I intend to use a 6 mm sqr cable spine or open ended run, with connection boxes to drop off a few 2.5 mm sqr spurs to single or double sockets.
I have been given the OK to do this by the local building inspectors, but I have been told by one electrician that it would not be possible to test, because it's non-standard i.e. the 'spine' is not a loop - just a long run with spurs.
Is he right? Or is there a facility in the test report to allow for non standard practice?
Surely, a single 6 mm cable would be better than a 2.5 mm loop, which equals 5 mm? I also intend to just cut back the insulation, leaving the 6 mm run unbroken in the junction box, and solder the 2.5 mm drops to the 6 mm cable, then clamp the cable under the screw terminals.
Not really any point in doing this as the MCB must be for the 'weakest' cable therefore 20 amps. So unless dealing with unusual cable runs make it all 2.5mm. If you want a 32 amp radial it must all be 6mm.
The correct name is radial circuit.
What on earth for? That doesn't conform to any regs. If the JBs are accessible, screw terminals are ok. If not you'd usually use crimped connections.
You have to protect the thinnest cable with the appropriate fuse. For a 2.5mm radial, thats a 20A mcb if my memory serves me right - for the whole circuit! As for the quality of your soldering directly onto the
6mm spine, will that take 20A through it all week, or will it cause problems and burn down your house? I'm not sure that soldering is acceptable.
Why not make you life simple by daisy chaining your radial circuit? Nothing stopping you using 6mm cable for this, though I believe 4mm would suffice, then you could use a 32A mcb (type B).
Ask you electrician if he has ever tested a radial lighting circuit. I doubt you told the inspector about your soldering plan?
On 30 Apr 2007 09:13:01 -0700, snipped-for-privacy@gmail.com mused:
4mm may still need a 20A, or 25A maybe. Depends on the run. IIRC 32A is pushing it for a 4mm on a decent run.
An electrician that can't work out how to test a radial with different sized cables isn't an electrician, it's a pillock that's done a 1 day part p qualification [sic] course.
Solder! What's wrong with the terminals in the junction box? Soldering a 6mm cable will require enough heat to cause the insulation to melt for a modest distance. I wouldn't recommend soldering onto it at all.
I'd recommend you just run a ring main. Remember that Part P testers are like PAT testers. Not necessarily electrically skilled, and therefore might get confused by odd wiring.
Rubbish. In any event if you are making a Part P application it's the building inspectors who have the responsibility for certifying the installation.
It is a non-standard circuit so compliance with the regulations must be proven by calculation from first principles. This would need to be done by someone competent (and preferably competent and qualified) in electrical circuit design, which many 'electricians' aren't. I think it's fair to surmise from your post that you aren't able to do this yourself.
You don't solder then clamp cables. Solder creeps under clamps.
It *might* be possible to demonstrate compliance of your proposed circuit - I am assuming you want to protect the circuit at 32A - if you can show that the short circuit protection for the 2.5mm segments is satisfied by a 32A MCB, while overload protection is provided by the 13A or 2x13A fuses on the load.
I really don't see the point, however. Subject to area and intended loading, a 20A 2.5mm radial, or a 32A 4mm radial, are both standard circuits and can provide an unlimited number of sockets. You are also not restricted to 'spine and spur' layout - you can spur from spur, etc.
As long as you restrict the sockets to one single or double per drop then you could protect the circuit at 32A (i.e. same situation with a spur from a conventional 2.5mm^2 ring). You are then splitting the responsibility for overload and fault protection into two separate protective devices rather than having both functions performed by the circuit protective device. The fuse in the appliance plug gives overload protection to the 2.5mm spur. The main circuit MCB will still offer adequate fault protection.
I'm a bit uncomfortable with your suggestion. Ring circuits are a peculiarly British affliction. In order to cope with their limitations, a spur to one single or double socket outlet, with effectively reduced cabling size is allowed. Relying on portable equipment to be properly maintained in order to ensure the safety of a fixed circuit seems to me even more stupid than ring circuits themselves. Indeed, even with portable equipment correctly fused, dangerous cable overheating could result. I think this might be why BS7671 defines a "spur" as "a branch from a _ring_ final circuit" if my memory serves me correctly (emphasis my own). While your suggestion, that a 32A circuit breaker, allied with correct fusing of portable equipment and diversity, will protect a 2.5mm cable, might appear to be in the spirit of the 16th Edition. I wouldn't rely on it as a defence if the OP's house burns down.
I was not trying to suggest that the proposed circuit design was particularly desirable, only that there are circumstances where it is acceptable and would meet the regs.
Personally I have no problems with ring circuits. since when you analyse the most common circuit failure modes, the ring circuit will usually perform better than the equivalent radial...
Even incorrectly fused (and I am excluding cropped off sections of six inch nail here), the maximum notional load is 2x13A. Generally unlikely to cause major damage to a 2.5mm^2 T&E (rated at 28A clipped direct or buried in building materials) in a short space of time. Obviously if you are designing for a situation where you know the cable will need to be heavily de-rated (i.e. buried in insulation etc) then you would not start with that design.
That is indeed the definition (BS7671 part 2), however a branch from a radial circuit does not really have a name, it is just a part of the radial.
It is much the same situation as wiring two 13A radials in 2.5mm^2 cable and connecting them both to the same 32A MCB. Gut feel says it looks wrong, although you can usually demonstrate by calculation that the 32A MCB does provide adequate protection to the cable in the presence of a fault current. It goes against what one perceives as "normal" since the job of overload protection is not also being carried out by the MCB. (normally if I were going to rely on this style of design, I would rather see the overload protection enforced in terminating FCUs rather than relying on the fuse in the appliance plugs)
You could get a similar situation on a ring circuit if you loaded up a double socket right at one end of the ring to maximum capacity.
I'd be interested to know what the most common failure modes are. The trouble with a ring circuit in my opinion is that if the ring is broken somewhere, it still works.
This is why BS7671 allows it. It's still relying on something outside the installation to protect the installation, which is a bad idea. It is _only_ permitted on ring circuits!
You aren't supposed to endanger any part of a radial circuit by reducing its cross-section. You are only allowed to do that on a ring. If you are bonkers enough to do this on a radial, then you've got to fuse it. Then it becomes a separate "circuit", as defined by BS7671 (I think).
I have to totally disagree with you here! It certainly is not.
The breaker is to protect against overcurrent as well, which it will not.
I hope you have never done this!
A ring circuit has no "end". That's why it's called a ring circuit. So where exactly is this end socket?
On 1 May 2007 01:43:59 -0700, snipped-for-privacy@gmail.com mused:
You obviously don't understand ring mains. The above scenario is exactly the same as 2 spurs from a ring main on the 2 legs of the ring from the mains.
You're scaring me now. You appear to be saying that the 2 "legs" of a ring main are the same thing as 2 radials run off the same circuit breaker. They certainly are not! One is anachronistic, the other downright dangerous! To make matters worse you equate two spurs with these non-compliant radials! I thought you were an electrician with Part Pee and everything??
In my experience the most common failure is a high impedance joint (loose terminal screw etc). Where breaks are concerned, then the most common conductor to suffer a break is the CPC (typically because it is physically less well protected having no individual insulation, and is also typically thinner than the other conductors. It is also often obscured by loosely fitting earth sleeving.
Hence the most common failure modes in order of likelihood seem to be:
1) high impedance Phase or Neutral connection,
2) high impedance CPC (where 2 is as likely as 1 probably)
3) broken CPC
4) broken phase / neutral
1) will result in localised heating and possibly fitting or cable damage. Depending on the location of the break you may be passing close the the full circuit current through the high impedance point on a radial. With a ring you have the advantage of an alternative path.
2) Will result in either prolonged or or complete failure to open a CPD in the event of a phase/earth fault, and also introduce the risk of high touch voltages in the presence of a fault. Possibly resulting in indirect contact injury. On a ring the performance is unlikely to be impaired.
3) Will result in a section of circuit having no earth connection at all on a radial and potential problems as per 2)
4) This is the situation where the radial performs better (and it seems is the rallying call of the "radials are better than rings" proponents!), in that the conductor rating will be sized for the full circuit load. With the ring circuit there is a risk of accelerated cable fatigue on a circuit loaded close to the limit.
That is not really true. The example I gave of two 16A circuits connected to a single 32A breaker would be one case. It also depends on your notion of "outside". It would certainly not be a good way to provide a pair of general purpose power circuits, however in more controlled circumstances (i.e. hardwired connections to fixed equipment via FCUs) it can work.
Again not really true. You can reduce cross section without fusing down at the reduction, if you size the main CPD such that it is appropriate for the smaller cable not the larger one. An exmaple of this happening where a section of damaged cable is replaced with one of a larger cross section - there would be no need to change the CPD rating at the circuit origin.
Fusing down as you describe is perfectly acceptable (and standard practice with fused spurs and sub mains etc)
How is it different? It is a situation that could occur should spurs be take from the ring at the CU (an acceptable place to take a spur if convenient).
No I think, this is the fundamental misunderstanding. The breaker
*usually* provides fault *and* over current protection. However (as was the thrust of my original post) this does not *have* to be the case. The fault protection must be at the supply end of the circuit. The overcurrent protection however does not, and can be at the destination end in some circumstances.
Personally I have never found the need, but I have come across installations where this has been done. One for example, had an immersion heater radial circuit, and a radial feeding a pump for a spa bath connected to a single 32A MCB. Each circuit wired in 2.5mm^2 T&E. If you do the sums then the circuit is ok. The 32A MCB provides adequate fault protection for the cables, the FCUs (or plug/socket as was the case at one point here) at the appliance connection points provide overload protection. Without modifying the circuit there is nothing you can realistically do to place the cable at risk of overload.
The weakness of installs like this was however nicely demonstrated in this particular house. At a later stage someone had come along and modified the setup to include a socket on the cable feeding the immersion - the intention only being to provide a connection point for a
3A shower pump, but it did in theory now provide a way of introducing an overload to one of the circuit cables.
The ends are where it connects to the CU. The first socket in one leg of the circuit may be (for example) 1.5m from the CU in one direction and
60m in the other. Hence the short leg will carry the lions share of the current.
On 30 Apr 2007 08:37:11 -0700 someone who may be Ren wrote this:-
You have yet to explain why you intend to do this, rather than a standard ring or radial circuit. Unless you are trolling there is presumably a reason for opting for this particular design.
He has if he has ever fitted a ring main. Copper is so cheap that 2.5mm2 ring mains are silly. Use common sense and fit a MCB that is below the rating of the cable like you would in any industrial installation.
On 1 May 2007 06:36:37 -0700, snipped-for-privacy@gmail.com mused:
I think you are misunderstanding something.
The non compliant installation as you put it is 2 spurs on a 32A MCB, which is exactly the same as 2 sours from a ring main. I know it's not technically correct and it's something I'd not do or condone, but technically there's no difference.
Most common I see is a poor terminal connection which causes heating under load. On a ring, generally only the load on that one socket contributes to this. Load from other sockets tends to avoid the bad connection and more goes the other way, so it remains safer than a radial where the full circuit load is trying to go through the poor connection.
I don't find broken CPCs as mentioned by someone else, but in the case of a broken CPC, all sockets remain earthed and safe. For high integrity earthed circuits, a ring requires very little different from the way it's normally wired, whereas a radial, in recognition of the increased likelyhood of an unsafe failure, requires a different earthing technique.
No, it's a general principle allowed in any circuit. Fault current protection must be at the origin of the circuit, but overload current protection can be anywhere along the circuit up to the load.
You aren't allowed to endager any circuit, but there's no blanket prohibition on reducing cross-section. You can use any layout which you can prove is safe. If you steer off the conventional path, then you won't come under any of the conventional circuits which can be taken-for-granted as safe, so you'll have to perform the relevant proof.
So you have a ring circuit, which has a couple of spur connections direct to the MCB, which is perfectly permissable and comes under standard conventional circuits. Why does this suddenly become unsafe if you disconnect the ring part of the circuit? I'm not saying this is how I would connect up two such sockets, but it doesn't suddenly become less safe than the two spurs off a ring circuit.
The plug fuses will do that. (Reference my comment again that overcurrent protection does not need to be at the origin of the circuit.)
It's done everytime someone creates an unfused spur.
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