Multiple FCUs

[JohnW2]

No problem - it's always nice to see indications that you actually are human, like the rest of us

Returning to a related topic, I wonder what you and others feel about the practice of connecting 13A BS1362 (rather than, say, 5A) sockets to a 6A lighting circuit - which seems not to be uncommon (e.g. in lofts to run TV amplifiers etc.). In this case, all the wiring is adequately protected by the 6A MCB, so are you happy for the socket(s) to be directly connected to the lighting circuit, without any intervening fuse (which would seem a bit redundant), despite the fact that one could obviously overload the circuit (and, ultimately, operate the MCB) via 1 or 2 '13A' socket outlets. I think I would be perfectly happy with that - although, in my usual fashion, I'm sure I would 'label' the sockets accordingly to indicate that they were only intended for small loads.

As for 'why 13A sockets' (rather than, say, 5A) ones, in many cases where this is required there could be a wallwart involved - so 13A sockets avoid the need for an 'adapter cable'.

Kind Regards, John

 

[Chri5]

[It's obviously impossible to "show that the connected load could not be increased", since it obviously could be increased by someone who had such a desire, just as a 6A MCB could be 'upgraded' to a 32A one!

John- Have you considered that a fused spur fuse can be changed for fault, failure or load by the 'user'. The user being a house holder aged 13-99 ??

The issue is that a mcb is a skilled task, the installation of the fspur could be and the changing of fuses in the spur by anyone.

The installer and designer have a duty of care. Providing a bank of any number beyond 1 x fused spur as a rf spur should be limited to one unit or limited by having a master fuse spur which controls the max load of the fs beyond it.

 

[Paul_C]

I also considered that - but does BS7671 actually say explicitly that such an arrangement is non-compliant - and, if so, where? Each of the fixed loads has an appropriate, separate, fuse, the only issue being the short length of 2.5mm² cable common to the feed of all of them (and the supply path is obvioulsy common to all three FCUs once one gets upstream of that.

Hmmm........ I'm not sure of the precise wording in the current edition, so perhaps it's changed. But I'm pretty sure it used to the case that the actual cable running from the ring to the collection of FCU's in your proposal would have been classed as an unfused spur, which was limited to feeding one fixed appliance. Obviously there would be fuses for each load in those FCU's, but they would be just the fuses required at the connection point for each appliance not connected by way of a (fused) plug & socket.

But on the later point, while somebody could come along some years into the future and connect higher-powered appliances, changing the fuses to 13A, I agree with you that it's not your job to foresee everything that somebody might do in the future which would create a danger. As you say, if that were the case we'd have to be designing shower circuits for about 11.5kW even if it was intended to connect only an 8kW shower at the present time, just "in case" somebody swapped the shower unit out later and changed the MCB. You wouldn't install a light fitting rating for 40W maximum "in case" somebody comes along later and fits a 100W lamp. And so on.

So as I've said about other aspects of electrical installations before, I don't believe it's your responsibility to design in a way which caters for somebody else making changes later without understanding the implications of those changes.

 

[JohnW2]

John- Have you considered that a fused spur fuse can be changed for fault, failure or load by the 'user'. The user being a house holder aged 13-99 ??

Sure, I've considered that - and if we were talking about 'plug-in loads' (also at the mercy of 13-99 year-olds), then I wouldn't be thinking the way I do. However, when we're talking about a hard-wired fixed load, I don't really see the difference from a shower.

JohnThe issue is that a mcb is a skilled task, the installation of the fspur could be and the changing of fuses in the spur by anyone.

As above, my thinking is that the changing of a hard-wired fixed load (whether a shower or anything else) is also a 'skilled task' - whereas, as you say, the changing of an FCU fuse certainly isn't.

The installer and designer have a duty of care. Providing a bank of any number beyond 1 x fused spur as a rf spur should be limited to one unit or limited by having a master fuse spur which controls the max load of the fs beyond it.

That's what everyone is saying, and probably what I'll do - but my feelings/comments remain.

I presume that you see the problem as the current-carrying capacity of the short length of spur cable feeding the three FCUs, rather than the concentration of theoretically potential load (if some changed the fuses and loads) in one part of the ring - i.e. that you would be happy with the three FCUs having their own separate spurs from the ring, joined to the ring just inches apart? If so, what would you say if there were just one spur feeding the three FCUs, but wired with 4mm² cable?

Kind Regards, John.

 

[Paul_C]

The installer and designer have a duty of care.

But not a duty to consider every possible thing that somebody else might do wrong in the future, otherwise he'd never be able to install anything.

The person who comes along later and wants to change 3 x 100W loads for 3 x 2kW loads has to accept responsibility for such a change and make sure that the existing circuit arrangements are suitable.

 

[JohnW2]

Hmmm........ I'm not sure of the precise wording in the current edition, so perhaps it's changed. But I'm pretty sure it used to the case that the actual cable running from the ring to the collection of FCU's in your proposal would have been classed as an unfused spur, which was limited to feeding one fixed appliance.

Not unreasonably, the regs do not 'allow' an unfused spur to feed anything, other than sockets into which fused plugs can be inserted - a fixed load can only be supplied by an FCU (although I agree one can debate what one calls the cable between ring and FCU).

But on the later point, while somebody could come along some years into the future and connect higher-powered appliances, changing the fuses to 13A, I agree with you that it's not your job to foresee everything that somebody might do in the future which would create a danger. As you say, if that were the case we'd have to be designing shower circuits for about 11.5kW even if it was intended to connect only an 8kW shower at the present time, just "in case" somebody swapped the shower unit out later and changed the MCB. You wouldn't install a light fitting rating for 40W maximum "in case" somebody comes along later and fits a 100W lamp. And so on.
So as I've said about other aspects of electrical installations before, I don't believe it's your responsibility to design in a way which caters for somebody else making changes later without understanding the implications of those changes.

Yes - exactly my point. The more one thinks about it, the sillier one could get in attempts to pre-empt changes which someone might make in the future. As with labelling a light fitting "40W maximum" (even though a 150W lamp would fit into the lampholder), I personally think that adequately labelling things (like my FCUs) should be adequate. If someone decides to change the fuses in all of my FCUs from 3A to 13A, and then wire 3kW loads into each of them, despite labels/warnings on them that the fuse should be "3A maximum", that is surely 'their problem'?

You can go to your high street pharmacy and buy a bottle which contains many times the lethal dose of, say, paracetamol - and no-one can stop a sufficiently stupid or determined person from downing the lot once they have bought them. However, labelling the bottle with the maximum permitted dose and "Do not exceed the stated dose" is considered to be an acceptable safeguard in that situation.

Kind Regards, John.

 

[EFLImpudence]

As far as the original proposal (daisy-chained FCUs on supply side) is concerned -

If this were a Radial FC then that is how it would be done.
A radial, though, would have a 20A mcb so the issue is overloading the spur but it cannot be overloaded because there are just three 3A fuses.

I think, therefore that this is a debate between a/my 'bog standard Ring FC without thinking' as per appendix 15 and having worked out something else which is sufficiently protected and equally safe.

If you were to install a 13A FCU before the other three, what would stop the future owner wrongly altering this?
Future errors by JohnW3 buying the house and connecting his hadron collider are not your responsibility.

 

[JohnW2]

As far as the original proposal (daisy-chained FCUs on supply side) is concerned - If this were a Radial FC then that is how it would be done. A radial, though, would have a 20A mcb so the issue is overloading the spur but it cannot be overloaded because there are just three 3A fuses.

If it were a radial circuit, the question would not really arise (certainly not in the same form), because no part of the cable could ever carry a current above the limit imposed by the circuit's OPD (regardless of what loads were connected), and cable size would have been selected such as to be adequate for that current.

As If you were to install a 13A FCU before the other three, what would stop the future owner wrongly altering this? Future errors by JohnW3 buying the house and connecting his hadron collider are not your responsibility.

That is, as you will realise, my view. I would, however, feel duty-bound to label the FCUs appropriately ("3A max"), as a guide to anyone who might subsequently contemplate changing the installation.

Kind Regards, John

 

[EFLImpudence]

because no part of the cable could ever carry a current above the limit imposed by the circuit's OPD (regardless of what loads were connected)

Nor can yours without changing an OPD.

 

[JohnW2]

because no part of the cable could ever carry a current above the limit imposed by the circuit's OPD (regardless of what loads were connected)

Nor can yours without changing an OPD.

Yes, that's true (and, of course, part of the basis of my views) - but I cannot disagree with the view that several people have expressed that fuses in FCUs are much more likely to be changed by 'unskilled people' than are MCBs.

Kind Regards, John.

 

[EFLImpudence]

I cannot disagree with the view that several people have expressed that fuses in FCUs are much more likely to be changed by 'unskilled people' than are MCBs.

So you still feel you are responsible for future (mis)use.
There are a myriad of possibilities for what may happen in the future.

Without the (restrictive and often unnecessary) manufacturers' instructions it would be acceptable to run all three items of equipment from the load side of one 13A fuse but I know you are uncomfortable if all items are not fused down as much as possible.
Having said that it begs the question why you originally posted the question to save one extra FCU.

I don't think you have said what the three items are. Would they perhaps be small enough to run off one 3A fuse?
Although if switches are needed they may as well be SFCUs.

 

[timbim]

I don't think you have said what the three items are. Would they perhaps be small enough to run off one 3A fuse?
Although if switches are needed they may as well be SFCUs.

For the greatest neatness, could you not have a 4 way grid with one fuse serving three switches?

Tim

 

[securespark]

....

Could you perhaps expand on that a little?

Kind Regards, John.

I can
.....

You beat me to it!!

I can appreciate your train of thought, John, but I would be tempted to remove any ambiguity or possibility of abuse & do what you suggested: put that 13A FCU in prior to the 3A ones.

 

[holmslaw]

..

 

[JohnW2]

I cannot disagree with the view that several people have expressed that fuses in FCUs are much more likely to be changed by 'unskilled people' than are MCBs.

So you still feel you are responsible for future (mis)use. There are a myriad of possibilities for what may happen in the future.

I'm just trying to find a sensible middle course. As I've said many times in the last 24 hours, I certainly don't feel any responsibility in relation to incompetent activities of people in the future doing things which only a 'skilled' person should be doing - although I do feel some responsibility to inform/guide them with appropriate labelling/documentation. However, I also recognise that there are things which people with limited knowledge/skill are more likely to attempt (wisely or unwisely) -and I think it's fair to say that Joe Public is far more likely to change a fuse in an FCU than change a MCB in a CU.

Without the (restrictive and often unnecessary) manufacturers' instructions it would be acceptable to run all three items of equipment from the load side of one 13A fuse ....

That is true (indeed, as you ask later, they'd probably be OK off one 3A fuse) - but there are three points here. Firstly, as you go on to surmise, I would ideally like separate switches for each of the loads, so might 'just as well use FCUs'. Secondly, even if I went without the separate switching, all three loads are connected by flexible cables, and the mechanics of FCUs (and their cable entries/clamps etc.) are far from ideal for three flexible cables. Thirdly ....

but I know you are uncomfortable if all items are not fused down as much as possible.

'Uncomfortable' is probably a bit stronger than the truth. However, I do feel that it makes sense to fuse down as far as possibly, partially because I believe that the fuses (if low-enough rated) can serve useful purposes beyond merely protecting the supply cable, and partially because this increases the chances of achieving discrimination from other upstream OPDs.

Having said that it begs the question why you originally posted the question to save one extra FCU.

A fair question. To be honest, the main reason for wanting to 'save one etra FCU' was a fairly trivial practical one - that whilst two additional FCUs would 'fit nicely' in the space available, a bit of re-arranging will be necessary to get three to fit! As I initially wrote, I was essentially happy with the design I had conceived (which is clearly totally OK so long as I am in residence) but suspected that others might not be, so thought I would canvass opinions as to what any future PIR/EICR might have to say about it!

Kind Regards, John.