4Mm Radial Off Ring Cct

[apprentice87]

Hi Steps,

I do not think there is anything that specifically says you coulod not feed a CU from a RF, but they DO say the following;

The load current in any part of the circuit should be unlikely to exceed for long periods the current-carrying capacity of the cable. This can generally be achieved by;

1, Locating socket-outlets to provide reasonable sharing of the load around the ring.

2, Not supplying immersion heaters, comprehensive electric space heating or loads of a similar profile from the ring circuit.

3, Connecting cookers, ovens and hobs with a rated power exceeding 2kW on their own dedicated radial circuit.

I think the idea is, as you have stated, to avoid a badly unbalanced ring...

I suppose you could fit a CU with a 16A MCB as you have suggested, but you would only be 3A better off than you would by using a BS1363 FCU. Still, the CU would make a handy junction box!!!

I would just go for a 13A fused spur, and then at least there is no question that it complies.. Explain to the client that this means that they are somewhat limited [more in theory than in practice] as to what they will be able to run in there, but if they really do envisage large load then they will have to have a dedicated 4mm 32A radial run out there.

I think i would just extend the ring myself if this were possible.

john...

 

[steptoe]

John,

I was just 'thinking aloud' as it were,

as for anything else,

Im not so sure, only an on site judgement could decide, I think.

 

[apprentice87]

Hi Steps, Yes i agree totally. A person would have to see for themselves, consult with the client and then use their best engineering judgement.

Different scenarios would/could all call for different solutions..

john..

 

[binky]

ah, but did you allow for PD over such a large area?

its not simply about Zs or VD , its about potential change,

and I'll bet you never measured that.

Voltages were fine from what I remember, but it was many years ago - I did check just to prove a point! The rooms were big, but not much in them, just a few sockets in usual corners etc etc, so few joins and not being kitchen was only on a 20A MCB. Quite simple stuff so cable lengths not excessive either.

 

[wozz]

ah, but did you allow for PD over such a large area?

its not simply about Zs or VD , its about potential change,

and I'll bet you never measured that.

can you explain this please.

 

[SLIPSHOD & SLAPDASH]

There was an article on unbalanced rings (and kitchen grids) published last year by either the NICEIC or the IEE ( cant remember which ?) I haven't got the intelligence or patience to work out all the variables off the current flow down different parts of the ring due to lengths and resistances of joints to see if its acceptable, although according to chr!s thats all wrong anyhow.

 

[SLIPSHOD & SLAPDASH]

Found this old thread whilst looking, read from post 4 onwards

http://www.theiet.org/forums/forum/messageview.cfm?catid=5&threadid=3177

 

[steptoe]

can you explain this please.

what part are you finding hard?

 

[hxsarge]

its like the whole argument over "ring mains" fed from a single 6mm. i see it a lot in ex council places, where kitchens are supplyed with 2 6mms one for cooker and one that splits into a 2.5 ring around the kitchen. ive heard this is no longer acceptable,(QS asked at last assesment) but i forget what the excuse is?

 

[wozz]

what part are you finding hard?

PD and potential change you mention.

Whats to check? what are the limits?

 

[steptoe]

PD and potential change you mention.

Whats to check? what are the limits?

you have to ensure the earth potential is constant throughout the installation,

faraday cage [?]

thats a similar reason to why TNC-S shouldnt be exported,

on larger installations it can change.

 

[SLIPSHOD & SLAPDASH]

its like the whole argument over "ring mains" fed from a single 6mm. i see it a lot in ex council places, where kitchens are supplyed with 2 6mms one for cooker and one that splits into a 2.5 ring around the kitchen. ive heard this is no longer acceptable,(QS asked at last assesment) but i forget what the excuse is?

Thats not correct. The NICEIC even published an article on this a short while back. They didnt call it a lolypop but something else.

 

[binky]

you have to ensure the earth potential is constant throughout the installation,

faraday cage [?]

thats a similar reason to why TNC-S shouldnt be exported,

on larger installations it can change.

do yuo ever check your self Steps. If exporting outside of building ie to shed I can see an argument for this, or possibly within an industrial unit. But even large domestic property, properly bonded etc etc, I would be 'surprised' to get a problem. 90m2 isn't that big an area

 

[wozz]

you have to ensure the earth potential is constant throughout the installation,

faraday cage [?]

thats a similar reason to why TNC-S shouldnt be exported,

on larger installations it can change.

So how do you test this? As in where do you put the probes? And how can this be avoided at the design stage?

 

[professional]

Hi all,

The regs, and their accompanying diagram make it all perfectly clear...

Unfused spur; "An unfused spur should feed one single or one twin socket-outlet only" Soooo, that is the end of the argument!!

As regards fused spurs, according to the regs and the diagram, they can be used to do a few different things [including fixed equipment], but the killer is, it clearly states " Fused connection unit [FCU] to BS 1363-4" ----- "maximum fuse 13A"

So, that is the end of that then!!!

john...

Absolute rubbish!

By spur, they are assuming a cable of the same csa as those that comprise the ring final CIRCUITi.e. must be capable of carrying a sustained current of 20A. Under certain circumstances, 4mm t&e is rated at 32A and is therefore protected by the cpd rated as same.

People do post such drivel, they really do.

I well remember 32A 4mm radial circuits disappearing from the OSG, only to reappear again when the appropriate committee had a brain cell inserted ....

 

[professional]

you have to ensure the earth potential is constant throughout the installation,

faraday cage [?]

thats a similar reason to why TNC-S shouldnt be exported,

on larger installations it can change.

Earth potential will NEVER be constant throughout an installation.

All than can be achieved is that the potential relative to adjacent metalwork is kept to acceptable limits.

 

[apprentice87]

Hi Professional,

With the greatest of respect, if you think i post complete drivel, i suggest you take it up with the IEE, because what i wrote was transcribed EXACTLY from the regs. If you think it is ok to spur off a ring and protect the spur with a 32A ocpd, then, if you are happy with that, then carry on, why would i care..

I never mentioned 4mm radials, cannot see the relevance to the original post...

john...

 

[philbas]

Not sure I should add to this after a fair amount of Dr Bells medicine, but we need to ensure the cable is protected we have 32A protection, 4mm is good for that, a balanced ring can handle that, what we have to ask is have we unbalanced the ring to much, only the man/women on the spot can decide that.

 

[SLIPSHOD & SLAPDASH]

 

[apprentice87]

The illustration in the regs clearly show the options available, and they clearly state either a single socket unfused spur, or a spur with more than one socket or feeding fixed equipment HAS to be via a FCU and they clearly state "13a maximum" If that is what they say, that is what they say....

Not my fault, i did not write it, but i presume they had their reasons when they did!!!!!

john..