Spinoffs from "Switched Live sleeving when looping at switch"

[JohnW2]

I just think the parts of Table 52.3 are so contradictory that it makes a nonsense of the contradicting parts.

We have agreed on many occasions that the Table is an abomination in all sorts of senses - particularly in relation to presentation/contradictions/lacks of clarity/definitions and also the fact that the whole concept seems to be without any electrical basis - the required CSA of a conductor obviously depends upon what current it has to carry (and on installation method, ambient conditions and other factors), but why on earth does there need (electrically) be a minimum which varies according to the 'type of circuit'??

However, NOTE 1 states "IT, signalling and control circuits intended for electronic equipment...".

It does, but you would have to ask them why they included that as a note rather than as an additional row in the table.

In BS7671 there is no definition of Control Circuit but the index has for Control Circuit states "see Voltage Band 1 definition". Make of that what you will.

Quite so. Your guess is as good as mine.

As for the different identification colours in Table 51 for "Control Circuits, ELV and other applications", do people think that the colours should be used in order. That is, if only one Live conductor then Brown would be used; for more than one then in the order stated....

Since it says nothing like that, anyone who thought' as you suggest would merely be guessing. In the absence of specific definitions of what any of the individual colours 'mean', I can't see it makes much difference which are used, or in what 'order'.

In a central heating system where the boiler and programmer are in the kitchen and valve and tank thermostat are in the airing cupboard, one might use (as I have) a 7-core cable with numbered black conductors. Obviously the numbers are adequate but the different colours might be used for easier identification.

Fair enough.

I would class that as a 230V control circuit, but it might not be what is meant.

As has been said (in the other thread), I think that's debatable - but, as you know, I would personally not call it a control circuit if the 'controlling devices' were merely switches (including relay contacts) which were switching the full loads (of boiler and valve).

For various reasons, I have a light switch on the ground floor which switches a light two floors above - would you call that a 'control circuit'?

Kind Regards, John

 

[EFLImpudence]

No.

 

[JohnW2]

No.

Thanks. As you know, neither would I.

What would you say (in your example) if there was just one 'item' in the airing cupboard (valve OR tank stat), with the boiler and programmer in the kitchen - would you still call the wiring which 'went upstairs' a control circuit?

Kind Regards, John

 

[EFLImpudence]

What would you say (in your example) if there was just one 'item' in the airing cupboard (valve OR tank stat), with the boiler and programmer in the kitchen - would you still call the wiring which 'went upstairs' a control circuit?

Yes. I don't think that would make any difference.

 

[JohnW2]

Yes. I don't think that would make any difference.

OK, so what is it that makes you say that, for example, a thermostat on a different floor represents a control circuit, whereas a light switch on a different floor does not? Is it a question of the nature of the 'switching device', or what?

Kind Regards, John

 

[EFLImpudence]

After all that has been said, I am not sure.

I just think that the wiring in, for example, a central heating system, after the isolator, is control wiring which can be done with small csa cable.

Perhaps it is because it is part of the general circuit to which other items will not be added.

 

[JohnW2]

After all that has been said, I am not sure. I just think that the wiring in, for example, a central heating system, after the isolator, is control wiring which can be done with small csa cable.

Are you perhaps simply thinking/saying that, since it will only ever carry a very small current (regardless of what one calls the type of circuit)? If so, I agree with you - as I recently wrote, my fundamental 'issue' with Table 52.3 is that I don't really understand why the 'type of circuit' should impose a minimum CSA. As I said, provided that the cable, as installed, was satisfactory for the current it was required to carry (and provided it was appropriately protected), then I don't see why there should be an (arbitrary) minimum acceptable CSA for any 'type of circuit'.

In other words, are you simply saying that a small CSA cable should be allowed, rather than that you think it should be called a 'control circuit'?

Perhaps it is because it is part of the general circuit to which other items will not be added.

Perhaps. However, as above, the apparent illogicality exists far more widely. What if, for whatever reason, I installed a dedicated circuit to supply just one very small (non-lighting!) load - does it make sense that the cable has to be at least 1.5mm², when 0.5mm² would probably be far more than was actually needed?

Kind Regards, John

 

[EFLImpudence]

No. I was not taking Table 52.3 into account in any way - just how I was thinking.

 

[JohnW2]

No. I was not taking Table 52.3 into account in any way - just how I was thinking.

Oh ... since you said that it (CH wiring) could "be done with small CSA cable" (with which I obviously agree, electrically speaking), I thought you were therefore wanting to call it a 'control circuit', so that Table 52.3 would then 'allow' you to use 0.5mm² (rather than 1.5mm²) cable.
'
Kind Regards, John

 

[EFLImpudence]

I agree with you; it is solely dependent on the current,

I just meant it could be done with small CSA cable as it is exclusive to the CH system, unlike your light switch with light a couple of floors up which could have other items added to it - albeit still unlikely to exceed the current but you know what I mean.

 

[ericmark]

I can see the point where all wiring to a gas or oil boiler is control wiring, the power comes from oil or gas, electric only controls that power.

As to at what point a valve or pump become not part of a control circuit can be debated, the same water pump which circulates central heating water to control how heat is transferred from boiler to house, could also circulate hot water around the house so there is no delay getting hot water at the taps, but the latter is not a control device.

There are many things in BS7671 which if taken as written are clearly wrong, there is no way we will fit a RCD to the 12 volt supply to door bell, but the rules on RCD do not state it does not include ELV supplies. Some times we just have to use common sense. And I would not install 230 volt wiring under 1 mm² LAN cables, telephone cables yes, but not 230 volt even if 0.5 mm² would do the job. However I will admit with the Nest heat link the space for cables is rather limited and I have used a wiring centre tight against it to allow multi wires to connect to one terminal. Had I used 0.5 mm² that wiring centre would not be required, so I can see why people would use thin cables.

Most of the central heating control is wifi and bluetooth today, using batteries to open and close motorised valves (TRV with electronic head is I would say a motorised valve) so why should the supply to a hard wired motorised valve be with thick cable? whole supply to central heating is normally from a 3 amp fuse.

 

[JohnW2]

I agree with you; it is solely dependent on the current, .... I just meant it could be done with small CSA cable as it is exclusive to the CH system ...

As you know, we're definitely agreed about that.

.... unlike your light switch with light a couple of floors up which could have other items added to it - albeit still unlikely to exceed the current but you know what I mean.

Yes, I know what you mean, but I don't think it really applies to my ligt switch - it's simply a 2-core 'switch drop' (the supply for the light coming locally at the location of the 'distant' light). Hence, other than putting additional lights in parallel with the existing one (which would be trivial, current-wise, anyway), if anything were added to the lighting circuit in question, the current of any additional load would not travel through the 'switch drop' in question - particularly given that it's an LED, electrically speaking, 0.5mm² would probably be definite overkill, and even 0.1mm² would probably be more than is actually needed!

However, it still seems to me that you're just talking about situations (like your CH one) where cable CSAs much smaller than 'required' by BS7671 would be electrically fine - whereas what you actually wrote was ...

In a central heating system where the boiler and programmer are in the kitchen and valve and tank thermostat are in the airing cupboard, one might use as I have) a 7-core cable with numbered black conductors. ..... I would class that as a 230V control circuit, but it might not be what is meant.

... whereas everything you have subsequently said has been about the adequacy of small CSA cable in such a situation (with which I totally agree). This is why I've been a bit uncertain as to whether you really were talking about whether or not it was a 'control circuit', rather than just the fact that a much smaller CSA than allowed by regs for a 'power circuit' would be (electrically) adequate.

Kind Regards, John

 

[EFLImpudence]

I can't say if it is a 'control circuit' as far as BS7671 is concerned because it doesn't define it.
I would call it a control circuit and you might call your switch drop a control circuit.

Table 52.3 mentions ELV electronics, so presumably our circuits cannot be control circuits, but how would the same thing at 230V be different?

Also I have just realised another anomaly in 52.3. The first part - minimum T&E size for lighting circuits - states 1mm². There is no smaller T&E so this is even more pointless.
So, if you delete that part, then all the table says is that 1.5mm² is the minimum for power circuits - what ever they are.
The section on flexible cable does not really say anything at all.

 

[JohnW2]

I can't say if it is a 'control circuit' as far as BS7671 is concerned because it doesn't define it.

Quite so.

I would call it a control circuit and you might call your switch drop a control circuit.

I might - but, as I've said, that would mean that the connections to any functional switch (or 'switching device' {e.g. timeswitch, thermostat etc.} probably even isolator) would qualify as 'a control circuit' - which would not only be silly (since it would include so many things), but is not really 'a circuit' (in any sense I know), since it's only 'part of a circuit'.

Table 52.3 mentions ELV electronics, so presumably our circuits cannot be control circuits, but how would the same thing at 230V be different?

I don't think that is what it's saying. The main part of the table says that the minimum CSA for "signalling and control circuits" (without qualification, so presumably including 230V as well as everything else) is 0.5mm² - so I read that as saying that one could use 0.5mm² for a 230V 'signalling or control circuit'). Note 1 merely extends that, and says that IF one is talking about ELV circuitry in electronic equipment, THEN the minimum falls to 0.1mm².

Also I have just realised another anomaly in 52.3. The first part - minimum T&E size for lighting circuits - states 1mm². There is no smaller T&E so this is even more pointless.

As we've discussed before, I think it's worse than that. Although the last section of the Table is called "Non-sheathed and sheathed flexible cables", the first section is just called "Non-sheathed and sheathed cables" (no mention of "non-flexible"). That means that, strictly speaking, flexible cables fall into both categories - such that it could be argued that the (higher) minimum CSAs in the first section 'trump' the (lower) minimum CSA figures in the last (flexible cables) section!

Returning to your specific point, that first section doesn't say anything about "T+E" (although that is what 'we' assume it to mean) - so, if taken literally, could be taken to apply to any type of cable, not just T+E.

Kind Regards, John

 

[EFLImpudence]

Ah, yes, so, even dafter.