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

[7029 dave]

Radial.

Break in CPC = socket outlet with no CPC = risk of electric shock = RCD disconnect when imbalance occurs.

Break in ring final = no detection of fault as only one line conductor has slipped out = 2 x 2.5 mm cables with a best case scenario current carrying capacity of 27A (surface clipped) protected by a 32A MCB = Overloaded cable = House Fire = Death

Bit ott mainline, don't recall millions of caused by ring circuit.

 

[mainline]

Bit ott mainline, don't recall millions of caused by ring circuit.

Me neither, it's probably something to do with cables overheating.

 

[Pretty Mouth]

Great thread @pc1966 .

The eagle-eyed reader will note in the OSG that for the RFC the loads are assumed to be distributed, for the radial it is end-point.

It does say in the OSG that the loads are assumed to be distributed for RFCs, however I believe the RFC has actually been designed with the entire load placed at the exact mid-point of the ring:

For the volt drop calculation, I treat the ring as if it were 2 radials, equal length, sharing the load equally. So a 106m ring (wired in 2.5) with a 26A load becomes 2 X 53m radials each with a 13A load.

53 X 13 X 0.018 = 12.4V

Next apply the correction factor for operating temperature from P429 of BS7671, using

tp=70 deg
Ca, Cg, Cs, and Cd all =1
Ib = 13A
It = 20A (from reg 433.1.204)

Gives:

(300-(1-(13²/20²))X(40))/300 = 0.923

12.4 X 0.923 = 11.45V, very close to the 11.5V limit allowed

 

[timhoward]

Radial.

Break in CPC = socket outlet with no CPC = risk of electric shock = RCD disconnect when imbalance occurs.

Break in ring final = no detection of fault as only one line conductor has slipped out = 2 x 2.5 mm cables with a best case scenario current carrying capacity of 27A (surface clipped) protected by a 32A MCB = Overloaded cable = Possibility of a fire.

Agreed. Though it's astonishing what well terminated 2.5 sq mm can actually carry if it has to. e.g. the shower I referred to earlier.

It's difficult to talk objectively about this, which is why I nearly didn't make this point. Every house and loading is different.
And strange anomalies happen even then - e.g. I've known a bride and her 4 bridesmaids all trying to use top-end hair dryers at the same time cause a bit of fun. (It was rather an enjoyable call-out so maybe we should all fit radials and hope for another instance of this, but that's another story)

I've had thought that a mid-point loose N on a radial could be a slightly more dangerous prospect than any loose conductor on a ring, but I won't die on my sword over this discussion!

 

[mainline]

I've had thought that a mid-point loose N on a radial could be a slightly more dangerous prospect than any loose conductor on a ring, but I won't die on my sword over this discussion!

A loose connection on a radial is more than likely going to cause problems with appliances etc downstream.

 

[123]

Once a Ring is Infected with numerous spurs on spurs on spurs , it can no longer be called a Ring and must be called a Spider circuit which needs to added to the 19th Edition assuming Rings aren't outlawed before that

Poor installation and botched alterations ruin anything and everything though.

A house I tested last year had bathroom shower and cooker joined together in a JB somewhere. Upstairs sockets off the lighting circuit. Utility room off ensuite shower in a JB somewhere. Immersion heater and garage now a bar with electric heating tied in together. You can't fix stupid.

 

[nicebutdim]

Poor installation and botched alterations ruin anything and everything though.

That about sums up the ring/radial debate. Every argument involves examples of bodged installation and none are based on objection to well designed and installed circuits.

No one would argue that radial circuits are inherently unsafe when sockets are found to be spurred of a shower supply in 1.5 T&E, so I don't get why a circuit design should be condemned on the basis of questionable alterations, made long after initial installation.

 

[pc1966]

Break in ring final = no detection of fault as only one line conductor has slipped out = 2 x 2.5 mm cables with a best case scenario current carrying capacity of 27A (surface clipped) protected by a 32A MCB = Overloaded cable = Possibility of a fire.

That is a bit dramatic. If you look at the worst case, say 50% above CCC of one leg then you are looking at 50% extra (x 1.5) on just one conductor of that leg, so instead of I2R heating of 125% more (1.5 * 1.5 = 2.25) you are looking at more like 1.625 more heating, and from the usual 30C to 70C cable design that puts your conductor probably just above 95C.

Is this good for the cable? Clearly not, but it is far far away from starting a fire. In fact it is well below the 160C upper limit taken for adiabatic heating without cable damage (e.g. Table 54.3) so what you are looking at really is a loss of useful cable life.

I have no idea what the factor needed for the Arrhenius equation is for PVC cable but usually it is a halving of component life for every 10-20C increase in temperature (assuming nothing dramatic happens like a fire!) so taking the cable on the faulty RFC as running 25C above specification you are looking at probably a quarter of its nominal 25 year life.

But who can afford to run their RFC at 32A or so for a long time? At today's cost that is something like 2 grand a month electric bill!

So in reality that ~6 years cable life is going to stretch out over a couple of decades as probably the high temperature is only for tens of minutes per day, not 24/7

 

[pc1966]

It does say in the OSG that the loads are assumed to be distributed for RFCs, however I believe the RFC has actually been designed with the entire load placed at the exact mid-point of the ring:

Thanks, that is good to see the workings.

It is actually not a bad design assumption as you could reach 32A limit on just two double sockets, so they might be close to the mid-point in the worst case.

 

[mainline]

That is a bit dramatic. If you look at the worst case, say 50% above CCC of one leg then you are looking at 50% extra (x 1.5) on just one conductor of that leg, so instead of I2R heating of 125% more (1.5 * 1.5 = 2.25) you are looking at more like 1.625 more heating, and from the usual 30C to 70C cable design that puts your conductor probably just above 95C.

Is this good for the cable? Clearly not, but it is far far away from starting a fire. In fact it is well below the 160C upper limit taken for adiabatic heating without cable damage (e.g. Table 54.3) so what you are looking at really is a loss of useful cable life.

I have no idea what the factor needed for the Arrhenius equation is for PVC cable but usually it is a halving of component life for every 10-20C increase in temperature (assuming nothing dramatic happens like a fire!) so taking the cable on the faulty RFC as running 25C above specification you are looking at probably a quarter of its nominal 25 year life.

But who can afford to run their RFC at 32A or so for a long time? At today's cost that is something like 2 grand a month electric bill!

So in reality that ~6 years cable life is going to stretch out over a couple of decades as probably the high temperature is only for tens of minutes per day, not 24/7

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

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