Unfused spurs

[mikhailfaradayski]

no takers for how Johnw2's idea may subject to criticism given 314.1?

I'm not quite sure whats being discussed here now? It started off as John guessing (in isolation) that most people would not want to have a new circuit simply to supply a few sockets, but now seems to be migrating into a whole new idea that seems would only really come into play for a new installation. Even then , I'm not convinced that the new idea will bring any added benefit when you consider the wealth of other options already available.

err? How did that happen?

314.1 anyone?

 

[holmslaw]

..

 

[JohnW2]

Also, a fused spur doesn't really cost very much, is it a big problem to just use one?

The reason for not having a fused spur would be nothing to do with cost but, as I said, would be to avoid the total capacity of the new sockets being limited to 13A.

Sorry, its just that somewhere in this thread you said that you could see some practical benefits to your concept, I thought you may have been talking about costs as a benefit (this seems to be supported by your further comment about people paying the bill)

Sorry, too, I think I've been unclear. Yes, convenience and cost of the work as a whole could be one of the practical benefits. I thought (perhaps stupidly) that you were just talking about the cost of the FCU. So, to re-answer your question, yes, it would be a 'big problem' (actually an insuperable problem) to use a fused spur if the requirement was (as I've been saying) for more than 13A total potential load on the extra sockets.

Given your example above of supplying a number of sockets which may be remote to the rest of the RFC, avoidance of fusing down and constructed in such a way that it would increase the probability for the overload and disconnection of the OPD (albeit it in a safe manner)....

Could you perhaps explain that bit?

sure, although it might be best if you told me what it is you want explaining further as it seemed quite straight forward to me.
But before we explore that, would you mind going back to my post to address the rest of it. Particularly the bit about 314.1

Since (despite the full stop in the middle) your question was really all one sentence, I can't really answer the bit about 314.1 without understanding the 'given that' bit (quoted above) which precedes it. What I would appreciate more explanation about is "constructed in such a way that it would increase the probability for the overload and disconnection of the OPD (albeit it in a safe manner)" - since I don't really recognise that as a feature of what I'm talking about. Thanks.

Kind Regards, John

 

[JohnW2]

It was originally a tongue-in-cheek suggestion, but it looks close to being a reasonable idea. I only came up with it last night in mid-post, so I cannot say it is thought through.

Indeed. As I said, it actually sounds if it could be the basis for a design that's potentially got quite a lot going for it. Many a brilliant idea has initially arisn in jest

Obviously with only a single active conductor, the heat generated will be localised, which can cause extra problems. However, we know that 2.5mm² T+E can carry 30 A in mid-air in both conductors, which is tantalisingly close to the required 32 A in a single conductor (dissipating 43% less heat overall). That suggests that the maximum insulation temperature (at the surface of the active conductor) might still be kept within bounds.

Agreed. Indeed, if one forgets 'ratings' for a moment, it would seem extremely likley that it would 'be OK' in practice (where 32A is very rarely going to flow).

IF it works at all, it would also be better able to accept point loads like a 32 A unfused spur at any point rather than just in the middle for a standard ring.

'Better able to' would seem to be a bit of an understatement. With such a design, it would be immaterial how loads (within the maximum) were distributed around the ring.

Just considering a 5% voltage drop at 18 mV/A/m:
230V × 5% ÷ 32A ÷ 18mV/A/m = 20m (radial length)
Which equates to a maxim ring length of 40m,....

Indeed, but 40m is still quite a usable ring length for domestic installs.

... which is a lot less than 106m for a standard ring.

That 106m doesn't relate to a 32A load, does it?

Kind Regards, John.

 

[mikhailfaradayski]

Also, a fused spur doesn't really cost very much, is it a big problem to just use one?

The reason for not having a fused spur would be nothing to do with cost but, as I said, would be to avoid the total capacity of the new sockets being limited to 13A.

Sorry, its just that somewhere in this thread you said that you could see some practical benefits to your concept, I thought you may have been talking about costs as a benefit (this seems to be supported by your further comment about people paying the bill)

Sorry, too, I think I've been unclear. Yes, convenience and cost of the work as a whole could be one of the practical benefits. I thought (perhaps stupidly) that you were just talking about the cost of the FCU.

Ah, no, to consider the cost of the FCU alone would be rather petty. You seem to have overlooked the bit about I mentioned about the cost differences between 2.5 and 4mm, this combined with other factors may dissuade a designer from using your idea (you mentioned that the additional sockets could be some distance from the rest of the ring)

So, to re-answer your question, yes, it would be a 'big problem' (actually an insuperable problem) to use a fused spur if the requirement was (as I've been saying) for more than 13A total potential load on the extra sockets.

What practical reason do you foresee which would make this a reality?

Given your example above of supplying a number of sockets which may be remote to the rest of the RFC, avoidance of fusing down and constructed in such a way that it would increase the probability for the overload and disconnection of the OPD (albeit it in a safe manner)....

Could you perhaps explain that bit?

sure, although it might be best if you told me what it is you want explaining further as it seemed quite straight forward to me.
But before we explore that, would you mind going back to my post to address the rest of it. Particularly the bit about 314.1

Since (despite the full stop in the middle) your question was really all one sentence, I can't really answer the bit about 314.1 without understanding the 'given that' bit (quoted above) which precedes it.

If you take a look at 314.1, it might enlighten you as to why I refer to it.

What I would appreciate more explanation about is "constructed in such a way that it would increase the probability for the overload and disconnection of the OPD (albeit it in a safe manner)" - since I don't really recognise that as a feature of what I'm talking about.

I wouldn't describe it as a feature of what you are talking about, more of a down-side.

Your idea seems to come from the need to find an easy way to extend a RFC safely and conveniently without using a fused spur and extend the area which that single circuit serves, with a view to it being subject to intermittent high loads.

If you have a RFC which has already been designed and installed and has been used in service, then you choose to add another branch to supply some sockets that are not limited to what current they can draw, then could end up with an undesirable consequence, e.g. The OPD may trip, causing loss to all sockets on that circuit. Due to the extended nature of your ring, this could be a significant inconvenience to the users of the installation.
If the designer/installer tasked with a work request like 'I want several sockets installed, some distance away from a RFC, I do not want the new sockets to be limited to 13A.' Then I'm not sure your idea would turn out to best the best solution. I'm not saying its unsafe or un-acceptable, just that it may be an obscure case where it might be the preferred method.

If this were the case and the work were to be scrutinised, how who you demonstrate that you have considered the guidance suggested by 314.1?

Thanks.

no problem

Kind Regards, John

 

[JohnW2]

Ah, no, to consider the cost of the FCU alone would be rather petty. You seem to have overlooked the bit about I mentioned about the cost differences between 2.5 and 4mm, this combined with other factors may dissuade a designer from using your idea (you mentioned that the additional sockets could be some distance from the rest of the ring)

We did the 2.5/4mm copper/cost difference to death in a thread a few weeks ago. The difference is really not enough to be a significant factor; I mentioned it here only 'in passing'.

I think you're probably making a bit too much of the 'some distance from the rest of the ring'. What I referred to was a situation in which incorporating the new sockets into the existing ring would appreciably increase the total length of the main ring - perhaps 'a pity' for the sake of sockets that one expects to be rarely used.

So, to re-answer your question, yes, it would be a 'big problem' (actually an insuperable problem) to use a fused spur if the requirement was (as I've been saying) for more than 13A total potential load on the extra sockets.

What practical reason do you foresee which would make this a reality

Given that one occasionally-used portable appliance (e.g. a fan heater) can use 10A or so, it's far from impossible that more than 13A worth of load would be plugged into a batch of several sockets.

... I can't really answer the bit about 314.1 without understanding the 'given that' bit (quoted above) which precedes it.

If you take a look at 314.1, it might enlighten you as to why I refer to it.

I know what 314.1 says, but couldn't answer your question without understanding the context you were specifying....

Your idea seems to come from the need to find an easy way to extend a RFC safely and conveniently without using a fused spur and extend the area which that single circuit serves, with a view to it being subject to intermittent high loads.

Yep, that's about it.

If you have a RFC which has already been designed and installed and has been used in service, then you choose to add another branch to supply some sockets that are not limited to what current they can draw, then could end up with an undesirable consequence, e.g. The OPD may trip, causing loss to all sockets on that circuit. Due to the extended nature of your ring, this could be a significant inconvenience to the users of the installation.

All you seem to be saying is that if one adds sockets (to any final circuit) without anything limiting what current can be drawn through those sockets, then there is an increased risk that a user may overload the circuit, with consequential OPD tripping and inconvenience. True though that is, it is no way specific to the design I was talking about. Exactly the same would be true if one added extra sockets to the ring, or added several unfused spurs each supplying one socket. Provided that the total number of sockets one ends up with on the circuit is reasonable, in terms of the floor area supplied and anticipated use, I do not see a problem with any of these approaches (including the 4mm² unfused spur) in terms of 314.1.

In fact, although I agree that avoiding inconvenience due to OPD tripping is desirable (and is, by implication, covered in other parts of the regs), I'm not at all sure that 314.1 is relevant to this, since none of things it mentions relate directly to circuit overload, unless it is due to a fault (when, techincally, it is 'fault', rather than 'overload')

If the designer/installer tasked with a work request like 'I want several sockets installed, some distance away from a RFC, I do not want the new sockets to be limited to 13A.' Then I'm not sure your idea would turn out to best the best solution. I'm not saying its unsafe or un-acceptable, just that it may be an obscure case where it might be the preferred method.

I agree that it may not always (indeed, may not ever) be the best solution. I have merely been asking for opinions about it being conssidered as one of the possibvle options.

If this were the case and the work were to be scrutinised, how who you demonstrate that you have considered the guidance suggested by 314.1?

Again, 314.1 does not seem to directly address the issue of inconvenience etc. due to circuit overload - but, that aside, assuming it could be justified (in terms of number of sockets/floor area, anticipated use etc.), I would demonstratew the reasonableness of the circuit in exactly the same was as if the same total number of sockets had been achieved by wiring them all into the ring or by having multiple unfused spurs each serving one socket.

Kind Regards, John

 

[cozycats]

The suggestion of only using one core of the ring would eliminate load balancing problems but wouldn't it produce induction problems and careful consideration as to where cables pass through metal objects such as backboxes?
The 'crossed' (as per r1r2 verification) suggestion would avoid that, but the confusion in marking the cables at the sockets would be immense?

 

[mikhailfaradayski]

Ah, no, to consider the cost of the FCU alone would be rather petty. You seem to have overlooked the bit about I mentioned about the cost differences between 2.5 and 4mm, this combined with other factors may dissuade a designer from using your idea (you mentioned that the additional sockets could be some distance from the rest of the ring)

We did the 2.5/4mm copper/cost difference to death in a thread a few weeks ago.

Did we? I don't recall.

The difference is really not enough to be a significant factor

It could be.

I mentioned it here only 'in passing'.

I think you're probably making a bit too much of the 'some distance from the rest of the ring'.

I don't know how much should be made from that, I was only responding to something you suggested

What I referred to was a situation in which incorporating the new sockets into the existing ring would appreciably increase the total length of the main ring - perhaps 'a pity' for the sake of sockets that one expects to be rarely used.

slightly confused with that, somebody wants sockets, they will be rarely used, but they want the supply to be capable of >13A ?

Seems an unusual request in a typical domestic setting.

Would the case for a dedicated circuit be strong in this instance?

So, to re-answer your question, yes, it would be a 'big problem' (actually an insuperable problem) to use a fused spur if the requirement was (as I've been saying) for more than 13A total potential load on the extra sockets.

What practical reason do you foresee which would make this a reality
Given that one occasionally-used portable appliance (e.g. a fan heater) can use 10A or so, it's far from impossible that more than 13A worth of load would be plugged into a batch of several sockets.

Possible, quite unlikely though, wouldn't you agree?

... I can't really answer the bit about 314.1 without understanding the 'given that' bit (quoted above) which precedes it.

If you take a look at 314.1, it might enlighten you as to why I refer to it.

I know what 314.1 says, but couldn't answer your question without understanding the context you were specifying....

Fair enough

Your idea seems to come from the need to find an easy way to extend a RFC safely and conveniently without using a fused spur and extend the area which that single circuit serves, with a view to it being subject to intermittent high loads.

Yep, that's about it.

If you have a RFC which has already been designed and installed and has been used in service, then you choose to add another branch to supply some sockets that are not limited to what current they can draw, then could end up with an undesirable consequence, e.g. The OPD may trip, causing loss to all sockets on that circuit. Due to the extended nature of your ring, this could be a significant inconvenience to the users of the installation.

All you seem to be saying is that if one adds sockets (to any final circuit) without anything limiting what current can be drawn through those sockets, then there is an increased risk that a user may overload the circuit, with consequential OPD tripping and inconvenience.

Yep, thats about it.

True though that is, it is no way specific to the design I was talking about.

It is

Exactly the same would be true if one added extra sockets to the ring, or added several unfused spurs each supplying one socket.

Of course it would, thats obvious

Provided that the total number of sockets one ends up with on the circuit is reasonable,

what do you mean? What number do you consider to be reasonable?

in terms of the floor area supplied and anticipated use, I do not see a problem with any of these approaches (including the 4mm² unfused spur) in terms of 314.1.

In fact, although I agree that avoiding inconvenience due to OPD tripping is desirable (and is, by implication, covered in other parts of the regs), I'm not at all sure that 314.1 is relevant to this,

It is

since none of things it mentions relate directly to circuit overload, unless it is due to a fault (when, techincally, it is 'fault', rather than 'overload')

Sorry John, but with respect, thats getting close to being pure gibberish.

Amongst other things, 314.1 calls for the division of circuits to minimize inconvenience in the event of a fault, simple. If the OPD disconnects, are you saying you don't consider that to be a fault? That might seem rather bizarre to some.

If the designer/installer tasked with a work request like 'I want several sockets installed, some distance away from a RFC, I do not want the new sockets to be limited to 13A.' Then I'm not sure your idea would turn out to best the best solution. I'm not saying its unsafe or un-acceptable, just that it may be an obscure case where it might be the preferred method.

I agree that it may not always (indeed, may not ever) be the best solution. I have merely been asking for opinions about it being conssidered as one of the possibvle options.

I think you've received a number of opinions about that.

If this were the case and the work were to be scrutinised, how who you demonstrate that you have considered the guidance suggested by 314.1?

Again, 314.1 does not seem to directly address the issue of inconvenience

Again, yes it does, have a second look at it. Actually, it might be worthwhile considering the whole of 314

etc. due to circuit overload - but, that aside, assuming it could be justified (in terms of number of sockets/floor area, anticipated use etc.), I would demonstratew the reasonableness of the circuit in exactly the same was as if the same total number of sockets had been achieved by wiring them all into the ring or by having multiple unfused spurs each serving one socket.

Kind Regards, John

 

[JohnW2]

What I referred to was a situation in which incorporating the new sockets into the existing ring would appreciably increase the total length of the main ring - perhaps 'a pity' for the sake of sockets that one expects to be rarely used.

slightly confused with that, somebody wants sockets, they will be rarely used, but they want the supply to be capable of >13A ?
Seems an unusual request in a typical domestic setting.

I personally wouldn't say so. As we've discussed in other threads, most end-users don't even realise/understand that there are limits to what they can plug into any circuit (assuming that each and every place they have to plug in a 13A plug can always provide 13A), so I think it would be a definite 'inconvenience' for them if they found that an FCU fuse blew when they plugged two or three (credible) things into various sockets. Indeed, it might well be an expensive inconvenience for them, because they quite probably wouldn't even be aware of the existance (let alone function/ significance) of a fuse in an FCU, so might well have to call out an electrician.

All you seem to be saying is that if one adds sockets (to any final circuit) without anything limiting what current can be drawn through those sockets, then there is an increased risk that a user may overload the circuit, with consequential OPD tripping and inconvenience.

Yep, thats about it.

True though that is, it is no way specific to the design I was talking about.

It is

Exactly the same would be true if one added extra sockets to the ring, or added several unfused spurs each supplying one socket.

Of course it would, thats obvious

If you agree that's obvious, why on earth did you answer 'It is' (specific to the design I was talking about) to the previous question. As you say, it's obvious that it applies to any socket circuit.

Provided that the total number of sockets one ends up with on the circuit is reasonable,

what do you mean? What number do you consider to be reasonable?

As I'm sure you know, this is a decision which the designer of any sockets final circuit has to make, depending on the particular circumstances. Although the regs (and OSG) give a bit of guidance it is necessarily a case-by-case decision.

In fact, although I agree that avoiding inconvenience due to OPD tripping is desirable (and is, by implication, covered in other parts of the regs), I'm not at all sure that 314.1 is relevant to this, since none of things it mentions relate directly to circuit overload, unless it is due to a fault (when, techincally, it is 'fault', rather than 'overload')

Sorry John, but with respect, thats getting close to being pure gibberish.
Amongst other things, 314.1 calls for the division of circuits to minimize inconvenience in the event of a fault, simple. If the OPD disconnects, are you saying you don't consider that to be a fault? That might seem rather bizarre to some.

It is not gibberish, bizarre or anything to do with what I consider to be a fault. You do not appear to be familiar with the very specific meaning of 'fault' within BS7671, so you may wish to look at the definition in Part 2 thereof. I still cannot see anything in 314.1 which relates to overcurrent/overload (also defined in Part 2).

Kind Regards, John.

 

[mikhailfaradayski]

What I referred to was a situation in which incorporating the new sockets into the existing ring would appreciably increase the total length of the main ring - perhaps 'a pity' for the sake of sockets that one expects to be rarely used.

slightly confused with that, somebody wants sockets, they will be rarely used, but they want the supply to be capable of >13A ?
Seems an unusual request in a typical domestic setting.

I personally wouldn't say so.

I guess we'll just have to agree to disagree.

As we've discussed in other threads, most end-users don't even realise

Yes, we discussed that. Part of that discussion was to do with that you seem to think that is the belief of the common man, but its not clear that anyone agreed with you

understand that there are limits to what they can plug into any circuit (assuming that each and every place they have to plug in a 13A plug can always provide 13A), so I think it would be a definite 'inconvenience' for them if they found that an FCU fuse blew when they plugged two or three (credible) things into various sockets.

Using your example here and your belief that many people might plug a 13A device into each socket outlet, does that mean that your new sockets should be capable of supplying 26A to 39A? (or would that be 52A to 72A if they were double sockets?)

Indeed, it might well be an expensive inconvenience for them, because they quite probably wouldn't even be aware of the existance (let alone function/ significance) of a fuse in an FCU,

Oh, please John, thats really quite condescending, give a little but more credit to the average person. I doubt there are many people out there who are utterly bewildered by fuses or what they do.

so might well have to call out an electrician.

I think most people of average intelligence have the ability to work out if fuses have blown or not without the assistance of a sparky.

All you seem to be saying is that if one adds sockets (to any final circuit) without anything limiting what current can be drawn through those sockets, then there is an increased risk that a user may overload the circuit, with consequential OPD tripping and inconvenience.

Yep, thats about it.

True though that is, it is no way specific to the design I was talking about.

It is

Exactly the same would be true if one added extra sockets to the ring, or added several unfused spurs each supplying one socket.

Of course it would, thats obvious

If you agree that's obvious, why on earth did you answer 'It is' (specific to the design I was talking about) to the previous question.

Because it has a bearing on the design choices available to the designer, those choices may include your idea. So it is specific to this idea, but also a general consideration for electricians

As you say, it's obvious that it applies to any socket circuit.

Provided that the total number of sockets one ends up with on the circuit is reasonable,

what do you mean? What number do you consider to be reasonable?

As I'm sure you know, this is a decision which the designer of any sockets final circuit has to make, depending on the particular circumstances. Although the regs (and OSG) give a bit of guidance it is necessarily a case-by-case decision.

Of course I'm aware of that, but if you were the designer, how many sockets would you consider reasonable to add to an existing RFC?

In fact, although I agree that avoiding inconvenience due to OPD tripping is desirable (and is, by implication, covered in other parts of the regs), I'm not at all sure that 314.1 is relevant to this, since none of things it mentions relate directly to circuit overload, unless it is due to a fault (when, techincally, it is 'fault', rather than 'overload')

Sorry John, but with respect, thats getting close to being pure gibberish.
Amongst other things, 314.1 calls for the division of circuits to minimize inconvenience in the event of a fault, simple. If the OPD disconnects, are you saying you don't consider that to be a fault? That might seem rather bizarre to some.

It is not gibberish, bizarre or anything to do with what I consider to be a fault. You do not appear to be familiar with the very specific meaning of 'fault' within BS7671,

I am familiar with definitions, but you seem to have turned the tables a bit there. Are you now suggesting that you must only consider words in the way the are laid out in BS7671? That seems to contradict what you mean't when you say that something like most electricians can only follow the BRB like a rule book?

so you may wish to look at the definition in Part 2 thereof. I still cannot see anything in 314.1 which relates to overcurrent/overload (also defined in Part 2).

Keep looking, it may also benefit you if you read the whole section.

Kind Regards, John.

I'm not sure where this thread is going now, but just to summarise it as i see it so far-

- You have an idea which doesn't appear to be unsafe, in terms of cable ratings and so on.
- You claim it may have benefits should it be an acceptable design.
- Some people argue whether it is compliant with current regs or not.
- Some people argue whether you need to be compliant in every respect.
- My point of view is that if it is safe, it is not excluded from use, then it may be a valid solution. However, I am not convinced of the real -life benefits it may provide, considering other options that would be available.

Does that sound about right to you?


John, just out of curiosity, I wonder if you might answer me a couple of questions?

-Are you a practising installer or designer? If not, would you mind telling me what it is that you do do?

-Have your ideas been born from necessity, a real scenario that you have encountered or are these ideas purely hypothetical?

If this is for a real situation, and you have considered all the options and decide your idea is the most appropriate solution, then go for it!

If this is just creating an idea for the purpose of enjoying the debate, then go for it!, but if thats all, then might i suggest that you have attempted to re-invent an old wheel, but created a white elephant instead.
You solution may be valid, but practically, i imagine it would only be the preferred solution to 1 out of 100 situations.

 

[JohnW2]

These messages are getting far too long (and time-consuming to respond to). I'm therefore going to be selective and only respond briefly to selected key points ...

Using your example here and your belief that many people might plug a 13A device into each socket outlet, does that mean that your new sockets should be capable of supplying 26A to 39A? (or would that be 52A to 72A if they were double sockets?)

No, I obviously did not mean that. However, I don't think that it's unreasonable to anticipate that more than one item might be plugged into a group of several double sockets.

Oh, please John, thats really quite condescending, give a little but more credit to the average person. I doubt there are many people out there who are utterly bewildered by fuses or what they do .... so might well have to call out an electrician..... I think most people of average intelligence have the ability to work out if fuses have blown or not without the assistance of a sparky.

I disagree. There are some fused spurs in my house. I've just asked the occupants (all of whom are graduates, so probably not below average intelligence) about them. None of them knew of the existance, location or function of FCUs, or that they contain fuses.

As I'm sure you know, this is a decision which the designer of any sockets final circuit has to make, depending on the particular circumstances. Although the regs (and OSG) give a bit of guidance it is necessarily a case-by-case decision.

Of course I'm aware of that, but if you were the designer, how many sockets would you consider reasonable to add to an existing RFC?

If you are aware of the fact that it is necessarily a case-by-case decision, you will also understand that I can't answer that question generically. It obviously depends upon how many sockets are already on the ring, how they are distributed, what floor area and types of rooms the sockets serve, and what is their anticipated use.

I am familiar with definitions, but you seem to have turned the tables a bit there. Are you now suggesting that you must only consider words in the way the are laid out in BS7671? That seems to contradict what you mean't when you say that something like most electricians can only follow the BRB like a rule book?

When interpreting a section in BS7671 (like 341.1 which you cited) which refers to 'faults', one obviously has to use the definition of 'fault' which is specified in the document; that's why such documents have definition sections ("For the purpose of the Regulations, the following definitions shall apply"). It is a rule book (albeit the 'rukes' are not legally mandatory). My comments to which you refer relate to people who cannot see beyond the examples and deemed-to-satisfy provisions to the underlying rules which are specified.

Are you a practising installer or designer? ..... Have your ideas been born from necessity, a real scenario that you have encountered or are these ideas purely hypothetical?

I am not a practising installer or designer. but I do sometimes get involved in briefing and approving the proposals of such people. All of the questions I have raised relate to real-world situations which exist (i.e. which I have encoutered).

If this is just creating an idea for the purpose of enjoying the debate, then go for it!,

As above, they are not 'created ideas'. However, I do enjoy (some of!) the debate, and I certainly would like to see a situation in which more of those who do design BS7671-complaint electrical installations were able and prepared to be more open to designs which can be shown to be compliant, even if they are not explicitly mentioned in the regs.

Kind Regards, John