Double socket spurred off of Ring.

[The Ghost]

Ok I agree BS1363 four hours 14a 6a and so on. But somehow I doubt it really especially on cheaper sockets. Maybe MK and some others but In a commercial kitchen I was working in where they are running loads of 3kw items and for at least several hours a day, it is hard to whole heartedly advise that is OK. But then I suppose we are talking domestic. And anyway I think they should have had a single socket for each appliance.

 

[westward10]

I agree we used to work for one of the leading fast food outlets, chicken to be precise and they had multiple radial 13A points for individual appliances. You could have six sockets at one point covered by six circuits.

 

[Mark42]

Hi CFTIS
A bit of a digression, but the vagaries of RFCs are something which have annoyed me for decades. I personally think they are obsolete now that WWII is over and copper is plentiful, but it appears I'm in a minority here.
See this thread for details:
Can I join 2 radials to make a ring? - https://www.electriciansforums.co.uk/threads/can-i-join-2-radials-to-make-a-ring.123486/
I always enjoy splitting rings at the furthest point and converting them to 2 x 20A radials.
I believe rings are potentially dangerous after only a single (and common) point of failure (ring break). It's the same intuition that made you question the (regulation-acceptable) issue of daisy-chained spurs on a 32A breaker. Well done for obviously thinking it through.
We will now get replies saying that no one would ever plug multiple 13A loads into a single ring. Sure, that may be true, but if we assume it is always true, why then do we ever need to protect plug outlet circuits at 32A? That's my point. You can't have the argument both ways! (And I'm assuming good installation practice with no heavy fixed loads - eg immersion heaters - spurred off the ring.)
I hold that just because the regulations say that something may be done in a certain way does not ALWAYS make it the best, or even the right, way in every situation. And the converse is also true

 

[ipf]

Hi CFTIS
A bit of a digression, but the vagaries of RFCs are something which have annoyed me for decades. I personally think they are obsolete now that WWII is over and copper is plentiful, but it appears I'm in a minority here.

Each to his own...

 

[M-Ty]

I suspect this has been done many times before, but given that the CPC is smaller than the others in T&E, it may be more prone to damage. Therefore an O/C live conductor in a ring is less likely than an O/C earth in a radial, and equally unlikely to be noticed. It is then arguable whether the typical loading on a broken ring (effectively two radials) is more likely to cause danger by exceeding the rating of the cable than the lack of an earth on a radial past the broken part.

 

[marconi]

I have wondered why the mcb/rcbo makers have not yet made a specific breaker for the ring circuit which has 2 pairs of LNE terminals for the legs to be connected to and internal functionality to detect a break in any of the 3 rings and then trip.

 

[ian.]

Marconi that's a really good idea.

 

[Mark42]

... It is then arguable whether the typical loading on a broken ring (effectively two radials) is more likely to cause danger by exceeding the rating of the cable than the lack of an earth on a radial past the broken part.

That's a very good point which I admit I had never thought of before.
That rings have - by accident - what is effectively a high integrity earth is useful, but not useful enough to convince me to use them, unless protected at 20 or 25A (which I have occasionally done.)
Your point about an unnoticed broken earth applies of course to all types of circuit.

 

[Mark42]

I have wondered why the mcb/rcbo makers have not yet made a specific breaker for the ring circuit ...

Probably because there's little market outside the UK.
I once installed some RFCs, using UK sockets, in Asia, but could not get the local contractors to understand why they were wired like that. They were completely puzzled, and thought I was a madman.

 

[Lucien Nunes]

why then do we ever need to protect plug outlet circuits at 32A

Diversity & flexibility. Suppose you have three 2.4kW loads (this is an artifical case to illustrate, I accept it is stretching the point, but 3kW appliances are not as common as they used to be). You can plug them all in anywhere on a 32A circuit without further thought. With 2x 20A although you have 8A more available in total, you can't plug those three loads in anywhere without technically overloading one circuit.

The 32A vs. 20A debate is a separate consideration to the ring vs. radial debate, as you can have 32A radials which might be the best of both worlds.

 

[Deleted member 26818]

As an RFC is essentially two conductors in parallel, why would you want two terminals at the breaker?

 

[Lucien Nunes]

Marconi's idea is to not connect the ends directly at the DB, but to insert a measurement shunt or balance transformer winding of low resistance, that can detect the current flowing in the two legs separately, or inject a tiny measurement current that flows around the ring. This would allow the breaker to detect an open-circuit in the ring even while all the connect loads can still see L, N & E. It wouldn't work reliably for the CPC though, as there may legitimately be parallel paths to various points that bypass the detection circuit or disturb its balance.

You could make a simple detection scheme for the L & N, that would respond to one conductor being broken, using a low-sensitivity balance transformer added to an ordinary RCD. One leg would be wired through it and one would bypass it, so that it would only see balance provided the current in L and N divided between the legs in the same ratio. Any single break would change the division ratio in that conductor but not the other, causing the detection transformer to trip the breaker. An ordinary balance transformer would still be needed in the unit to provide shock protection. It would not detect both conductors broken in the same place, as the two disconnected legs would inherently be in balance again.

Kitchen table project for Marconi... Mock this up using a 4-pole RCD. Both legs through poles 1 & 2 for shock protection, one leg only through poles 3 & 4 as well for ring break detection. Adjust sensitivity and trim balance by shunting poles 3 & 4.

FWIW some of the early lamp failure detection schemes in cars used the difference in currents between left and right circuits to trigger the warning with a differential relay. Both lamps out would not be detected.

 

[Mark42]

... Suppose you have three 2.4kW loads (this is an artifical case to illustrate, I accept it is stretching the point, but 3kW appliances are not as common as they used to be). You can plug them all in anywhere on a 32A circuit without further thought. ...

Indeed you can, and that's exactly my point: if one ring conductor is open circuit, you then have a potential (and entirely invisible) cable overload, especially at the far end of long rings.
I agree both our arguments are stretching the point, as I don't believe many people do actually plug in multiple 2.4kW / 3.0kW loads any more. But if that's so, one doesn't need the 'flexibility' of 32A.

 

[Mark42]

.

 

[w0z]

Indeed you can, and that's exactly my point: if one ring conductor is open circuit, you then have a potential (and entirely invisible) cable overload, especially at the far end of long rings.
I agree both our arguments are stretching the point, as I don't believe many people do actually plug in multiple 2.4kW / 3.0kW loads any more. But if that's so, one doesn't need the 'flexibility' of 32A.

It's the middle of winter and minus 5 outside, your boiler breaks so off you go to find some 2.4kW heaters to keep your family warm. You have 2 , you borrow another 2 off a mate....

 

[Dave OCD]

Then the MCB trips off and you realise you're being a silly arze.

 

[Toneyz]

Probably because there's little market outside the UK.
I once installed some RFCs, using UK sockets, in Asia, but could not get the local contractors to understand why they were wired like that. They were completely puzzled, and thought I was a madman.

It will never work Mr Pete