Ring Final Circuit (RFC) versus Radial. Yes, again.

[Lucien Nunes]

Post No.28 above is important, because it debunks one of the most common incorrect arguments used against the 2.5sq.mm. 32A ring. It is often said that if one leg becomes disconnected or the circuit is badly arranged leaving all or most of the 32A load on one leg of 2.5, that cable presents a fire risk through overload. It does not.

As stated in the above post, the most likely result is a minor reduction in cable service life, and the worst reasonably likely result is a more significant reduction in cable service life. A length of 2.5 carrying any level of current that a 32A MCB will give it, will not burst into flames nor set anything else on fire. It will be hot to the touch and a bit floppy and that is all. It will withstand perhaps 80A before anything dramatic happens, and even then that is not generally fire.

To re-state that in practical terms, a defective ring of 2.5T+E with spurs off spurs, inappropriate load distribution and one leg open circuit, will support four 12A heaters plugged into a row of adjacent sockets anywhere until the 32A OCPD trips, without anything catching fire and probably without any further damage of any sort.

Fire does not start with marginally-overloaded conductors, which rise to an elevated but not extreme temperature and then reach equilibrium, never to get hotter. Much more likely to start a fire is the uncontrolled temperature that can be reached by arcing connections, since these can just go on getting hotter and hotter as their resistance rises. Where a connection fails high-resistance, the parallel paths available in the RFC can replace a potential arcing scenario with a marginal conductor overload one, in which case it probably reduces the risk of fire. It does then reduce the likelihood of fault detection, but the very fact that we do find burnt terminals still in service shows that detection is not by any means guaranteed in use.

 

[mainline]

Post No.28 above is important, because it debunks one of the most common incorrect arguments used against the 2.5sq.mm. 32A ring. It is often said that if one leg becomes disconnected or the circuit is badly arranged leaving all or most of the 32A load on one leg of 2.5, that cable presents a fire risk through overload. It does not.

Even in a properly connected joint, when heated could cause a fire risk.

Much more likely to start a fire is the uncontrolled temperature that can be reached by arcing connections, since these can just go on getting hotter and hotter as their resistance rises.

Where a connection fails high-resistance, the parallel paths available in the RFC can replace a potential arcing scenario with a marginal conductor overload one, in which case it probably reduces the risk of fire. It does then reduce the likelihood of fault detection, but the very fact that we do find burnt terminals still in service shows that detection is not by any means guaranteed in use.

Is this where the debate of AFDDs when used in an RFC may not detect the series arc fault.

 

[pc1966]

That's a relief, no need to worry about overloaded broken ring finals causing a fire.

Again, over the last 6 decades, how often do you hear a spark saying "oh, found another burnt out RFC today"?

Burnt out plugs/sockets, yes, but that is independent of the RFC/radial choice.

I would now assume that counts for 4 mm radials as well.

It would.

But there is a slight difference in designing to meet the regs (choice of cable, install method, and OCPD, etc) and worrying about a fault developing on a design that meets the regs.

 

[pc1966]

Incidentally, did John Ward not do a video once that tried to burn out 2.5mm cable? Maybe testing the quality of Wagos, etc?

I vaguely remember it taking something quite large like 60 odd amps, etc.

 

[pc1966]

Is this where the debate of AFDDs when used in an RFC may not detect the series arc fault.

As others have pointed out, the AFDD not detecting a series arc in the RFC is down to such an arc being practically impossible to start (as only ten or so volts difference if the ring breaks) and if on microscopic gap it does, it is not big enough to be a serious risk.

From your own experience, how often do you find burnt out joints, and of those you have, how many were on sockets of either system versus showers?

 

[mainline]

Again, over the last 6 decades, how often do you hear a spark saying "oh, found another burnt out RFC today"?

Burnt out plugs/sockets, yes, but that is independent of the RFC/radial choice.

It would.

But there is a slight difference in designing to meet the regs (choice of cable, install method, and OCPD, etc) and worrying about a fault developing on a design that meets the regs.

But surly in the design process the risk of a particular fault developing in the circuit choice has to be taken into account.

 

[Dustydazzler]

Incidentally, did John Ward not do a video once that tried to burn out 2.5mm cable? Maybe testing the quality of Wagos, etc?

I vaguely remember it taking something quite large like 60 odd amps, etc.

He did , I think he got around 60-70amps up a 2.5 before it started to get warm and around 80-90amps before it started smoking / melting. Yes it was a short length clipped direct but it shows how much overloading a cable clipped direct can actually take

 

[Dustydazzler]

I think Bic Nundy did a similar test again with a bit of old 2.5, iirc he did it with different types of cable pvc / lsf etc
Again kept pumping more amps up it until it started smoking. again it took a serious overload to get it to smoke

 

[mainline]

As others have pointed out, the AFDD not detecting a series arc in the RFC is down to such an arc being practically impossible to start (as only ten or so volts difference if the ring breaks) and if on microscopic gap it does, it is not big enough to be a serious risk.

From your own experience, how often do you find burnt out joints, and of those you have, how many were on sockets of either system versus showers?

Rare at sockets to be honest, but many JBS and shower pull cord types, mostly due to not using ones that have decent terminations.

 

[Lucien Nunes]

As mentioned above, a 'series arc' in one leg of a ring won't really arc because there's hardly any voltage across it. If it doesn't arc, the AFDD won't find it. If it does arc, the AFDD will react and disconnect it. A loose terminal in a socket will arc on a ring just as as on a radial and the AFDD will find that too.

 

[Dustydazzler]

Rare at sockets to be honest, but many JBS and shower pull cord types, mostly due to not using ones that have decent terminations.

Its nearly always a burned out joint / terminal that is the issue , as you say usually a cheap carp accessory that burns out and not the actual cable itself.

I'm not anti-Ring, a well installed 30amp ring in 2.5 is bullet proof and will last a lifetime, its the gimps that infect the ring with spurs on spurs where the design faulters

 

[mainline]

I think Bic Nundy did a similar test again with a bit of old 2.5, iirc he did it with different types of cable pvc / lsf etc
Again kept pumping more amps up it until it started smoking. again it took a serious overload to get it to smoke

Maybe it should be tried with a 100 mtrs of 2.5 with terminated sockets jointed in.

 

[Dustydazzler]

Maybe it should be tried with a 100 mtrs of 2.5 with terminated sockets jointed in.

We did something at college with a 100mtr roll of 2.5 and pumped I think iirc 100amps up it on the roll and it took ages to get warm to the touch. It was to show a massive overload won't damage the insulation on the cable providing automatic disconnection of supply is achieved quickly.

 

[Dustydazzler]

I reiterate a 2.5mm 30amp ring is almost bullet proof if its actually a ring and its on a 30/32amp device.
I just find the ring concept a bit old fashioned and in a modern world of cheap cable and cheap consumer units the point of a ring to save money on cable no longer holds up imo

 

[mainline]

I reiterate a 2.5mm 30amp ring is almost bullet proof if its actually a ring and its on a 30/32amp device.
I just find the ring concept a bit old fashioned and in a modern world of cheap cable and cheap consumer units the point of a ring to save money on cable no longer holds up imo

When you go into a rewired property and there are 3 RFCs one up one down.

One in the kitchen with drops for the appliances.

Kettle. Microwave, dishwasher, washing machine, usb sockets, toaster, coffee machine etc etc.

Is this where the debate of segregation of circuits come in.

 

[Dustydazzler]

When you go into a rewired property and there are 3 RFCs one up one down.

One in the kitchen with drops for the appliances.

Kettle. Microwave, dishwasher, washing machine, usb sockets, toaster, coffee machine etc etc.

Is this where the debate of segregation of circuits come in.

From an circuit isolation POV one could possibly argue a ring with everything on it a poor design but in the real world it really isn't that big an issue. Maybe in a commercial kitchen setting where you only want to isolate part of a kitchen.

 

[mainline]

Its nearly always a burned out joint / terminal that is the issue , as you say usually a cheap carp accessory that burns out and not the actual cable itself.

I'm not anti-Ring, a well installed 30amp ring in 2.5 is bullet proof and will last a lifetime, its the gimps that infect the ring with spurs on spurs where the design faulters

Of the 21 cottages I look after, I only found 2 with RFCs that tested ok.

All the others have/had faults due to add-ons over the years.

Nothing against RFC as such, but I will avoid using where I can, and I think most will.

 

[Dustydazzler]

Of the 21 cottages I look after, I only found 2 with RFCs that tested ok.

All the others have/had faults due to add-ons over the years.

Nothing against RFC as such, but I will avoid using where I can, and I think most will.

Nearly all sparks I speak to over the past few years who do daily domestic and Eicrs IF they come across a broken down Ring they no longer bother trying to find the break they just change the MCB to a 20amp and in effect create a 20amp spider radial.
I rarely install rings anymore if adding a new circuit, I just do a 20amp in2.5 or 32amp in4mm.
I can't actually remember the last time I installed a brand new 30amp ring in a house,

 

[mainline]

From an circuit isolation POV one could possibly argue a ring with everything on it a poor design but in the real world it really isn't that big an issue. Maybe in a commercial kitchen setting where you only want to isolate part of a kitchen.

Difficult to comply with 531.3.2 though.

 

[brianmoooore]

Incidentally, did John Ward not do a video once that tried to burn out 2.5mm cable? Maybe testing the quality of Wagos, etc?

I vaguely remember it taking something quite large like 60 odd amps, etc.

Anyone with a conventional arc welder can recreate this. I've connected up odd lengths of various size cables to my 180A Pickhill when I've been bored.
Try it in a well ventilated area though. The gasses given off are lethal.

 

[mainline]

Incidentally, did John Ward not do a video once that tried to burn out 2.5mm cable? Maybe testing the quality of Wagos, etc?

I vaguely remember it taking something quite large like 60 odd amps, etc.

It was about a meter of 1mm.
At 50 amps it caught fire quite quickly, It seemed fine at twice its rating.

I would like to see one where the cables are in insulation, along with terminations etc and run at twice the rating for a sustained amount of time.

 

[happysteve]

I mostly do additions and alterations, infrequently new circuits, very rarely (partial) rewires. I've installed RFCs in new installs where voltage drop would have otherwise been an issue (e.g. upstairs sockets on large area 4-bed house), but otherwise install radials where I can.

The issue with RFCs, even correctly-installed ones, is you never know what you're going to find, which makes it harder to do a quote for a trivial job of adding another socket, because you never know (visually) if an existing socket it on the ring, or a spur. Yes, you can turn the circuit off, have a look behind, and check r1 rn r2, but that's a fair bit of faff when you're just popping round to do a quote. If it's a B20 or B16 supplying a radial circuit, then I smile because I just do a quick Zs check at the socket I'm going to extend from and - other than insulation resistance - the number of unknowns is massively reduced.

A typical case will be adding an outside socket on the back wall of the house, where there's an existing socket in the kitchen cupboard (or behind the washing machine). If it's on the ring - great! Add a DP isolator (or 13A SFCU), drill through the back wall, and away you go. If it's a spur? Sure, could add a 13A FCU to feed the existing socket, plus the new outside one... but what if the existing spur is a 2G socket that feeds a dishwasher and washing machine (or worse, tumble dryer)? The 13A FCU will end up working beyond its capacity, whereas the unfused spur supplying the two appliances on the existing 2G socket would have been fine.

So I propose a new Regulation:

433.1.205: Unfused spurs on a RFC should be clearly marked as such, either using a permanent label, or a small indication in the bottom right hand corner of the plate that can be seen with a UV light or similar​

I sleep soundly at night adding a 3A SFCU feeding an outside LED light (or a doorbell) to an unfused spur (with an existing 1G or 2G socket on it)... doesn't match the guidance in Appendix 15 (Informative), but does comply with 433.1.204 as the few extra mA of current are not going to make a jot of difference to the safety of the unfused spur.

 

[mainline]

 

[pc1966]

Yes, that is what I remembered seeing. Thanks!

 

[pc1966]

So I propose a new Regulation:

433.1.205: Unfused spurs on a RFC should be clearly marked as such, either using a permanent label, or a small indication in the bottom right hand corner of the plate that can be seen with a UV light or similar​

Sadly most of the problem additions will be done by folks who have never read the regs at all!

I sleep soundly at night adding a 3A SFCU feeding an outside LED light (or a doorbell) to an unfused spur (with an existing 1G or 2G socket on it)... doesn't match the guidance in Appendix 15 (Informative), but does comply with 433.1.204 as the few extra mA of current are not going to make a jot of difference to the safety of the unfused spur.

Indeed, while not how things should be done it actually presents no risk of overload/overheating.