Mains smoke and heat detectors

[PBC_1966]

No, you don't have to!

O.K., don't have to, but would like to!

Since the BS7671 table is clearly trying to make a distinction between a lighting circuit and a power circuit, it might be considered important to understand the intended meanings of those terms.

They're simple statements. Which word(s) don't you understand?

NOTE 4 In the UK, 1,0mm2 cable is allowed for use in lighting circuits.
NOTE 5 In the UK 1,0 mm2 copper cable is allowed for fixed installations utilizing cables and insulated conductors for power and lighting circuits.

Ignoring that we're not clear on the difference between a lighting circuit and a power circuit, and assuming that somehow the intended meaning is that the latter does not include the former, note 4 starts off clear enough if read by itself, inferring that 1.0 is the minimum for a lighting circuit and 1.5 the minimum for anything else (presumably a power circuit, whatever that may be).

Note 5 then seems to suggest that the U.K. allows 1.0 for both lighting and power circuits (but only if copper). But what are we supposed to understand from the phrase "utilizing cables and insulated conductors" in this context?

As I mentioned before, does "cables" mean composite cables such as T&E, and "insulated conductors" mean singles for conduit use, perhaps? That interpretation would mean that with T&E you can use 1.0 for lighting circuits, but need 1.5 for power circuits (however defined), but if wiring in singles in conduit you can use 1.0 for power circuits as well as lighting circuits. But that would make "cable" in note 5 mean any conductor in whatever form in its first use and only a composite cable in its second.

What other interpretations might we get from note 5?

 

[stillp]

Well, a cable can be single core, but assuming by "composite" you mean multicore, I would agree with your interpretation that 1,5mm² is the minimum for power circuits unless conduit is used, when 1.0mm² is permitted.

 

[JohnW2]

Well, a cable can be single core, but assuming by "composite" you mean multicore, I would agree with your interpretation that 1,5mm² is the minimum for power circuits unless conduit is used, when 1.0mm² is permitted.

That may be what is intended (and would seem to make reasonable sense), but it requires an awful lot of 'assuming' (or guessing!). Is this not appalling for a Standard?

Kind Regards, John

 

[PBC_1966]

Well, a cable can be single core, but assuming by "composite" you mean multicore, I would agree with your interpretation that 1,5mm² is the minimum for power circuits unless conduit is used, when 1.0mm² is permitted.

As John says though, it's a pretty convoluted route of assumptions and trying to second-guess what was intended to get there. And from what's been posted so far regarding the BS7671 table, it doesn't seem to agree with the latter anyway.

 

[EFLImpudence]

Appliances are outside the scope of BS7671.

They are. However, it still begs the question as to whether or not a circuit supplying 2A or 5A BS 546 sockets, originally "intended" to be used for lights, qualifies as a 'lighting circuit', since equipment other than lighting could be plugged into the sockets.

Does your house meet the requirements for a motorway, just in case somebody changes it from the designer's intent?

As with a ring final, you cannot design a circuit with sockets as far as the loading is concerned.
It just has to have an opd to protect the cable - so, with 1mm², 16A and 1.5mm², 20A (method C).

Why would a 16A (or 6A) circuit have to have 1.5mm²?

If, as you say, (is there anything in writing?) these requirements are for mechanical protection, why does it not apply to lighting circuits?
If it stated 10mm² cable was the minimum would you not query it and want to know why?

 

[stillp]

If, as you say, (is there anything in writing?) these requirements are for mechanical protection, why does it not apply to lighting circuits?

I should have added that the table is preceded by this sentence:

524.1 For mechanical reasons, the cross-sectional area of line conductors in a.c. circuits
and of live conductors in d.c. circuits shall be not less than the values given in Table 52.2.

I could guess at why the requirement for lighting circuits is different, but it would only be a guess.

 

[EFLImpudence]

Then - do you think it may be unnecessary?

Is your Table 52.2 exactly the same as BS7671 Table 52.3?

 

[JohnW2]

I could guess at why the requirement for lighting circuits is different, but it would only be a guess.

We're very used to seeing guesses and personal opinions here - would you be prepared to share yours?

Kind Regards, John

 

[JohnW2]

Does your house meet the requirements for a motorway, just in case somebody changes it from the designer's intent?

That's getting a little silly!

As I often comment, given the absence of crystal balls, it's impossible to 'properly design' any sockets circuit. However, do you seriously believe that it would be acceptable (or, at least, sensible) to design a circuit feeding, say, multiple BS1363 sockets on the basis of the "intent" (of designer and current householder/user) that the total load on all the sockets would never exceed, say, 6A or 10A ??

Kind Regards, John

 

[stillp]

I could guess at why the requirement for lighting circuits is different, but it would only be a guess.

We're very used to seeing guesses and personal opinions here - would you be prepared to share yours?

Kind Regards, John

My guess is that lighting circuits are mostly out of reach during normal service, whereas circuits serving socket-outlets, cooker circuits, circuits supplying machines, and similar, are not.

 

[stillp]

Does your house meet the requirements for a motorway, just in case somebody changes it from the designer's intent?

That's getting a little silly!

As I often comment, given the absence of crystal balls, it's impossible to 'properly design' any sockets circuit. However, do you seriously believe that it would be acceptable (or, at least, sensible) to design a circuit feeding, say, multiple BS1363 sockets on the basis of the "intent" (of designer and current householder/user) that the total load on all the sockets would never exceed, say, 6A or 10A ??

Kind Regards, John

No, I personally don't, but it seems quite common on this forum when a poster asks for advice on a supply to an outbuilding.

 

[JohnW2]

My guess is that lighting circuits are mostly out of reach during normal service, whereas circuits serving socket-outlets, cooker circuits, circuits supplying machines, and similar, are not.

... but we're talking about the fixed wiring, aren't we, and that is usually no less "out of reach during normal service" for sockets/cooker circuits etc. than for lighting ones, is it?

Kind Regards, John

 

[stillp]

Then - do you think it may be unnecessary?

Is your Table 52.2 exactly the same as BS7671 Table 52.3?

By "my" table, do you mean the one in IEC 60364-5-52? If so, then the provisions are substantially the same.

 

[JohnW2]

No, I personally don't, but it seems quite common on this forum when a poster asks for advice on a supply to an outbuilding.

Fair enough, but the issue with an outbuilding is only a subset of the problem with any sockets circuit, and one has to give some weight to what the (current) user says about usage/loading. It would, after all, be physically possible to plug 200A-300A worth of load into many a 'normal' ring final!

Kind Regards, John

 

[PBC_1966]

My guess is that lighting circuits are mostly out of reach during normal service, whereas circuits serving socket-outlets, cooker circuits, circuits supplying machines, and similar, are not.

But as John has asked, what does that have to do with the minimum size for fixed wiring supplying those various devices? I don't see your line of reasoning here.