CHESTNUT

[ebee]

I meant planning it out for first fix.
Getting a reading once cables installed should be more accurate (dependent on the meter, amb temp etc etc basic error plus field errors) but by then it would be too late , you are rather committed to positions.

Not impossible but might be difficult to say the least

 

[JohnW2]

I meant planning it out for first fix. Getting a reading once cables installed should be more accurate (dependent on the meter, amb temp etc etc basic error plus field errors) but by then it would be too late , you are rather committed to positions.

True - but, in reality, I doubt whether anyone in their right mind would be designing sich a circuit! In any event, being exactly at the mid-point would not be all that critical - 20A (quite probably more, depending on installation method) is significantly more than half of 32A.

Kind Regards, John.

 

[ban-all-sheds]

yes, in theory 434.2 can be satisfied without invoking the 3m limit if you want to do the adiabatic calculation in 434.5.2,.

do you now agree that ... the 3m limit does not apply?

I did say, earlier, "yes, in theory 434.2 can be satisfied without invoking the 3m limit if you want to do the adiabatic calculation in 434.5.2...".

...so, provided ... then there is no 3m limit, ... Do you agree?

If I have succeeded in convincing BAS (he hasn't actually said so yet!)

yes, in theory 434.2 can be satisfied without invoking the 3m limit if you want to do the adiabatic calculation in 434.5.2,.

I did say, earlier, "yes, in theory 434.2 can be satisfied without invoking the 3m limit if you want to do the adiabatic calculation in 434.5.2...".

 

[ban-all-sheds]

2) Re the overload protection - it makes no difference to the requirements for fault protection, so I do still struggle to understand why, given that you do have overload protection as per 433.2.2 you would wish to perversely ignore that and claim that you didn't need it as per 433.3.1.

Well, yes, you can argue in one of two possible ways

One of which is perverse.

but, either way, there is no 3m length limit. Firstly, as you have been doing, one can argue that overload protection is being provided by the downstream fuses per 433.2.2 - but, if you have adequate fault protection (see above), you can utilise 433.2.2(i) [rather than 433.2.2(ii)] and therefore not have a 3m limit. Alternatively, given adequate fault protection, you can invoke 433.3.1(ii) to demonstrate that overload protection does not have to be provided. 433.2.2 (which is about positioning of an OPD, if one has one) and, with it, the 3m limit in 433.2.2(ii), then ceases to be applicable ...

What that all boils down to is this.

With overload protection you need the cable to be ≤ 3m etc, or to have fault protection.

Without overload protection you need an overload to be unlikely and the cable to be ≤ 3m etc, or to have fault protection.

You have overload protection.

Why would you wish to discount that, say that you don't actually need it to be there, just to end up with the same outcome that you have by recognising that it is there?

 

[JohnW2]

With overload protection you need the cable to be ≤ 3m etc, or to have fault protection.

Indeed.

Without overload protection you need an overload to be unlikely and the cable to be ≤ 3m etc, or to have fault protection.

I don't get that one. Shouldn't it read "Without overload protection you need an overload to be unlikely and to have fault protection." ? Where do you get the <3m requirement, or the "...or fault protection", from?

I think I can see where some of the confusion (at least, in my mind) has come from. I have been assuming that adequate fault protection would always be required (thereby satisfying either of the above). Under what circumstances (& regs) do you envisage that cables in a final sockets circuit would not be required to have adequate fault protection?

Kind Regards, John.

 

[scousespark]

Incredible!!!! 6 pages discussing whether the regs allow an unfused spur longer than 3m

 

[JohnW2]

Incredible!!!! 6 pages discussing whether the regs allow an unfused spur longer than 3m

Indeed - but, although not part of the regs, there could actually be a far more constraining guideline.

It's not clear (at least not to me) as to what 7.2.2 of the OSG means by "the furthest point of the ring" (furthest from the CU, furthest from the spur, or what?) but, whatever it means, this guidance could result in the recommended maximum length of an unfused spur being pretty short, certainly <3m and maybe even <1m for small rings.

Kind Regards, John.

 

[EFLImpudence]

It's not clear (at least not to me) as to what 7.2.2 of the OSG means

The reason for that could be this extract from the preface of the OSG (red one).

However, this guide does not ensure compliance with BS 7671. It is a simple guide to the requirements of BS 7671, and electricians and electrical installers should always consult BS 7671 to satisfy themselves of compliance.

I do note that there are no regulation reference numbers in the margin of 7.2.2 or many of the other sections of section 7.

So, perhaps it (7.2.2) doesn't mean much.


I would say that it is obvious, in my opinion, that the farthest point of a ring is half way round the circuit cable.
After all, what bearing could geography have on the electrical consequences of any actions?

 

[JohnW2]

However, this guide does not ensure compliance with BS 7671. It is a simple guide to the requirements of BS 7671, and electricians and electrical installers should always consult BS 7671 to satisfy themselves of compliance. ... So, perhaps it (7.2.2) doesn't mean much.

I did say that it was guidance and not the regs - but one can but assume that the IET think that their 'rule of thumb' is at least a good idea.

I would say that it is obvious, in my opinion, that the farthest point of a ring is half way round the circuit cable. After all, what bearing could geography have on the electrical consequences of any actions?

We know that it means half way round the circuit cable (not half way round in geographical terms), because it says so! However, as I asked, 'half way round' as measured from where - the CU, the spur, or where?

Kind Regards, John

 

[EFLImpudence]

We know that it means half way round the circuit cable (not half way round in geographical terms), because it says so! However, as I asked, 'half way round' as measured from where - the CU, the spur, or where?

Well. that's even more obvious, isn't it?

Half way round the ring is the farthest point from the CU.
There would be no point the farthest point being half way round from the spur as that would just be half the length of the ring from wherever.
It then need only state that a spur's length shall not exceed 1/16 the length of the ring.
Why would 'half the length of the ring' be termed as 'the farthest point'?

Also, it is 'farthest' not 'furthest' - so it's not written correctly so why take so much notice of its content and it also tells you to consult BS 7671 to make sure?
Why not just make sure first?

 

[JohnW2]

Well. that's even more obvious, isn't it? Half way round the ring is the farthest point from the CU.

Have you thought that 'obvious' suggestion through? .... do you believe that this 'rule of thumb' in the OSG is suggesting that if, say, you connect a spur 0.8m from the point in the ring which is farthest from the CU (i.e. virtually at the middle of the ring), the length of that spur cable should be limited to a maximum of 0.1m ? !!

As for furthest/farthest, I may have all sorts of degrees coming out of my earholes, but I never get that one right (none of my degrees is in English - you need my younger daughter for that, but she can't spell for Adam ).

Kind Regards, John

 

[EFLImpudence]

[Have you thought that 'obvious' suggestion through? .... do you believe that this 'rule of thumb' in the OSG is suggesting that if, say, you connect a spur 0.8m from the point in the ring which is farthest from the CU (i.e. virtually at the middle of the ring), the length of that spur cable should be limited to a maximum of 0.1m ? !!

Yes, I did, but I thought it may spoil my argument.

Perhaps it spoils the OSG rule of thumb.

However, pretending I am wrong - regarding the length of spurs, what could be the relationship between them and an eighth of anything?
Is there a reason why spurs could be longer nearer the CU (assuming circuit measurements are within limits)?

As for furthest/farthest, I may have all sorts of degrees coming out of my earholes, but I never get that one right (none of my degrees is in English - you need my younger daughter for that, but she can't spell for Adam

That's alright but such mistakes in official documents make me wonder about the rest of the content.

Furthermore, farther away.

 

[JohnW2]

[Have you thought that 'obvious' suggestion through? .... do you believe that this 'rule of thumb' in the OSG is suggesting that if, say, you connect a spur 0.8m from the point in the ring which is farthest from the CU (i.e. virtually at the middle of the ring), the length of that spur cable should be limited to a maximum of 0.1m ? !!

Yes, I did, but I thought it may spoil my argument. Perhaps it spoils the OSG rule of thumb.

It certainly spoils something. Whatever they intended, it surely was not what your 'obvious' interpretation leads to?

However, pretending I am wrong - regarding the length of spurs, what could be the relationship between them and an eighth of anything?
Is there a reason why spurs could be longer nearer the CU (assuming circuit measurements are within limits)?

Not that I can think of. If you look back a few pages in this thread, you'll see that one of the few things BAS and I agreed about was that neither of us could think of any reason why the recommended length of a spur cable should vary according to where it was attached to a ring.

That's alright but such mistakes in official documents make me wonder about the rest of the content.

Agreed, up to a point - although an awful lot of official documents, even laws, these days are full of grammatical, spelling and typographical errors, even when the actual 'content' is generally reasonable! However, the point is surely this. As we are agreed, BS7671 says nothing about this 'length of a spur' stuff, so there was absolutely no complusion on the IET to say anything about it in the OSG. The fact that they did include this very odd 'rule of thumb' must mean that someone understood what it was meant to mean, and also thought that there was some good reason for recommending it!

Kind Regards, John.

 

[EFLImpudence]

I can't add anything at the moment but would just ask -

On a ring with no spurs, surely you would agree, the farthest point would be half way round from the CU.
Therefore, is it plausible that the farthest point can move?

 

[ban-all-sheds]

According to Sir Ernest Gowers it's particularly official documents which are full of errors...

Haven't forgotten your post of 01:37 - been busy and haven't had time to give it the necessary attention.

Re the OSG ROT - I haven't looked, but I wonder if the worst-case length of a spur from a ring of maximum length for a 3036 on a TN-S supply works out at 3m.....