Rcd protection

[piz]

Hello again
I know this might sound stupid bit it aint if you dont know
I do some Electrical work for one of the national kitchen diy stores only just started and on my paper work it says
install RCD protection to existing ring main
ok I can obtain a rcd shower/garage unit with an rcd and 32amp mcb a henley block double pole and some 25mm tails break in to the main tails screw on my mini unit remove the ring from the old wylex board connect it to the new breaker and then it is protected not to forget the earth
is there any other way someone said it can be done with some 6mm cable but i was unsure what he meant
guess this is a long winded way any ideas or better ways would be great
Cheers

 

[Mark.W]

It's been a long week ok, well that's my excuse anyway, and I'm sticking to it.

 

[Darkwood]

Then why 6mm cable? a ring is surely designed using 2.5mm cable, so wouldn't you use 2.5mm?

2.5mm twin and earth isnt rated for 32amp but in a ringmain you can use it because the load is shared between the incomming and outgoing legs proportional to the distance from the board each leg is , e.g. if you put a 13amp load on the exact midpoint of the ring in theory the load would split 50/50 6.5 amps one way and 6.5amps the other, with other loads on the ring this sum gets quite complicated.
In theory you could carry your ring through the rcd unit to the fuseboard if thats what you mean but not a method i use.

 

[DeanoZX7R]

Think you getting mixed up here the cable from c.u. to rcd unit has to be able to carry the same as the ring main was designed to carry and be rated for the 30a fuse/ 32amp mcb that is fitted, the rcd is a monitoring device for earth leakage only and can be rated anything above the mcb, so a 63amp rcd unit still only requires a 6mm cable.

Sorry darkwood, you are quite right, I meant cable of sufficient capacity to withstand the rated trip/rupture of the relevant circuit mcb/fuse.

 

[electricAl]

Sorry darkwood, you are quite right, I meant cable of sufficient capacity to withstand the rated trip/rupture of the relevant circuit mcb/fuse.

aka "big enough"

but that`s a bit of a mouthful

 

[DeanoZX7R]

I know.......I know...... to think I was seriously considering becoming a solicitor... lmao

 

[electricAl]

I know.......I know...... to think I was seriously considering becoming a solicitor... lmao

Rrrrrr...& they charge for each letter.

Ya cud ha made a fortune Deano

 

[lister]

6mm cable is capable of carrying 32A comfortably.

2.5 is only rated at around 27A.

Come on now, basic stuff!!!!

so why not use 6 inches of 4mm cable

 

[Spudnik]

so why not use 6 inches of 4mm cable

I wondered how long it would be before someone mentioned 4mm

 

[lister]

Sorry

 

[Darkwood]

so why not use 6 inches of 4mm cable

Generally 4mm is made with a 1.5 earth which would effectively half the ring main csa fault path, having said that its been overlooked by all including myself that 6mm t&e comes with a 2.5 earth so to keep the earth size right you may need to run a additional 1mm earth or just do it all in singles to start with.