Fused spur or ring main

[EFLImpudence]

That in a nutshell is the only problem with them

It's quite a big one.

but correctly installed connections generally don't fail.

Make up your mind.

A break in a radial results in dead sockets.

Therefore noticeable - and safe.

The ring has 5mm of copper conductors. The loading aspect causing problems isn't a simple as many people seem to think. In short any load anywhere will have more than 2.5mm available to carry the current.

Not if one is unknowingly broken/disconnected.

The people that dreamt it up were not stupid and gave it a lot of thought,

The reasons for dreaming it up no longer exist.

 

[ajohn]

The reasons for dreaming it up no longer exist

I'm always intrigued by the reasons people mention so wonder what your is? One is sometimes the war - copper shortage so use 5mm^2 instead of 4. Really ????

Some prefer laying 4mm providing it's stranded. 2.5mm isn't bad with single wire.

It's not unusual for electricians to moan about the testing a ring needs. It takes a little longer - tough.

The reasoning behind it is this

Kirchhoff's Current Law. Kirchhoff's Current Law (KCL) is Kirchhoff's first law that deals with the conservation of charge entering and leaving a junction. ... In other words the algebraic sum of ALL the currents entering and leaving a junction must be equal to zero as: Σ IIN = Σ IOUT.

Cables warming due to current flow comes into it as well. It also unlike a radial averages the voltage drop where ever the load is.

 

[bernardgreen]

The reasons for dreaming it up no longer exist.

The basic laws of physics and electricity still exist and have not changed.

Other than than the possibility of single fault going un-detected the ring final was, in the 1960's, seen as better than the radial.

 

[securespark]

If I were to rewire a house today, I wouldn't install any ring finals.

 

[EFLImpudence]

The reason is far simpler and less convoluted - not sure what Kirchhoff has to do with it.
Namely: the introduction of MCBs.
(At the time a 30A radial would have required a 42A cable.)

433.1.204 is still written with BS 3036 30A fuses in mind so, obviously, is not the optimal for a 32A MCB - using too large a cable or too low an MCB.
Were there a 32A cable (3mm²?) introduced at the same time as MCBs, then the ring would be redundant anyway.

As it is - with 2.5mm² - two 25A radials would allow more usage.

 

[ajohn]

Rewireable fuses have different blow characteristics to enclosed ones. I came across this looking at live to armour shorts on SWA. At max length rewirable would blow in the permitted time. No other I could find would. Similar problem with MCB's. No problem with 5x rating for L to N but and RCD needed for L to armour. Increasing cable sizes doesn't help because the currents just go up proportionally so may as well use an MCB and an RCD and a cable that is just about capable of meeting the voltage drop specs. One answer is also to not worry about live to armour shorts but it is supposed to be all about failure modes not just specific ones. The faults can only in real terms happen if cable is damaged and when that happens some at least will need replacing however things are wired and what ever with.

Exporting earth on swa has it's interesting points a well - in some cases a PEN might be a better option but comes to grief on a TN-S supply as neutral voltage isn't well defined. I assume this is why DNO's wont fix crap Ze on TN-S by converting to combines earth and neutral.

Kirchhoff is pretty simple if viewed another way. Say it's a 40m long ring and a load is placed on it at 10m. So current flows according to resistance back to the source. Resistance is 1x in one direction and 3x in the other. The sum of these currents = the load. I'm not going to hurt my head what that means in splitting the current flow. This is why sockets should be reasonably balanced on them but going back to the 1x direction it will warm up more which will increase it's resistance causing more to flow down the 3x direction. Highest resistance for a short is at 20m formed by 2 20m lengths in parallel so 1/2 that of just 20m on it's own. I'm not going to check that rigorously. Kirchhoff makes that tricky but would hope that the guide at least clarifies that if it needs considering.

The faster blow of re wireable at lower over currents surprised me but seems to be fact going on charts in the regs. Rapid blow times at suitable current levels don't seem to differ much when other things are used, MCB's or cartridge fuses of similar ratings.