D Skelton]

Time delayed RCDs can be used to protect submains to achieve a specific disconnection time allowing for discrimination with other RCDs further on in the circuit. They cannot be used for additional protection, only 30mA RCDs and supplementary bonding can be used for additional protection.

If you can protect the T+E with an earthed metallic plate at least 5mm thick then do that, if you can't, you either put up with having the whole submain on one RCD or rip out the T+E and replace with SWA or similar.

 

[davidcaulfield]

Dark wood i find you a bit rude and insulting.

I have not installed the submain

The will be not be exporting the earth as it it's a granny flat connected to the existing house. Basically a big extension?

My old boss used a time delay RCD and I was just after more information on this method as I've never done it myself. (Obviously he was not doing it correctly)

I came on here for a bit of friendly advice that's all. In future I think I will not ask questions to improve my knowledge and try to keep learning just incase I get this sort of response

Thanks for putting me off this site

 

[Darkwood]

Apologies David i actually mixed your query up with another thread!


I suggest you throw away your teachings from your old boss as it sounds like his --- and elbow must have looked very similar to himself, the supply cable while it stays as is requires rcd protection 30mA - No time delay

Take the board out and fit a standard board mainswitch with mcb set up's or replace the submain supply with a mechanically protected cable like swa.

 

[davidcaulfield]

Take the board out and fit a standard board mainswitch with mcb set up's or replace the submain supply with a mechanically protected cable like swa.

Does this not mean there's no dual RCD protection?

Every time the RCD trips they have to go to the garage to re set it as opposed to going to the local consumer unit inside the granny flat?

 

[D Skelton]

Yep, that is the nature of your predicament.

 

[Darkwood]

Can i just make sure we have this set-up correct...

Main house with built on garage- mains supply comes up in garage and you are sub-maining to a built on granny flat extension, effectively structurally all one building then?
No gaps between the buildings?

Sorry but re-reading the thread you gave us a different impression when you mention garage and submaining to granny flat thus exporting earth would be needed to be assessed.

 

[davidcaulfield]

Hi the granny flat is directly connected to the main house. The garage is connected to the main house Aswell. Sorry for any confusion

Reading other threads they say exporting the earth doesn't really make a diffrence whether is Tns or tncs. I do need to swat up on this.

I understand it if its an outhouse less than 2metres away with no extraneous conductive parts it doesn't need a tt system? Such as an outhouse with a socket and light

 

[davidcaulfield]

D Skelton thanks for you advice and knowledge.

I am looking to do a job of 30 bed sits

I don't want to use swa ( not enough room to run them all through the joists)

I will have to use 2 30ma rcds inline again?

 

[Darkwood]

This is the info we lacked when you progressed through your thread and we have to assume by what you post ....hence a few of us brought up the exporting earth ...if it was a garage several meters away and of a exposed steel contruct then this would be a possible situation for TT-ing the garage. We now know this is all irrelevant as its not the case... but i think you are now clear with where your issues lie regarding needing to protect the submains with both overload and 30mA rcd protection.

Yes your in a corner if the rcd trips then to the garage it is.... only way around it is to mechanically protect the existing sub supply or replace the existing with SWA.... you have your options now put your argument forward for replacing the sub mains cable.

You could just have the set up as is with 63amp mcb and 30mA submain protection on the 17th edition board and yes poor design and not professional to leave as such but if not your design just argue your corner and do within reason what they ask.

 

[Amp David]

Hello David.

The RCD is there for the protection of personnel who may inadvertently come into contact with live a.c. mains. This will not happen on a sub-distribution cable, so no RCD is required for this. The cable itself requires protection from short circuit conditions, and this is what an MCB does. A correctly-rated MCB will be required at the supply end. If personnel protection is deemed necessary for the final sub circuits, this should be provided at the remote end of the supply cable. Nuisance trips become an issue if this is installed at the supply end. If anyone sticks a drill or similar into the cable, the MCB will oprerate and disconnect it.

Regards,

Colin Jenkins.

What a load of balls. Never heard so much rubbish in a single post

 

[Darkwood]

D Skelton thanks for you advice and knowledge.

I am looking to do a job of 30 bed sits

I don't want to use swa ( not enough room to run them all through the joists)

I will have to use 2 30ma rcds inline again?

No you won't if this is the case the design is wrong and you should change it, if you have other trades on site you have a site meeting you express the need to run x amount of swa cables maybe a riser or ducting can be formed by the builders as a cable run.

Ive just done 20 block of flats all flats have a swa DB supply the cable runs were discussed before pricing and the layout of the room plans had to be altered to allow for my supplies.

Dont box yourself into a corner you can't get out off ..... your the one they will ring when the front end rcd trips especially if the tenants don't have access to say the landlords meter room.

 

[Darkwood]

What a load of balls. Never heard so much rubbish in a single post

Read on further he's already taken the firing squad and admitted the dunce award.

 

[Darkwood]

Lastly a point no one has brought up ....because the submain is a twin and earth cable the earth would be required to be the same size as the live conductor as it supplies earthing for multiple circuits .... if you haven't run additional earthing you will have to show the earth complies to 543.1.2 (i) ---i.e. with use of the adiabatic equation ....

the above is something to remember if you doe run the proposed flats in T&E..

 

[GLENNSPARK]

So you have to use 2 30ma Rcds inline with each other?

this is not competance david....
its babyish...

 

[Guitarist]

Dark wood i find you a bit rude and insulting.

I have not installed the submain

The will be not be exporting the earth as it it's a granny flat connected to the existing house. Basically a big extension?

My old boss used a time delay RCD and I was just after more information on this method as I've never done it myself. (Obviously he was not doing it correctly)

I came on here for a bit of friendly advice that's all. In future I think I will not ask questions to improve my knowledge and try to keep learning just incase I get this sort of response

Thanks for putting me off this site

Both darkwood and D Skelton have spent a lot of time trying to help you with something which, as you have accepted this job, you should really already know. Be grateful.

 

[Darkwood]

Both darkwood and D Skelton have spent a lot of time trying to help you with something which, as you have accepted this job, you should really already know. Be grateful.

In his defence this comment was justified as i wrote a now deleted post where i mixed my threads up by accident and replied in essence of another thread.

 

[Aindowsimon]

Wot is the issue with a 30mA type s rcd ? All I know is that supplementary protection is desired by the sub-main and by a 30mA rcd is there a reg on that the type s rcd ar not suitable for additional protection ? . Why don't you surface wire the t+e ? I understand it isn't aesthetically pleasing , neither is it convenient to lose all you're circuits to a lamp failure

 

[D Skelton]

Good luck trying to find a 30mA Type-S!