Unfused spurs

[JohnW2]

Unfortunately in our trade there are those that cannot see past BS7671 and the fact that a circuit has been designed outside the scope of it's guidance will send some calling for the electrical police.

Exactly - and, as I'm sure you understand, that is the reason for many of the questions I've been asking here. One would hope that the training of those in question would enable them to properly understand circuit design, quite apart from 'details' in the regs (which are often only really intended as 'examples'). After all, we're not talking about things which are not compliant with BS7671 - merely things that require a bit of knowledge/thought/work to demonstrate that they are essentially compliant.

Granted it might throw some working in the domestic scenario, but I come across many commercial ring finals in 4mm² and would not have a problem with a multi socket radial spur from them. (Depending on all other circuit parameters)

Indeed, I'm sure that it would be very easy to justify the design of such a spur in many/most cases - but I again agree that it might 'surpise' some who were only used to standard domestic installations. Appendix 15 of the regs obviously doesn't even attempt to go there, since it only talks about 2.5mm² RFCs.

Kind Regards, John.

 

[JohnW2]

Unfortunately in our trade there are those that cannot see past BS7671 and the fact that a circuit has been designed outside the scope of it's guidance will send some calling for the electrical police.

For such people, I perhaps should have added that the word (let alone spirit) of BS7671 would seem to 'allow' multiple unfused spurs (each feeding one double socket), wired in 2.5mm cable, to be attached to the same point (if one could fit all the cables in), or very close points, on a ring circuit - and I can't see that what I'm proposing could be regarded as any worse than that.
Kind Regards, John.

 

[RossR]

Whilst I am pretty sympathetic to the concept of radials, I am not very comfortable with 20A ones (since it's only too easy to exceed a 20A load on multiple sockets)

In a domestic setting and outside of the kitchen, I'd dispute that. I've run 16A radial power circuits, where reference method 101 was required and a rewire in 4mm² was not feasible.

In fact I've lived in a rented house that only had a single 20A circuit for all the sockets including the kitchen, and in 3 years it never tripped.

 

[ajrobb]

Agreed - in fact, 'worse', since there is nothing in the regs preventing a 20A unfused spur actually being connected to the CU itself, and then loading it to a full 20A! Mind you, again one has to keep things in perspective. What you describe is no worse than plugging 20A worth of load into a double socket on the ring which is very close to the CU.

Without an unfused spur on the first socket, its load would nominally be limited to 20 A (let's not start another double-socket thread, please). With an unfused spur though, the socket and the spur together could load the full 32 A onto the ring at the first socket!

I can't see anything wrong with adding a 20 A unfused spur onto the ring at the CU as it doesn't load either leg of the ring.

I'd still like a B32 RCBO with 4 parallel 20 A thermal elements (one for each conductor) as well as an electromagnetic B32 element and 30 mA RCD in a double-width package. Thus a ring could once again support a distributed 40 A load (as with a 30 A semi-enclosed fuse) but without overloading any conductor or terminal.

 

[JohnW2]

Whilst I am pretty sympathetic to the concept of radials, I am not very comfortable with 20A ones (since it's only too easy to exceed a 20A load on multiple sockets)

In a domestic setting and outside of the kitchen, I'd dispute that. I've run 16A radial power circuits, where reference method 101 was required and a rewire in 4mm² was not feasible.

I'm not sure that 'dispute' is really the right word, since I was merely saying that I am 'not very comfortable' with 16A/20A multi-socket circuits. Outside of a kitchen/utility room, one certainly will usually 'get away with it, but one is relying on the fact that no more than one high-powered portable device (e.g. fan heater etc.) will be simultaneously used on the circuit - hence my less-than-complete 'comfort' about such situations. I do, indeed, have some 16A socket circuits in my house, but they are in areas in which such overloading is very unlikley.

In fact I've lived in a rented house that only had a single 20A circuit for all the sockets including the kitchen, and in 3 years it never tripped.

I don't doubt that (as I said, one will usually get away with it) - but, apart from anything else, lack of tripping is obviously not really a very good indicator - since a 20A Type B MCB will allow a current considerably in excess of 20A (more like 30A) for appreciable periods (an hour or so) without tripping. OK, I'm sure that the cables are quite capable of coping with that - but I imagine you will agree that it's nevertheless not good practice (nor, I would say, compliant with the regs) to have a circuit which is likely to quite often carry a current (even for relatively short periods) which is appreciably above the rating of the circuit's protective device (and probably also the cables).

Kind Regards, John.

 

[JohnW2]

Without an unfused spur on the first socket, its load would nominally be limited to 20 A (let's not start another double-socket thread, please). With an unfused spur though, the socket and the spur together could load the full 32 A onto the ring at the first socket!

Well, yes, but you are talking about very unlikely situations, and there is a limit to the extent to which one can (or needs to) design for such extremes. It could equally be the case that (even without spurs) the second socket on the ring was very close to the first) - with the same possible (although very unlikley) consequences as you describe..

I can't see anything wrong with adding a 20 A unfused spur onto the ring at the CU as it doesn't load either leg of the ring.

That's an interesting statement. Do I therefore take it that you'd have no problem with an unfused 'spur' supplying multiple sockets if it was wired in 4mm directly from the CU? I put 'spur' in quotes since some people would describe this arrangement as a ring circuit and radial circuit both connected to the same MCB at the CU.

I'd still like a B32 RCBO with 4 parallel 20 A thermal elements (one for each conductor) as well as an electromagnetic B32 element and 30 mA RCD in a double-width package. Thus a ring could once again support a distributed 40 A load (as with a 30 A semi-enclosed fuse) but without overloading any conductor or terminal.

So long as we have ring circuits, that would certainly be the theoretical ideal, but I suspect that such devices do not (yet) exist as such (although some 3-phase devices must get pretty close to satisfying that hypothetical specification). Whether or not it is 'necessary' is perhaps a different question, since the system/devices/circuit designs we currntly have seem to be doing an adequate job in the vast majority of cases. Even if a ring gets broken near one end (which is probably about the worst-case scenario), I would imagine that it would be extremely rare for any great harm, let alone fires, to result from the whole of an RFC's load going through a single run of 2.5mm cable - but maybe I'm wrong.

Kind Regards, John

 

[ajrobb]

The point is that a ring is susceptible to carrying more than 20 A in a cable without exceeding 32 A load. You talk about overloading a radial, the situation could get much more dangerous with a ring. With two radials, you have a total capability of 40 A without overloading anything. Given that a ring is generally designed for distributed loads, you'll be gaining the ability to support point loads with radials. If you distribute the load from a 32 A ring onto two 20 A radials, your safety margin should go up rather than down. Also your 20 A sockets will be better protected from prolonged 26 A overloads.

 

[JohnW2]

The point is that a ring is susceptible to carrying more than 20 A in a cable without exceeding 32 A load. You talk about overloading a radial, the situation could get much more dangerous with a ring. With two radials, you have a total capability of 40 A without overloading anything. Given that a ring is generally designed for distributed loads, you'll be gaining the ability to support point loads with radials. If you distribute the load from a 32 A ring onto two 20 A radials, your safety margin should go up rather than down.

In an (electrical circuit designer's !!) ideal world, all loads would be fixed-wired, so that the distribution of loads between circuits would be under the full control of the designer - who could achieve just what you describe. However, in the real world we have plugs and sockets (which must be the bain of designers' lives!), which means that the designer loses a lot of control. If one "distributes the load from a 32A ring onto two 20A radials", and if the designer's intelligent guesses about 'what will be plugged into which socket' prove to be correct, then one could end up with the improved situation you describe. However, it's all probabilistic ('gambling' in everyday language!), so it's also possible that the end-user would put most of the load on one of the 20A radial circuits. Hence, the potential benefit of the two radials would be dependent on a gamble that users would not do this.

Also your 20 A sockets will be better protected from prolonged 26 A overloads.

I certainly can't argue with that, given that a 30A/32A protective device would obviously offer no guaranteed protection against a prolonged 26A overload (if 'overload it is ) on one double socket. In a theoretician's ideal world, I guess that sockets, as well as plugs, might each have their own inbuilt fuse - although I very much doubt that would protect a socket against 26A (any more than a 20A MCB necessarily would).

Kind Regards, John.

 

[ajrobb]

The gambles with one 32 A ring and two 20 A radials are completely different:
radials: will a CB trip?
ring: will a cable melt?

Which gamble would you rather lose?

 

[EFLImpudence]

I feel that people are falling into the same trap as I did in one of John's previous topics. Namely, because have been mentioned - 20A radials, 20A rated socket outlets and 32A rings we seem to be thinking that these are the current carrying capacities of the cable.

If installed by reference method C (this would have been considered as part of the design process should any of the above problems have been expected) then a 2.5mm² T&E can carry 27A thus the actual current carrying capacity of the cable in the ring is 54A. By regulation, and probably precisely for the reasons discussed, a 2.5mm² ring is limited to a maximum of 32A.

By these figures it follows that one leg of the ring could 'safely' carry 84% of the maximum load.

The regulations are just that - regulations - they are rules we follow and (Appendix 15 for example) cannot be expected to cover every possible circuit configuration. Figure 15A contains no reference to 1mm² cable but this must be a frequent occurrence when connecting lights or fans from a RFC. On the left of the diagram it has a 2.5mm² cable to the FCU when the rest of the circuit is 1.5mm. This circuit could, by regulation, all be in 1.5mm². So, because it is not mentioned specifically in Appendix 15 doesn't mean it is forbidden. As long as the circuit tests are all within the limits allowed and not expressly forbidden then surely it must be acceptable.

I, personally, do not see anything wrong with John's original suggestion but, to be honest, I would be uncomfortable installing it in a customer's house. Probably because, if a dispute did arise, I would not be confident that the adjudicator, whoever that may be, would be intelligent or knowledgeable enough to work it out rather than blindly follow what he can see and what he has or has not been told or read.

 

[JohnW2]

The gambles with one 32 A ring and two 20 A radials are completely different:
radials: will a CB trip?
ring: will a cable melt?
Which gamble would you rather lose?

It sounds as if your thinking may be going way beyond situations I was considering. When you said "If you distribute the load from a 32A ring to two 20A radials....", I assumed that you were talking about a 32A ring which was not overloaded (i.e. 32A max total current, in practice rarely anything like that). Even in the worse fault case of ring broken at one end, you are not going to get anywhere near melting 2.5mm² cable with 32A. With Method C, 2.5mm² cable is 'rated' at 27A, and the regs recognise that it can 'safely' cope with at least 45% more than that (i.e. about 39A) for at least an hour - and I'm sure there is a reasonably generous 'safety margin' built into that. Even in a true 'overload' situation (which I was not considering), a 32A MCB would not allow more than about 46A for an hour, and I'm pretty sure that even that wouldn't get close to melting 2.5mm² cable.

Kind Regards, John.

 

[JohnW2]

I feel that people are falling into the same trap as I did in one of John's previous topics. Namely, because have been mentioned - 20A radials, 20A rated socket outlets and 32A rings we seem to be thinking that these are the current carrying capacities of the cable.

Indeed so.

If installed by reference method C (this would have been considered as part of the design process should any of the above problems have been expected) then a 2.5mm² T&E can carry 27A thus the actual current carrying capacity of the cable in the ring is 54A. By regulation, and probably precisely for the reasons discussed, a 2.5mm² ring is limited to a maximum of 32A.
By these figures it follows that one leg of the ring could 'safely' carry 84% of the maximum load.

Quite, and to go even further, the regs implicitly recognise that cable can carry at least 1.45 times its Iz for at least an hour without adverse effect. On that basis, one leg of a 2.5mm² (Method C) ring could 'safely' carry about 39A (about '122%' of the maximum load) for at least an hour.

The regulations are just that - regulations - they are rules we follow and (Appendix 15 for example) cannot be expected to cover every possible circuit configuration. .....As long as the circuit tests are all within the limits allowed and not expressly forbidden then surely it must be acceptable.

Indeed,. 'The regulations' are really not quite what many people seem regard them as - namely those people who restrict their thinking to the 'examples' given, and circuits/situations considered in the regs. The regulations do, however, generally state the underlying requirements - enabling the person with adequate knowledge and ability to design for situations/circuits not explicity considered/addressed in 'the red book'

I, personally, do not see anything wrong with John's original suggestion but, to be honest, I would be uncomfortable installing it in a customer's house. Probably because, if a dispute did arise, I would not be confident that the adjudicator, whoever that may be, would be intelligent or knowledgeable enough to work it out rather than blindly follow what he can see and what he has or has not been told or read.

That, needless to say, is much of the thinking behind most of my questions here. Even though I regard it as a definite nuisance to have to go through this because of the existance of those 'adjudicators' with insufficient intelligence or knowledge, I am attempting to formulate and document design justifications for those aspects of my house's (large and complex) electrical installation which may beyond any of those 'blind followers' who are faced with it in the future. In that way, any such people who appear on the scene will have to find a way (if it is within their capabilities) to discredit that documented justification of the engineering design if they wish to 'adjudicate against' what they find!

Kind Regards, John.

 

[ban-all-sheds]

On the left of the diagram it has a 2.5mm² cable to the FCU when the rest of the circuit is 1.5mm. This circuit could, by regulation, all be in 1.5mm².

No it couldn't - read the 2nd sentence of 433.1.5.

 

[JohnW2]

On the left of the diagram it has a 2.5mm² cable to the FCU when the rest of the circuit is 1.5mm. This circuit could, by regulation, all be in 1.5mm².

No it couldn't - read the 2nd sentence of 433.1.5.

I don't think I agree with that, per se. 433.1.5 is merely describing a circuit which is deemed to satisfy 433.1.1. As I see it, that doesn't preclude the theoretical possibility that other arrangments (e.g. different cable sizes) might be compliant, if one can (oneself) demonstrate that they satisfy the requirements of 433.1.1.

However, I think that in terms of 'the bottom line', you are right, and EFLImpudence is therefore probably wrong. I would say that 1.5mm ² cable from the ring to the FCU would not be compliant - not because of 433.1.5, but because such a design would leave that (albeit probably very short) piece of 1.5mm² cable inadequately protected by the RFC's (probably 30/32A) protective device. Put another way, I think it would be non-complaint with the requirement of 433.2.1 for a protective device to be present where the current-carrying capacity reduced (i.e. where the 1.5mm² cable joined the 2.5mm² ring).

That's how I see it, anyway.

Kind Regards, John.

 

[EFLImpudence]

Does this not come under 433.2.2?