Double socket spurred off of Ring.

[CFTIS]

Ok so I'd like your opinions on this as I seem to be the minority on this.

I'm fairly newly qualified and obviously still learning everyday.

Electricians I work with and everywhere on the web says that spurring a double socket from another that is on a ring final circuit is fine, as long as it's just the one!

Ok so let's set the scene, one double socket spurred off of another via 1x 2.5 t and e, the source socket used is in a ring final circuit on a 32a mcb.

With my understanding 1 single 2.5mm t and e has a ccc of say 27a ref method c, so you could potentially plug 2x 3kw heaters into your spurred socket on full power and thus over load the cable but not the mcb causing potential danger ??

I know any danger is highly unlikely as you'd nearly never plug anything in to draw such a load but surely the potential to is enough for this not to be good practise ??

I must be missing something as everything and everyone I ask seems to say it's fine but surely you'd just extend the ring circuit or add a fused spur first before the additional socket ??

thoughts?

TIA.

 

[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