RFC and Radials for Computers

[Buzz1980]

Hi,

I have a couple of questions regarding RFC and Radial circuits.

Regarding the radial circuits. I have been reading a few post lately regarding the use of a 16amp MCB radial in 2.5mm T&E that when installed on a 16amp MCB that there should be only one socket connected to the circuit. Is there a regulation that states this ? I thought it would be unlimited. As I have done before ran a radial circuit in 2.5mm T&E to feed a router, PC/CCTV, intruder alarm and fibre box and installed on a 16amp MCB type C. This was a temp install but didn’t think there would be any problem as the load would be so low ??

Also in the process of supplying sockets for a school through another company. I have been reading that when supplying sockets that are dedicated for computers they should be wired in RFC with also a high Integrity earth also installed and should be wired in 4mm if not mechanically protected ? Is this correct
Any advice will be much appreciated thanks in advance

 

[Pete999]

Hi,

I have a couple of questions regarding RFC and Radial circuits.

Regarding the radial circuits. I have been reading a few post lately regarding the use of a 16amp MCB radial in 2.5mm T&E that when installed on a 16amp MCB that there should be only one socket connected to the circuit. Is there a regulation that states this ? I thought it would be unlimited. As I have done before ran a radial circuit in 2.5mm T&E to feed a router, PC/CCTV, intruder alarm and fibre box and installed on a 16amp MCB type C. This was a temp install but didn’t think there would be any problem as the load would be so low ??

Also in the process of supplying sockets for a school through another company. I have been reading that when supplying sockets that are dedicated for computers they should be wired in RFC with also a high Integrity earth also installed and should be wired in 4mm if not mechanically protected ? Is this correct
Any advice will be much appreciated thanks in advance

The high integrity earthing applies to circuits supplying IT stuff \,with possible high earth leakage, the is information in the OSG and BS 7671 which needs to be read before continuing with your installation.Sockets with two earth terminals are required ,and in the case of radials the cpc must be wired as a ring, see the diagrams in these books the are self explanatory.

 

[pc1966]

As @Pete999 says this sort of thing is covered in the regs and OSG (pages 86-87), also a quick search will find examples on IET and professional electrician web sites.

While IT stuff is often quite low power, you can get quite a high inrush so you might find the likes of a 16A B-curve a bit trip-happy. You can have multiple sockets off it since it is safe from overload, but it is not good practice due to the likely tripping under what most folk would consider is normal use.

Also you will very likely reach the leakage limit for the RCD side long before you run in to load limits. So with lots of computers in, say, an open plan office area you would probably want to divide it up so you only have 5-10 computer set-ups per RCBO-fed circuit.

Even without the sockets featuring twin CPC terminals, having the ring final arrangement of a looped CPC gives you a significant reliability advantage. It is probably just as easy to put in a RFC as it is to put in radials with an extra CPC to loop at the last socket. Using, say, a 20A C-curve RCBO would make 2.5mm radials (with extra CPC) perfectly reasonable from a cable overload protection pint of view, and surge trip is slightly more than a 2.5mm RFC or 4mm radial on 32A B-curve since 20*(5..10) = 100...200A magnetic trip versus 32*(3...5) = 96...160A

 

[Buzz1980]

Hi thanks for the reply.

Yeah the circuits are feeding several computers so will be installing a ring in 2.5mm but didn’t know if the 1.5mm CPC ring is adequate or have to install a further 4mm ring around all sockets for high integrity earthing ?

Plus the radial on a 16amp MCB have been reading other threads that are saying if on 16amp MCB then can only feed one socket. Where my thinking is that it can feed as many as you want as long as load taking into consideration?

Thanks in advance ?

 

[Pete999]

Hi thanks for the reply.

Yeah the circuits are feeding several computers so will be installing a ring in 2.5mm but didn’t know if the 1.5mm CPC ring is adequate or have to install a further 4mm ring around all sockets for high integrity earthing ?

Plus the radial on a 16amp MCB have been reading other threads that are saying if on 16amp MCB then can only feed one socket. Where my thinking is that it can feed as many as you want as long as load taking into consideration?

Thanks in advance ?

Use sockt outlets with 2 cpc terminals and connect the cpcs to 2 different terminals on the earth busbar

 

[pc1966]

Look at the OSG or regs for the details, but the 4mm (or 10mm) minimum earth is for a single conductor with (or without) mechanical protection.

For a RFC then the normal 1.5mm CPC is fine as you have two paths. Ideally you would also use the sockets with "high integrity" double earths so no single fault can allow one to become floating. That matters if folks plug in extension leads and so one "socket" really could have as many as 4-8 IT things attached leading to significant leakage.

Examples of such dual-earth sockets:

K2746WHI MK Logic Plus 13A 2 Gang DP Switched Socket with Dual Earth Terminals - White | YESSS Electrical

www.yesss.co.uk

K2744WHI MK Logic Plus 13A 2 Gang DP Switched Socket with USB & Dual Earth Terminals | YESSS Electrical

www.yesss.co.uk

 

[pc1966]

A lot of IT folk prefer the sockets with "outboard" switch positions as less risk of accidentally switching off both when stabbing behind the desk for one switch.

 

[pc1966]

Plus the radial on a 16amp MCB have been reading other threads that are saying if on 16amp MCB then can only feed one socket. Where my thinking is that it can feed as many as you want as long as load taking into consideration?

As above, a 16A MCB feeding several sockets is not unsafe - no difference from a fused spur off a RFC - but the risk of nuisance tripping means it is not very good practice. The magnetic trip of the MCB being more likely to go than a 13A fuse.

Unless you have a very good reason for that, going for 20A C-curve is better able to deal with typical usage.

 

[Mike Johnson]

FYI European standards where Radials are the norm are :
16amp @ 1.5mm including CPC maximum eight sockets.
20amp @ 2.5mm including CPC maximum twelve sockets, except in a kitchen where the maximum is six.
Double sockets count as two etc.

 

[Buzz1980]

Thanks for all replies guys much appreciated ??