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?

 

[Richard Burns]

A 100mA RCD might trip before a 30mA RCD depending on the type of fault and the characteristics of the RCDs.
To ensure discrimination the 100mA RCD would need to be time delayed i.e type S.
If I were installing I might tend to go for an upfront time delayed 100mA RCD and then a split load Dual RCD board, though obviously RCBOs would be better if you can get them in double pole versions.

If you had good Ufer earthing and the resistance to earth was then low enough to trip your MCBs then you would be OK with just RCBOs.

 

[Deleted member 26818]

You can use single pole RCDs with TT.
There just has to be Double Pole isolation somewhere.
The 100mA should be an S type to be used with 30mA RCDs.

 

[Yr ys]

How low would the resistance to earth have to be to trip a neutral to earth fault? My home has a radial foundation 50cm thick concrete and rebar. I'd have to break the concrete to access the rebar and then connect a 16 mm2 earth cable about 2 meters from my consumer unit. Then reseal the concrete to prevent corrosion.

My power is coming from a 10000kva inverter with a 60 meter long 16 mm2x2 10mm2 earth cable just for your info.

 

[Deleted member 26818]

10 million VA?

 

[Yr ys]

Correction, 10000VA