Ring circuit cable length

[Yr ys]

Hi all

I have a ring circuit with 13 sockets. 2.5 mm 3 core cable. My floor area is 60 sqm My question is what is the maximum wire length for a single ring circuit? I used 92 meters of wire for the whole ring circuit.

Also I only had a c type rcbo available at my local electrician. Will this be OK or do I need a type b? It's a TT system. Another question is, do I need a rcd in TT systems as the rcbo is a single pole? Would a 63a 100ma rcd be sufficient?

Thank you

 

[jay sean]

If I remember a ring final cct can serve an area up too 100m2
about the c/ b type MCB, this will depend on the Zs of the installed ring main

 

[The Ghost]

I am guessing the C type RCD will not meet disconnection times but then you are in Nicosia? Might be entirely different there. Have you tested it?

 

[Yr ys]

Hi and thank you for your reply .

There really isn't much regulation going on but I want it to be safe for my family. So I want to do what's best. I will have to find type b rcbos for the sockets but I think they will be OK for 6amp lighting circuits.

 

[westward10]

Type characteristics are not really overly relevant as you have TT system.

 

[Deleted member 26818]

The 63A 100mA RCD is ok for circuits which don’t require 30mA RCD additional protection.
Sockets require 30mA RCD additional protection.

 

[westward10]

He is in Nicosia.

 

[Deleted member 26818]

He is in Nicosia.

Isn’t that in Cyprus?
Cyprus uses BS7671 as far as I’m aware.

 

[westward10]

Isn’t that in Cyprus?
Cyprus uses BS7671 as far as I’m aware.

No idea to be honest.

 

[telectrix]

yourtype C Rcbos will be fine as long as (for a 32A) the Zs is < 0.54 for the circuit.

 

[Richard Burns]

In accordance with BS7671 and in the UK table 7.1 in the on site guide the maximum length of cable when using a 32A type C RCBO would be limited to 82m on the basis of the current flowing on a line neutral short circuit overheating the cable where the resistance of the Line Neutral external loop was 0.35Ω. If you were using a type B then the limit would be 106m. However if the L to N loop resistance were lower then the disconnection time would be faster and less strain on the cable.
Using a single pole RCBO on a TT system would not clear a neutral to earth fault and this would progress back to the next RCD. For BS7671 then there should be double pole isolation at least somewhere in the installation.

 

[Yr ys]

Thank you for extensive answer

Is it a good idea to use a 100ma rcd for isolation ?

 

[telectrix]

re. ^^^^^.i would only use one if i had to e.g TT earthing, as if it trips, youlose the whole installation. rotten fish in frezer etc.

 

[Deleted member 26818]

Frozen fish in Cyprus?
I’d expect fresh fish daily.

 

[Yr ys]

Yes we eat our fish fresh here but I have to get my Cumberland sausages frozen so it would be good to avoid a whole house fault trip.

But I can't use single pole rcds with tt earthing. What if it was ufer earthing? Also is it good to use 100ma main rcd with 30 ma rcbos?